CERNCourier2016Apr Digitaledition

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CERNCOURIERIN T E R N A T I O N A L J O U R N A L O F H I G H - E N E R G Y P H Y S I C S

AWAKE

The plasma cell is in

its final position

p10

ACCELERATORMILESTONE

Japan’s SuperKEKB

achieves “first turns” p11

DZero collaboration

discovers a new

particle

p13

TETRAQUARKS

A strong belief

Welcome to the digital edition of the April 2016 issue ofCERN Courier.

The issue went to print while the LHC resumed operation after the technicalstop that started in December 2015. At the same time, in another accelerator,SuperKEKB in Japan, beams have completed their rst turns. However, the

spotlight these days is not so much on accelerator physics as on the discoveryof gravitational waves by the LIGO interferometers in the US, which is gainingthe interest of the whole scientic community. The interview with Barry

Barish, one of the founding fathers of LIGO, proves just how hard scientic

endeavours can sometimes be. Hard, but also extremely rewarding. Comingback to closer universes, this issue also features ar ticles about CERN’s neutron

facility, ALICE’s new TPC, and science carr ied out with a PET cyclotron.Last but not least, Interactions & Crossroads brings you information aboutinteresting conferences in physics and related elds around the world.

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CERN Courier – digital edition

W E L C O M E

WWW.

EDITOR: ANTONELLA DEL ROSSO, CERN

DIGITAL EDITION CREATED BY JESSE KARJALAINEN/IOP PUBLISHING, UK

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C ER N C o ur i e r A pr i l 2 0 1 6

Contents


Covering current developments in high-energyphysics and related fields worldwide

CERN Courier is distributed to member-state governments, institutes and laboratoriesaffiliated with CERN, and to their personnel. It is published monthly, except forJanuary and August. The views expressed are not necessarily those of the CERNmanagement.

Editor Antonella Del RossoBooks editor Virginia GrecoCERN, 1211 Geneva 23, SwitzerlandE-mail [email protected] +41 (0) 22 76 69070Web cerncourier.com

Advisory board Luis Alvarez-Gaume, Peter Jenni, Christine Sutton, Claude Amsler,Roger Bailey, Philippe Bloch, Roger Forty

Laboratory correspondents:Argonne National Laboratory (US) Tom LeCompteBrookhaven National Laboratory (US) P YaminCornell University (US) D G CasselDESY Laboratory (Germany) Till Mundzeck EMFCSC (Italy) Anna CavalliniEnrico Fermi Centre (Italy) Guido PiraginoFermi National Accelerator Laboratory (US) Katie YurkewiczForschungszentrum Jülich (Germany) Markus BuescherGSI Darmstadt (Germany) I PeterIHEP, Beijing (China) Tongzhou XuIHEP, Serpukhov (Russia) Yu RyabovINFN (Italy) Antonella VaraschinJefferson Laboratory (US) Steven CorneliussenJINR Dubna (Russia) B StarchenkoKEK National Laboratory (Japan) Saeko OkadaLawrence Berkeley Laboratory (US) Spencer KleinLos Alamos National Laboratory (US) Rajan GuptaNCSL (US) Ken KingeryNikhef (Netherlands) Robert FleischerNovosibirsk Institute (Russia) S EidelmanOrsay Laboratory (France) Anne-Marie LutzPSI Laboratory (Switzerland) P-R KettleSaclay Laboratory (France) Elisabeth LocciScience and Technology Facilities Council (UK) Jane BinksSLAC National Accelerator Laboratory (US) Farnaz KhademTRIUMF Laboratory (Canada) Marcello Pavan

Produced for CERN by IOP Publishing LtdIOP Publishing Ltd, Temple Circus, Temple Way,Bristol BS1 6HG, UK

Tel +44 (0)117 929 7481

Publisher Susan CurtisProduction editor Lisa GibsonTechnical illustrator Alison ToveyGroup advertising manager Chris ThomasAdvertisement production Katie GrahamMarketing & Circulation Angela Gage

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AdvertisingTel +44 (0)117 930 1026 (for UK/Europe display advertising)or +44 (0)117 930 1164 (for recruitment advertising);E-mail: [email protected]; fax +44 (0)117 930 1178

General distribution Courrier Adressage, CERN, 1211 Geneva 23, SwitzerlandE-mail: [email protected] certain countries, to request copies or to make address changes, contact:China Jiang Ya'ou, Library, Institute of High Energy Physics,PO Box 918, Beijing 100049, People’s Republic of ChinaE-mail: [email protected] Antje Brandes, DESY, Notkestr. 85, 22607 Hamburg, GermanyE-mail: [email protected] Mark Wells, Science and Technology Facilities Council, Polaris House, North Star

Avenue, Swindon, Wiltshire SN2 1SZE-mail: [email protected] US/Canada Published by Cern Courier, 6N246 Willow Drive,St Charles, IL 60175, US. Periodical postage paid in St Charles, IL, USFax 630 377 1569. E-mail: [email protected]: send address changes to: Creative Mailing Services, PO Box 1147,St Charles, IL 60174, US

Published by European Organization for Nuclear Research, CERN,1211 Geneva 23, SwitzerlandTel +41 (0) 22 767 61 11. Telefax +41 (0) 22 767 65 55

Printed by Warners (Midlands) plc, Bourne, Lincolnshire, UK

© 2016 CERN ISSN 0304-288X

V O L U M E 56 N U M B E R 3 AP R IL 2016

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CERNCOURIERINTERNATIONAL J OURNAL OF HIGH -ENERGY PHYSICS

AWAKE

The plasma cell is inits final positionp10

ACCELERATORMILESTONE

Japan’s SuperKEKBachieves “first turns”p11

DZero collaborationdiscovers a newparticlep13

TETRAQUARKS

AstrongbeliefOn the cover: Artwork showing how our Sun and Ear th warp space and time.Gravitational waves are created when massive bodies accelerate through space andtime. (Image credit: T Pyle/Caltech/MIT/LIGO Lab )

5 V I EWP O INT

7 NEW S

• The LHC is restarting • Searches with boosted topologies at Run 2• Anisotropic ow in Run 2• CMS hunts for supersymmetryin uncharted territory • LHCb awards physics prizes for its Kagglecompetition • Another important step for the AWAKE experiment• BESIII makes rst direct measurement of the Λc at threshold• ‘First turns’ for SuperKEKB• TPS exceeds design goal of500 mA stored current• DZero discovers new four-avour particle• Testing of DUNE tech begins

15 SC IENC EWA TC H

17 AS TR O WA TC H

FEA TU R ES18 LIGO: a strong belief

Twenty years of hard work with a strong beliefthat the endeavour would lead to a historicbreakthrough.

21 Science with a medical PET cyclotron Medical PET cyclotrons can be used for multidisciplinary research.

23 ALICE selects gas electron multipliers for its new TPCThe experiment plans a major upgrade for its TPC.

25 Neutrons in full flight at CERN’s n_TOF facilityThe facility features two beamlines and two experimental halls.

29 INTER A C T IO NS & CR O S S R O A DS

33 FA C ES & PL A C ES

36 REC R U ITM ENT

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Viewpoint

By Sijbrand de Jong

Interest in CERN has evolved over the years. At itsinception, the Organization’s founding member statesclearly saw the new institution’s potential a s a centre ofexcellence for basic research, a driver of innovation, aprovider of rst-class education and a catalyst for peace.After several decades of business as usual, CERN isagain on the radar of its member-state governments.This is spurred on partly by the public interest thathas made CERN something of a household name. Butwhether in the public spotlight or not, it is incumbenton CERN to spell out to all of its sta keholders why itrepresents such good value for money today, just asit did 60 years ago. Even though the reasons may befamiliar to those working at CERN, they are not alwaysso clear to government ofcials and policy makers, andit’s worth setting them out in detail.

First and foremost, CERN has made majorcontributions to how we understand the world that welive in. The discovery and detaile d study of weak vectorbosons in the 1980s and 1990s, and the recent discovery

of the Higgs boson, messenger of the Brout–Englert–Higgs mechanism, have contributed much to ourunderstanding of nature at the level of the fundamentalparticles and their interactions: now rightfully calledthe Standard Model of particle physics. This on itsown is a major cultural achievement, and it has taughtus much about how we have arrived at this point inhistory: right fr om the moment it all began, 13.6 billionyears ago. Appreciation of this cultural contributionhas never been higher than today. More than 100,000people visit CERN every year, including hundreds of journalists reaching millions of people. None leaveCERN unimpressed, and all are, without a doubt,culturally enriched by their experience.

Educating and innovatingCERN’s second major area of impact is education,having educated many generations of top-levelphysicists, engineers and technicians. Some haveremained at CERN, while others have gone on to

pursue careers in basic research at universities andinstitutes elsewhere, therefore contributing to top-leveleducation and multiplying the effect of their ownexperience at CE RN. Many more, however, have madetheir way into industry, fullling an important missionfor CERN – that of providing skilled people to advancethe economies of our member states and collaboratingnations. More than 500 doctoral degrees are awardedannually on the basis of work carried out at CERNexperiments and accelerators. In 2015, more than400 doctoral, technical and administrative students

were welcomed by CERN, usually staying for betweenseveral months and a year. The CERN summer-studentand teacher programmes, which provide short sti ntsof intensive education, also welcome hundreds ofstudents and high-school teachers every year.

A third important contribution of CERN is theinnovation that results from research that requirestechnology at levels and in areas where no one hasgone before. The best-known example of CER Ntechnology is the World Wide Web, which hasprofoundly changed the way that our society worksworldwide. But the web is just the tip of t he iceberg.Advances in fields such as magnet technology,cryogenics, electronics, detector technology andstatistical methods have also made their way intosociety in ways that are equally impactful, althoughless obviously evident. While the societal benetsof techniques such as advanced photon and leptondetection may not seem immediately relevantbeyond the realms of resear ch, the impact they havehad in medical imaging, for instance, is profound.

Often not very visible, but no less effective in

contributing to our prosperity and well-being,developments such as this are a vital part of theresearch cycle. CERN is increasingly taking aproactive approach towards transferring its innovation,knowledge and skills to those who can make thesecount for society as a whole, and this is generallywell appreciated. Recent initiatives include public–private partnerships such as OpenLab, Medipix andIdeaSquare, which provide low-entry-thresholdmechanisms for companies to engage with CERNtechnology. In return, CERN benefits throughstimulating the kind of industrial innovation thatenables next-generation accelerators and detectors.

The recent Viewpoint by CERN Director-General,Fabiola Gianotti (CERN Courier, March 2016 p5) givesa superb outline of the opportunities and challengesfor particle physics during the coming years. Clearly itwill require great dexterity to juggle the continuationof a state-of-the-art research programme at the LHCand a diverse range of other facilities, with greater

engagement with important activities beyond CERN,such as the US neutrino programme, while at the sametime preparing for future accelerators and detectors.This will stretch CERN’s capabilities to the limit.But it is precisely this challenge that will motivate theOrganization to do better and innovate in all areas,with inevitable benets for society. Scientic cultureand societal impacts advancing hand-in-hand throughcutting-edge research: it is this that makes CERNworthy of the support it r eceives from governmentsworldwide.

The CERN effectTransferring innovation and knowledge so that cutting-edge research can benefit society.

A student from CERN’s Beamline for Schoolscompetition explains histeam’s experiment tomembers of Council. Beamline for Schools is animportant part of CERN’seducational work.

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News

The LHC, last among all of CERN’saccelerators, is resuming operation withbeam while this issue goes to press. Theyear-end technical stop (YETS) started on14 December 2015. During the 11 weeksof scheduled maintenance activities,several interventions have taken place inall of the accelerators and beamli nes. Theyincluded the maintenance of the cryogenicsystem at several points; the replacement of18 magnets in the Super Proton Synchrotron(SPS); an extensive campaign to identifyand remove thousands of obsolete cables;the replacement of the LHC beam absorbersfor injection (TDIs) that are used to absorbthe SPS beam if a problem occurs, providingvital protection for the LHC; and 12 LHCcollimators have been dismantled andreinstalled after modication of the vacuumchambers, which restricted their movement.

The YETS also gave the experimentsthe opportunity to carry out repairs andmaintenance work in their detectors. Inparticular, this included xing the ATLAS

vacuum-chamber bellow and cleaning thecold box at CMS, which had caused problemsfor the experiment’s magnet dur ing 2015.

Bringing beams back into the machineafter a technical stop of a few weeks isno trivial matter. The Electrical QualityAssurance (ELQA) team needs to test theelectrical circuits of the superconductingmagnets, certifying their readiness foroperation. After that, the powering tests canstart, and this means about 7000 tests in12 days – a critical task for all of t he teamsinvolved, which will rely on the availabilityof all of the sectors. About four weeks afterthe start of commissioning, the LHC isready to receive rst beams and for them tocirculate for several hours in the machine(stable beams).

The goal of this second part of Run 2 isto reach 2700 bunches pe r beam at 6.5 TeV

and with nominal 25 ns spacing. In 2015, themachine reached a record of 2244 bunchesin each beam, just before the beginningof the YETS. In 2016, the focus of theoperators will be on ensuring maximumavailability of the machine. For this, pipescrubbing will be performed severaltimes to keep the electron cloud effectsunder control. Thanks to the experienceacquired in 2015, the operators will be ableto improve the injection process and to

perform ramping and squeezing at the same

time, therefore reducing the time neededbetween two successive injections.

In addition to several weeks ofsteady standard 13 TeV operation with2700 bunches per beam and β* = 40 cm,the accelerator schedule for 2016 includesa high-β* (~ 2.5 km) running periodfor TOTEM/ALFA dedicated to themeasurement of the elastic proton–protonscattering in the Coulomb–nuclearinterference region. The schedule alsoincludes one month of heavy-ion run.Although various congurations (Pb–Pband p–Pb) are still under consideration,the period – November – has already beendecided. As usual, the heavy-ion run willconclude the 2016 operation of the LHC,while the extended year-end technical stop(EYETS) will start in December and willlast about ve months, until April 2017.

Several upgrades are already planned by theexperiments during the EYETS, includinginstallation of the new pixel system at CMS.

The goal for the second part of Run 2 is toreach 1.3 × 1034 cm –2 s –1 of luminosity, whichwith about 2700 bunches and 25 ns spacingis estimated to produce a pile-up of 40 eventsper bunch crossing. This should give anintegrated luminosity of about 25 fb –1 in2016, which should ensure a total of 100 fb –1 for Run 2 – planned to end in 2018.

The LHC is restartingA C C E L E R A T O R S

Sommaire en françaisLe LHC redémarre 7

Exploitation 2 : des recherches visent des 8

particules dopées

Mesure de flux anisotrope de l’exploitation 2 8

CMS explore des terres inconnues en quête 9

de la supersymétrie

LHCb décerne des prix aux lauréats de son 9

concours « Kaggle »

Nouvelle étape pour AWAKE 10

BESIII : première mesure directe du Λc 11au seuil

« Premiers tours » au SuperKEKB 11

Le TPS dépasse sa capacité nominale de 12

stockage de 500 mA

DZero découvre une particule à quatre 13saveurs

Premiers tests des technologies destinées 13

à DUNE

Le retour de l’incandescence 15

L’excès de rayons gamma observé ne vient 17

pas de la matière noire

The wall current monitors are installed in the PS ring. Several maintenance activities werecarried out in the whole CERN accelerator chain during the year-end technical stop,in preparation for the general restart.

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News

The CMS collaborationis continuing its hunt forsigns of supersymmetry(SUSY), a popular extensionto the Standard Model

that could provide a weakly interactingmassive-particle candidate for dark matter,if the lightest supersymmetric particle(LSP) is stable.

With the increase in the LHCcentre-of-mass energy from 8 to 13 TeV, theproduction cross-section for hypotheticalSUSY partners rises; the rst searches tobenet are those looking for the stronglycoupled SUSY partners of the gluon (gluino)and quarks (squarks) that had the moststringent mass limits from Run 1 of t he LHC.By decaying to a stable LSP, which does notinteract in the detector and instead escapes,SUSY particles can leave a characteristic

experimental signature of a large imbalancein transverse momentum.

Searches for new physics based on nalstates with jets (a bundle of particles) andlarge transverse-momentum imbalance aresensitive to broad classes of new-physicsmodels, including supersymmetry. CMS hassearched for SUSY in this nal state usinga variable called the “stransverse mass”,MT2, to measure the transverse-momentumimbalance, which strongly suppresses fake

contributions due to potential had ronic-jetmismeasurement. This allows us to controlthe background from copiously producedQCD multi-jet events. The remainingbackground comes from Standard Modelprocesses such as W, Z and top-quark pairproduction with decays to neutrinos, whichalso produce a transverse-momentumimbalance. We estimate our backgroundsfrom orthogonal control samples in datatargeted to each. To cover a wide variety ofsignatures, we categorise our signal eventsaccording to the number of jets, the numberof jets arising from bottom quarks, the sumof the transverse momenta of hadronic jets(HT), and MT2. Some SUSY scenariospredict spectacular signatures, such as fourtop quarks and two LSPs, which would givelarge values for all of these quantities, whileothers with small mass splittings produce

much softer signatures.Unfortunately, we did not observe any

evidence for SUSY in the 2015 data set.Instead, we are able to signicantly extendthe constraints on the masses of SUSYpartners beyond those from the LHC Run 1.The gluino has the largest productioncross-section and many potential decaymodes. If the gluino decays to the LSP anda pair of quarks, we exclude gluino massesup to 1550–1750 GeV, depending on the

quark flavour, extending our Run 1 lim its bymore than 300 GeV. We are also sensitive tosquarks, with our constraints summarisedin gure 1. We set limits on bottom-squarkmasses up to 880 GeV, top squarks up to

800 GeV, and light-flavour squarks up to600–1260 GeV, depending on how manystates are degenerate in mass.

Even though SUSY was not waiting for usaround the corner at 13 TeV, we look forwardto the 2016 run, where a large increasein luminosity gives us another chance atdiscovery.

● Further reading CERN-EP-2016-067.

CMS hunts for supersymmetryin uncharted territory

Fig. 1. Exclusion limits as a function ofsquark and LSP masses, for squark pair

production with different avours andmass-degeneracy scenarios.

q = t

~

400 800 1200mq (GeV)

0

200

400

600

800

1000

q = qL + q

R (u, d, s, c)

CMS preliminary

SUS-15-003, 0-lep (M T2)

pp→ qq, q → q χ1)

expectedobserved~ ~

~ ~

q = b~ ~

~ ~ ~ ~ ~

q = one light q~ ~

~~ ~ ~

2.3 fb–1 fb–1 (13 TeV)

~

m χ 1

( G e V )

0

0

C M S C o l l a b o r a t i o n

Machine learning, alsoknown in physics circlesas multivariate analysis,is used more and more in

high-energy physics, most visibly in dataanalysis but also in other applications such astrigger and reconstruction. The communityof machine-learning data scientists organises“Kaggle” competitions to solve difcult andinteresting challenges in different elds.

With the aim being to developinteractions with the machine-learningcommunity, LHCb organised such acompetition, featuring the search forthe lepton-flavour violating decay,

LHCb awards physics prizes for its Kaggle competition

▲

increase measured in the integrated flow(gure 1) reflects the increase of the meantransverse momentum. Further comparisonsof differential-flow measurements andtheoretical calculations will provide a

unique opportunity to test the validity ofthe hydrodynamic picture, and the powerto further discriminate between variouspossibilities for the temperature dependenceof the shear-viscosity to entropy density

ratio of the produced matter in heavy-ioncollisions at highest energies.

● Further reading J Adam et al. ALICE Collaboration, arXiv:1602.01119.

LHCb Kaggle-competition participants.

M a r c i n C h r z a s z c z

8


News

The rst LHC runwas highlightedby the discovery ofthe long-awaited

Higgs boson at a ma ss of about 125 GeV, butwe have no clue why nature chose this mass.Supersymmetry explains this by postulating

a partner particle to each of the StandardModel (SM) fermions and bosons, but thesenew particles have not yet been found. Acomplementary approach to address this issueis to widen the net and look for signaturesbeyond those expected from the SM.

Searches for new physics in Run 1 foundno signals, and from these negative resultswe know that new particles may be heavy.For this reason, their decay products, such astop quarks, electroweak gauge bosons (W, Z)or Higgs bosons, may be very energetic andcould be highly boosted. When such particlesare produced with large momentum and decayinto quark nal states, the decay productsoften collimate into a small region of thedetector. The collimated sprays of hadrons(jets) originating from the nearby quarksare therefore not reliably distinguished.Special techniques have been developed to

reconstruct such boosted particles into jetswith a wide opening angle, and to identifythe cores associated with the quarks using

soft-particle-removal procedures (grooming).ATLAS performed an extensive optimisationof top, W, Z and Higgs boson identication,exploiting a wide range of jet clustering andgrooming algorithms as well as kinematicproperties of jet substructure before thesecond LHC run. This led to a factor of twoimprovement in W/Z tagging, compared with

the technique used previously in terms ofbackground rejection for the same efciencyfor W/Z boson transverse momenta around

300–50 0 GeV.ATLAS exploited the optimised boson

tagging in the search for heavy resonancesdecaying into a pair of two electroweakgauge bosons (WW, WZ, ZZ) or of a gaugeboson and a Higgs boson (WH, ZH) at13 TeV collisions. Events are categorised into

different numbers of charged/neutral leptons,and all possible combinations are consideredexcept for fully leptonic and fully hadronicWH or ZH decays. For the Higgs boson,only the dominant decay into b quarks isconsidered. Figure 1 shows the results of WZsearches with a 2015 data set correspondingto 3.2 fb –1, presented as the lower limits on theproduction cross-section times the branchingfraction for a new massive gauge boson withcertain mass. No evidence for new physics hasbeen found with these preliminary searches.

The boosted techniques have evolved intoa fundamental tool for beyond SM searchesat high energy. ATLAS foresees that thesearch will be greatly enhanced by t hetechniques, and seeks opportunities to adaptthem in uncharted territory for the upcomingLHC run.

● Further reading See papers and notes at https://twiki.cern.ch/twiki/ bin/view/AtlasPublic/December2015-13TeV .

Searches with boosted topologies at Run 2L H C E X P E R I M E N T S

Fig. 1. Expected and observed limits on thecross-section times branching fraction toWZ for a new heavy-vector boson, W´, at

√ s = 13 TeV. The different limit curvescorrespond to different decay modes for theW and Z bosons.

500

10

1

10

–1

10–2

1 00 0 1 50 0mw' (GeV)

2 00 0 2 50 0 3 00 0 σ ( p p → W

' ) × B R ( W ' → W

Z ) [ p b ]

HVT model A, g v = 1

95% C.L. exclusion limitse xp ec te d obs er ve d

llqq l νqq

ννqq qqqq

HVT W' →WZ

ATLAS preliminary

√s = 13 TeV, 3.2 fb–1

Exploiting the data collectedduring November 2015with Pb–Pb collisions atthe record-breaking energyof √ sNN= 5.02 TeV, ALIC E

measured for the rst time the anisotropicflow of charged particles at this energy.

Relativistic heavy-ion collisions are thetool of choice to i nvestigate quark–gluonplasma (QGP) – a state of matter wherequarks and gluons move freely over distancesthat are large in comparison to the typical

size of a hadron. Anisotropic flow, whichmeasures the momentum anisotropy ofnal-state particles, is sensitive on the onehand to the initial density and to the initialgeometry fluctuations of the overlap region,and on the other hand to the transportproperties of the QGP. Flow is quantied bythe Fourier coefcients, νn, of the azimuthaldistribution of the nal-state chargeparticles. The dominant flow coefcient,ν2, referred to as elliptic flow, is related to

the initial geometric anisotropy. Highercoefcients, such as triangular flow (ν3)and quadrangular flow (ν4), can be relatedprimarily to the response of t he producedQGP to fluctuations of the initial energydensity prole of the par ticipating nucleons.

Figure 1 shows the centrality dependenceof flow coefcients, both for 2.76 and5.02 TeV Pb–Pb collisions. Compared withthe lower-energy results, the anisotropicflows ν2, ν3 and ν4 increase at the newlymeasured energy by (3.0±0.6)%, (4.3±1.4)%

and (10.2±3.8)%, respectively, in thecentrality range 0–50%.The transport properties of the created

matter are investigated by comparing theexperimental results with hydrodynamicmodel calculations, where the shear-viscosityto entropy density ratio, η/s, is the dominantparameter. Previous studies demonstratedthat anisotropic flow measurements arebest described by calculations using a valueofη/s close to 1/4π, which corresponds to

the lowest limits for a quantum fluid. It isobserved in gure 1 that the magnitude andthe increase of anisotropic flow measured atthe higher energy remain compatible withhydrodynamic predictions, favouring aconstant value for η/s going from √ sNN= 2.76to 5.02 TeV Pb–Pb collisions.

It is also observed that the results of thepT-differential flow are comparable for bothenergies. This observation indicates that the

Anisotropic flow in Run 2

0.05

0.00

0.10

0.15

v n

5.02 TeV 2.76 TeV 5.02 TeV

ALICE Pb–Pb hydrodynamics

0 10 20 30 40centrality percentile

50 60 70 80

v2(2,|Δη| > 1)v2(2,|Δη| > 1)v3(2,|Δη| > 1)v3(2,|Δη| > 1)

v4(2,|Δη| > 1)

v2(2,|Δη| > 1)

v3(2,|Δη| > 1)

v4(2,|Δη| > 1)v2(4)v2(4)

v2(6)v2(8)

Fig. 1. Anisotropic ow, vn, as a function ofevent centrality.

A L I C E C o l l a b o r a t i o n

A T L A S C o l l a b o r a t i o n

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6/24


11


News

The charmed baryon, Λc, was rst observedat Fermilab in 1976. Now, 40 years later, theBeijing Spectrometer (BESIII) experimentat the Beijing Electron–Positron Collider II(BEPCII) has measured the absolutebranching fraction of Λc

+→ pΚ−π+ atthreshold for the rst time.

Because the decays of the Λc+ to hadronsproceed only through the weak interaction,their branching fractions are key probes forunderstanding weak interactions inside of abaryon. In particular, precise measurementsof the decays of the Λc

+ will provide importantinformation on the nal-state stronginteraction in the charm sector, therebyimproving the understanding of quantumchromodynamics in the non-perturbativeenergy region. In addition, because most ofthe excited baryons of the Λc and Σc types, aswell as the b-flavoured baryons, eventuallydecay into a Λc

+, studies of these baryons aredirectly connected to understanding theground state, Λc+.

Most decay rates of the Λc+ are measured

relative to the decay mode, Λc+→ pΚ−π+, but

there are no completely model-independentmeasurements of the absolute branching

fraction for this decay mode. Moreover, mostmeasurements of the ground-state Λc

+ weremade more than 20 years ago.

In 2014, BESIII accumulated a data sampleof e+e− annihilations with an integratedluminosity of 567 pb−1 at a centre-of-massenergy of 4.599 GeV. This is about 26 MeVabove the mass threshold for a Λc

+Λ –

c−, so no

additional hadrons accompanying the Λc+Λ –

c−

are produced.The BESIII collaboration measures

hadronic branching fractions at theΛc

+Λ –

c− threshold using a double-tagging

technique that relies on fully reconstructed

Λc+Λ –

c− decays. This technique obviates

the need for knowledge of the luminosityor the Λc

+Λ –

c− production cross-section.

To improve precision, BESIII combines12 Cabibbo-favoured decay channelsand implements a global least-squares tby considering their correlations. This

leads to a result for the branching fractionfor Λc+→ pΚ−π+ of B(Λc+→ pΚ−π+) =(5.84±0.27±0.23)%.

This is the rst measurement of theabsolute branching fraction of the decayΛc

+→ pΚ−π+ at threshold, and it has theadvantage of incorporating an optimalunderstanding of model uncertainty. Inaddition, BESIII has made signicantlyimproved measurements of the other11 Cabibbo-favoured hadronic-decaymodes.

In 2015, based on the same data set,BESIII also measured the absolutebranching fraction of the semi-leptonicdecayΛc+→ Λe+νe, using a missing-neutrinotechnique. In future, a larger Λc

+ thresholdsample will help to improve furtherunderstanding of the properties of the Λc

+.

● Further reading BESIII Collaboration 2015 Phys. Rev. Lett. 115 221805.BESIII Collaboration 2016 Phys. Rev. Lett. 116 052001.

C H A R M D E C A Y S

BESIII makes first direct measurement of the Λc at threshold

B E S I I I C o l l a b o r a t i o n

The gure shows the beam-constrainedmass M BC (M BC c

2= √ (E 2beam − p2c2))

distribution for the simple tag samples. Themass peak is at (2286.46 ± 0.14) MeV/c2.

1500

Λc →pK –π

+

1500

500

2.25 2.26 2.27

MBC(GeV/c2)

2.28 2.29 2.30

0

e v e n t s / ( 1 . 0

M e V / c 2 )

+

N E W F A C I L I T I E S

On 10 February, the SuperKEKB electron–positron collider in Tsukuba, Japan, succeededin circulating and storing a positron beammoving close to the speed of light through1000 magnets in a narrow tube around the3 km circumference of its main ring. And on26 February, it succeeded in circulating andstoring an electron beam around its ring ofmagnets in the opposite direction.

The achievement of “rst turns”, whichmeans storing the beam in the ring throughmany revolutions, is a major milestone forany particle accelerator.

SuperKEKB, along with the Belle IIdetector, is designed to search for new physicsbeyond the Standard Model by measuring raredecays of elementary particles such as beautyquarks, charm quarks and τ leptons.

Unlike the LHC, which is the world’shighest-energy machine, SuperKEKB/

Belle II is designed to have the world’shighest luminosity – a factor of 40 higherthan the earlier KEKB machine, which holdsmany records for accelerator performance.SuperKEKB will therefore be the leadingaccelerator on the “luminosity frontier”.

The Belle II detector at SuperKEKBwas designed and built by an international

collaboration of more than 60 0 physicistsfrom 23 countries. This collaborationis working closely with SuperKEKBaccelerator experts to optimise the machineperformance and backgrounds.

At the same time as rst turns wereachieved, the BEAST in its cave at TsukubaHall awakened from its slumber. TheBEAST II detector is a system of detectorsdesigned to measure the beam backgroundsof the SuperKEKB accelerator. The para siticradiation produced by electromagneticshowers when the beam collides with thewalls of the vacuum pipe not only obscurethe signals that we wish to observe, but can

also damage the detector. Therefore, whenoperating the new accelerator, these beambackgrounds must be well understood.

The BEAST II detector will collect datain the unique environment produced bySuperKEKB’s rst beams, allowing Belle IIto safely roll into t he beam in 2017. ● Further reading https://twitter.com/belle2collab and https://www.facebook.com/belle2collab/ .

‘First turns’ forSuperKEKB

S u p e r K E K B

100

80

60

40

20

0

0 50time (μs)

100 150 200 250

a m p l i t u d e ( a r b i t r a r y u n i t s )

signal channel A

signal channel B

revolution clock 10 μs

injection trigger

The three middle “spikes” are signals from

CLAWS, a subsystem of the BEAST II detectortriggered by the SuperKEKB injection signal.

10


News

By harnessing the power of wakeeldsgenerated by a proton beam in a plasmacell, the AWAKE experiment at CERN(CERN Courier November 2013 p17)aims to produce accelerator gradients thatare hundreds of times higher than thoseachieved in current machines.

The experiment is being installed in

the tunnel that was previously used by theCERN Neutrinos to Gran Sasso facility. InAWAKE, a bea m of 400 GeV protons fromthe CERN Super Protron Synchrotron willtravel through a plasma cell and will generatea wakeeld that, in turn, will accelerate anexternally injected electron beam. A laserwill ionise the gas in the cell to becomea plasma and seed the self-modulationinstability that will tr igger the wakeeld.The project aims to prove that the plasmawakeeld can be driven with protons and thatits acceleration will be extremely powerful – hundreds of times more powerful than thatachieved today – and eventually to provide adesign for a plasma-based linear collider.

The AWAKE tunnel is progr essivelybeing lled with its vital components. In itsnal conguration, the facility will featurea clean room for the laser, a dedicated area

for the electron source and two new tunnelsfor two new beamlines: one small tunnelto hold the laser beam, which ionises theplasma and seeds the wakeelds, and asecond, larger tunnel that wil l be home

to the electron beamline – the “witnessbeam” accelerated by the plasma. At thebeginning of Februar y, the plasma cell waslowered into the tunnel and moved to itsposition at the end of the proton line. Thecell is a 10 m-long component developedby the Max Planck Institute for Physicsin Munich (Germany). A rst prototype

successfully completed commissioningtests in CERN’s North Area in the autum nof 2015. The prototype allowed the AWAKEcollaboration to validate the uniformity ofthe plasma temperature in the cell.

AWAKE is a collaborative endeavourwith institutes and organisationsparticipating around the world. Thesynchronised proton, electron and laserbeams provided by CERN are an integralpart of the experiment. After installationof the plasma cell, the next step willbe installation of the laser, the vacuumequipment and the diagnostic system forboth laser and proton beams.

Beam commissioning for the protonbeamline is scheduled to start this summer.The programme will continue withinstallation of the electron line, with the aimof starting acceleration tests at the end of 2017.

● Further reading https://cds.cern.ch/journal/CERNBulletin/2016/07/News%20Articles/2131154.

Another important step for theAWAKE experiment

AWAKE’s 10 m-long plasma cell in the experiment tunnel.

C E R N

τ→ μμμ. This decay is (almost) forbiddenin the Standard Model, and therefore itsobservation would indicate a discovery of“new physics”, which is now the key goalof the LHC. This Kaggle challenge (https://www.kaggle.com/c/flavours-of-physics)was conceived by a group of scientistsfrom CERN, the University of Warwick,the University of Zürich and the YandexSchool of Data Analysis. It was nanciallysupported by the Yandex Data Factor y,Intel and the University of Zürich. The

competition took place over three monthsbetween July and October 2 015. More than700 people competed to achieve the bestsignal-versus-background discriminationand to win the prize awarded to the r stthree ranked solutions, totalling $15,000.

This particular challenge, using both“real” and simulated LHCb data, has beenrecognised by the community as morecomplicated than usual challenges, andtherefore a refreshing problem to try andsolve. The winners of the competitionwere awarded their prizes in Decemberat one of the main conferences of themachine-learning community – theTwenty-ninth Annual Conference on NeuralInformation Processing Systems (NIPS).

In addition to the prizes for the best-rankedsolutions, another prize was foreseen for thesolution that is the most interesting from a

physics point of view. In the event, LHCbdecided to award two of these physics prizesof $2000 each to Vincens Gaitan (a formermember of the ALEPH collaboration atCERN’s Large Electron–Positron collider)and Alexander Rakhlin. Their solutions areinnovative and particularly suitable for caseswhere the size of the samples used to trai nthe multivariate operator is limited and whenthe training samples do not perfectly matchthe real data.

The two awardees collected their prizeat a three-day workshop organised at theUniversity of Zürich on 18–21 February,as a follow-up to the Kaggle challenge.This workshop brought together 55 peoplefrom the LHC and the machine-learningcommunities, and interesting ideas have beenexchanged. The general conclusion fromdiscussions at this event was that the exercise

had been a very positive one, both for LHCband those that entered the competition.

● Further reading https://indico.cern.ch/event/433556/ .

A C C E L E R A T I O N T E C H N I Q U E S

Les physiciens des particules du monde entier sont invités à apporter leurscontributions aux CERN Courier, en français ou en anglais. Les articles retenusseront publiés dans la langue d’origine. Si vous souhaitez proposer un article,faites part de vos suggestions à la rédaction à l’adresse [email protected] .

CERN Courier welcomes contributions from the internationalparticle-physics community. These can be written in English or French,

and will be published in the same language. If you have a suggestion for

an article, please send proposals to the editor at [email protected].

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7/24CERNCOURIER V O L U M E 5 6 N U M B E R 3 A P R I L 2 0 1 6

13


News

Scientists from the DZero collaboration atthe US Department of Energy’s Fermilab

have discovered a new particle – the latestmember to be added to the exotic species ofparticles known as tetraquarks.

In 2003, scientists from the Belleexperiment in Japan reported the rstevidence of quarks hanging out as afoursome, forming a tetraquark. Sincethen, physicists have glimpsed a handful ofdifferent tetraquark candidates, includingnow the recent discovery by DZero – therst observed to contain four differentquark flavours.

DZero scientists rst saw hints of thenew particle, cal led X(5568), in July 2015.After performing multiple cross-checks, thecollaboration conrmed that the signal couldnot be explained by backgrounds or known

processes, but was evidence of a new particle.And the X(5568) is not just any new

tetraquark. While all other observedtetraquarks contain at least two of the sameflavour, X(5568) has four different flavours:up, down, strange and bottom.

Four-quark states are rare, and although

there is nothing in nature t hat forbids theformation of a tetraquark, scientists do notunderstand them nearly as well as they dotwo- and three-quark states. This latestdiscovery comes on the heels of the rstobservation of a pentaquark – a ve-quarkparticle – announced last year by the LHCbexperiment at the LHC.

The next step will be for DZero scientiststo understand how the four quarks areput together. Indeed, the quarks could be

scrunched together in a tight ball, or theymight be a pair of tightly bound quarksrevolving at some distance from the otherpair. Scientists will sharpen the picture ofthe quark quartet by making measurementsof properties such as the way that X(5568)decays or how much it spins on its ax is.As with previous investigations of thetetraquarks, studies of the X(5568) willprovide another window into the workingsof the strong force that holds these particlestogether.

Seventy-ve institutions from 18 countriescollaborated on this result from DZero.

● Further reading arxiv.org/abs/1602.07588.

The planned Deep Underground NeutrinoExperiment (DUNE) (CERN Courier December 2015 p19) will require70,000 tonnes of liquid argon, making it thelargest experiment of its kind – 100 timeslarger than the liquid-argon particledetectors that came before. Scientistsrecently began taking dat a using a 35 tonnetest version of their detector – a signicantstep towards building the four massivedetectors at the Sanford UndergroundResearch Facility (SURF), which will holdthe 70,000 tonnes of liquid argon.

Built at the Department of Energy’sFermi National Accelerator Laboratory,

the 35 tonne prototype allows researchersto check that the various detector elementsare working properly and to start formalstudies. Scientists also use the prototypeto assess detector components that havenot been tried before. The new partsinclude redesigned photodetectors – longrectangular prisms with a special coatingthat changes invisible light to a visiblewavelength and bounces the collected lightto the detector’s electronic components.

DUNE scientists are also paying specialattention to the prototyp e’s wire planes –pieces that hold the thin wires strungacross the detector to pick up electrons. Toensure the frames wil l t down the narrowmineshaft at SURF and avoid having tostretch the wires across the long DUN Edetectors, risking sagging, scientists planto use a series of independent 6 m-longand 2.3 m-wide frames. These wire planes

should measure tracks in the liqu id argon,both in front of and behind them, unlikeother detectors.

Engineers have also moved some of thedetector’s electronic parts inside the cryostat,which holds liquid argon at –184°C.

Much like the full detectors, developmentof the components of the 35 tonneprototype depends on teamwork. For theprototype, Brookhaven and SLAC nationallaboratories in the US provided much of the

electronic equipment; Indiana University,Colorado State University, Louisiana StateUniversity and Massachusetts Institute ofTechnology worked on the light detectors;and the universities of Oxford, Sussex andShefeld helped to make special digitalcameras that can survive in liquid a rgon,and wrote the software to make sense ofthe data. Fermilab was responsible for thecryostat and cryogenic support systems.

Scientists will use what they learnfrom this small prototype version tobuild one of the full-scale modules fora larger, 400 tonne prototype currentlyunder construction at the CERN NeutrinoPlatform. A second 400 tonne module usingdual-phase technology will also be bui lt atCERN. These will be the nal tests b eforeinstallation of the four huge detectors atSURF for the actual experiment, which isscheduled to start in 2021/2022.

DZero discoversnew four-flavourparticle

Testing of DUNEtech begins

N E W P A R T I C L E S

N E W E X P E R I M E N T S

The m(B0sπ ± ) distribution together with the

background distribution and the t resultsafter applying the cone cut.

The DUNE

35 tonne prototype,a test version of the future DUNEdetector.

60

40

20

0

0

10

–10

80data

N

e v e n t s / 8 M e V / c 2

D0 Run II, 10.4 fb–1

fit with background shape fixed

background

signal

5.5 5.6 5.7 5.8 5.9

(GeV/c2)m (B0 π±)

r e s i d u a l s ( d

a t a - f i t )

s

D Z e r o C o l l a b o r a t i o n

R e i d a r

H a h n

12


News

Magnetic precision has a name www.metrolab.com

w w w . a g e n c e - a r c a . c o m -

P h o t o : S c o t t M a x w e l l ,

M a s

t e r fi l e

PT2026 NMR Precision Teslameter

Reach new heightsin magnetic fieldmeasurement The Metrolab PT2026 sets a new

standard for precision magnetometers.

Leveraging 30 years of expertise building

the world’s gold standard magnetometers,

it takes magnetic field measurement to

new heights: measuring higher fields with

better resolution.

The PT2026 offers unprecedented flexibility

in the choice of parameters, interfacing

and probe placement, as well as greatly

improved tolerance of inhomogeneous

fields. And with Ethernet & USB interfaces

and LabVIEW software, it fits perfectly into

modern laboratory environments.

L I G H T S O U R C E

TPS exceeds design goal of 500 mA stored currentIn December last yea r, the 3 GeV TaiwanPhoton Source (TPS) of the NationalSynchrotron Radiation Research Center(NSRRC) stored 520 mA of electron currentin its storage ring, a nd gave the world a

bright synchrotron light as the InternationalYear of Light 2015 came to an end. Thisis the second phase of commissioningconducted after the ve-month preparationwork set to bring the electron current of

TPS to its design value of 5 00 mA (CERNCourier June 2010 p16 and April 2015 p22).

After the rst light of TPS shone on31 December 2014, the beam injectionstored an electron current greater than100 mA with the efciency of the boosterto storage ring exceeding 75% using Petra

cavities. To overcome the instability ofthe electron beam, high chromaticity anda vertical feedback system were appliedto damp the vertical instability at a highcurrent, in this case close to 100 mA,whereas the longitudinal instabilityappeared when the beam current reachedaround 85 mA. Subsequently, the dynamicpressure of the vacuum conditioningreached 10 –7 Pa at 100 mA after feedi ng35 amps-per-hour beam dose. At thisstage, the TPS was ready for the upgradeimplementation scheduled for theremainder of 2015.

Several new components were installedduring this phase, including new undulatorsand superconducting cavities, whilethe cryogenic and control systems werecompleted.

The upgrade activities also involved

the injection system and the transfer linebetween booster and storage, to i mprovethe injection efciency and the stability ofthe system. In addition, 96 fast-feedbackcorrector magnets were placed at both endsof the straight sections, as well as upstreamof the dipole magnets.

After several test runs in the fourthquarter of 2015, an unusual and unfamiliarphenomenon began to emerge, preventingthe electron current from progressingbeyond 230 mA. The pressure of thevacuum chamber located in the rst dipoleof the second arc section in the storage ri ngrepeatedly surged to more than 300 timesthe normal value of 10 × 10 –9 Pa when thebeam current increase d to 190 mA. A smallmetal–plastic pellet that contaminated thevacuum environment was removed and thestaff performed flange welding on the spot.

After the vacuum problem had beensolved, commissioning of TPS wentsmoothly, ramping from 0 to 520 mA in11 minutes on 12 December.

While the TPS was ramping up to itsstored-current target value, two beamlines –the protein microcrystallography beamline(TPS-05) and the temporally coherent X-raydiffraction beamline (TPS-09) – were in thecommissioning phase. The TPS beamlineswill be open for use in 2016.

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15


SciencewatchC OM P I L ED BY J OH N SW A I N , N O R T H E A S T E R N U N I V E R S I T Y

A new approach to incandescent bulbspromises to beat L EDs for efciency.Ognen Ilic and colleagues at MIT replace dthe traditional light-bulb lament witha flat tungsten ribbon. They then coatedglass sheets with a photonic crystal madeof alternating layers of tantalum oxideand silicon dioxide, with thicknessesdetermined by computer modelling, and

sandwiched the tungsten between the two.The photonic crystals are transparent to

visible light but reflect infrared photons

back onto the lament so that they reheat itinstead of being radiated. The efciency sofar is 6.6%, triple that of conventional lightbulbs, but 40% may be reachable, whichwould far outstrip compact fluorescentbulbs (7% to 13%) and LED’s (5% to 15%).

● Further reading O Ilic et al. 2016 Nature Nanotechnology doi:10.1038/nnano.2015.309.

The brightest supernova

A new supernova is more than twice asluminous as any seen before, at its peak beingbrighter than 570 billion suns. Subo Dongof Peking University in Beijing, China, andcolleagues found the supernova – dubbedASASSN-15lh – at a redshift of 0.2326,apparently in a luminous galaxy with littlestar formation. The power it has radiated inthe rst four months post-detection strainsconventional models for its power source, soin addition to breaking a record, it poses an

astrophysical puzzle.

● Further reading S Dong et al. 2016 Science 351 257.

Google plays GoA computer has nally beaten a human atthe ancient board game Go – and r atherdramatically. Developed by GoogleDeepMind in London, the program AlphaGouses deep neural networks to get around thetraditional problems of a simple search dueto the size of the search space. The networktraining was from games with human expertsand from random games of self-play, soin a sense there was a bit of human i n thesoftware, which some might nd consolingbecause the score wasn’t close: AlphaGo 5,European Go champion 0. Expectations had

been that this would not happen for at leastanother decade.

● Further reading D Silver et al. 2016 Nature 529 484.

Babylonians tracked Jupiter The idea of obtaining distance by integratingspeed with respect to time is an elementaryone today for anyone who has studied

high-school calculus, but it now appears thatthe idea was used by astronomers in ancientBabylon to calculate the position of Jupiter.Mathieu Ossendrijver of Humboldt Universityin Berlin, Germany, studied four cuneiformtablets, part of a collection of some 450 frombetween 400 and 50 BC. It’s not full calculus,but it is the trapezoidal rule, far earlier thananyone thought possible. Jupiter held specialsignicance to the Babylonians because itrepresented Marduk, their patron god.

● Further reading M Ossendrijver 2016 Science 351 482.

Back to nine planets?If you are unhappy about Pluto losing itsofcial status as a planet, the good newsis that there may be a 9t h one that we’vemissed so far, orbiting out beyond Pluto witha period of 10,000–20,000 years. While notyet seen directly, Konstantin Batygin andMichael E Brown of Caltech argue that justsuch a planet, with a mass of around 10 Earth

masses or more, explains an otherwisemysterious clustering seen in Kuiper-beltobjects. It spends much of the time veryfar from the Sun, so would be hard to seedirectly, but the Subaru Telescope in Hawaiihas a chance, as does the Large SynopticSurvey Telescope in Chile, which shouldstart operating within 10 years.

● Further reading K Batygin and M E Brown 2016 The AstronomicalJournal 151 22.

A plant that countsRemarkably, the Venus flytrap can count tove. Erwin Neher of the Max Planck Institutefor Biophysical Chemistry in Göttingen,Germany, and colleagues, recorded electricalimpulses from the plant in response to oneto 60 touches. Two touches close the trap,but after only ve touches the plant starts tomake the enzyme that digests its prey, and toincrease production of a sodium transporterused to absorb nutrients.

● Further reading J Böhm et al. 2016 Current Biology 26 286.

First castesSocial insects have castes – queens, workers,and soldiers – and the origin of this structure

has been tracked back to ancient termites.Michael Engel of the University of Kansas i nLawrence and colleagues found six termitespecies preserved in amber from Myanmar,showing evidence of castes and dating back100 million years. The previous oldest castesoldiers were just 17 million years old.

● Further reading M S Engel et al. 2016 Current Biology doi:10.1016/j.cub.2015.12.061.

Incandescent return Newincandescentbulbs using a

at tungstenribbon promise tobeat LEDs

for efciency.

When trees break

Data suggest that trees break at a criticalwind speed of about 42 m/s, regardless ofthe characteristic s of any given tree. This hasnow been explained by Christophe Clanetof LadHyX, of the Ecole Polytechnique inPalaiseau, France, and colleagues, usingHooke’s law, the Griffiths criterion for cracks,and tree allometry and modelling trees asfragile rods. The maximum wind speeds onEarth are about 50 m/s, so this may be part of

why trees are so long-lived.

● Further reading E Virot et al. 2016 Phys. Rev. E 93 023001.

C h u c k C o k e r ( C C )

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17


AstrowatchC OM P I L ED BY M AR C TÜ R L E R, ISDC AN D OB S ER VA TOR Y OF TH E U N I V E R S I T Y OF G E N E V A , AN D CHIPP, U N I V E R S I T Y OF ZU R I C H

An excess of gamma rays at energ ies of afew GeV was found to be a good candidatefor a dark-matter signal (CERN Courier April 2014 p13). Two years later, a pair ofresearch articles refute this interpretation

by showing that the excess photons detectedby the Fermi Gamma-ray Space Telescopeare not smoothly distributed as expected fordark-matter annihilation. Their clusteringreveals instead a population of unresolvedpoint sources, likely millisecond pulsars.

The Milky Way is thought to be embeddedin a dark-matter halo with a density gradientincreasing towards the galactic centre. Thecentral region of our Galaxy is thereforea prime target to nd an electromagneticsignal from dark-matter annihilation. If darkmatter is made of weakly interacting massiveparticles (WIMPs) heavier than protons,such a signal would naturally be in the GeVenergy band. A diffuse gamma-ray emissiondetected by the Fermi satellite and havingproperties compatible with a da rk-matterorigin created hope in recent years of nallydetecting this elusive form of matter more

directly than only through gravitationaleffects.Two independent studies published

in Physical Review Letters are nowdisproving this interpretation. Usingdifferent statistical-analysis methods, thetwo research teams found that the gamm arays of the excess emission at the galacticcentre are not distributed as expected fromdark matter. They both nd evidence for

a population of unresolved point sources

instead of a smooth distribution.The study, led by Richard Bartels of theUniversity of Amsterdam, the Netherlands,uses a wavelet transformation of theFermi gamma-ray images. The techniqueconsists of a convolution of the photoncount map with a wavelet kernel shapedlike a Mexican hat, with a width tuned nearthe Fermi angula r resolution of 0.4° inthe relevant energy band of 1– 4 GeV. The

intensity distribution of the derived waveletpeaks is found to be inconsistent with thatexpected from a truly diffuse origin of theemission. The distribution suggests insteadthat the entire excess emission is due to

a population of mostly undetected pointsources with characteristics matching thoseof millisecond pulsars.

These results are corroborated by anotherstudy led by Samuel Lee of the BroadInstitute in Cambridge and PrincetonUniversity. This US team used a newstatistical method – called a non-Poissoniantemplate t – to estimate the contribution ofunresolved point sources to the gamma-rayexcess emission at the galactic centre. T heteam’s results predict a new population ofhundreds of point sources hiding below thedetection threshold of Fermi. The possibilityof detecting the brightest ones in the yearsto come with ongoing observations wouldconrm this prediction.

In the coming decade, new facilities at radiofrequencies will be able to detect hundreds ofnew millisecond pulsars in the central region

of the Milky Way. This would denitivelyrule out the dark-matter interpretation of theGeV excess seen by Fermi. In the meantime,the quest towards identifying the nature ofdark matter will go on, but little by little thepossibilities are narrowing down.

● Further reading R Bartels et al. 2016 Phys. Rev. Lett. 116 051102.S K Lee et al. 2016 Phys. Rev. Lett. 116 051103.

Picture of the month

This faint dwarf galaxy called IC 1613 was discovered in 1906 in theconstellation of Cetus. It is a member of the Local Group of galaxies andlies just over 2.3 million light-years away. The image, captured with theOmegaCAM camera on ESO’s VLT Survey Telescope in Chile, shows manydetails of this irregular galaxy containing very little c osmic dust. Thisproperty allowed astronomers to use it decades ago to study precisely

variable stars such as Cepheid and RR Lyrae, which have the specialproperty that their period of brightening and dimming is linked directlyto their intrinsic brightness. By measuring the period, astronomers canderive the actual luminosity of the star and hence – using the observedbrightness – its distance. It is thanks to such peculiar stars that the cosmicdistance ladder could be constructed. The extension of this ladder deeperand deeper into space with t he use of new “standard candles” such as TypeIa supernovae led to the Nobel-prize-winning discovery of the acceleratingexpansion of the universe (CERN Courier November 2011 p5).

Gamma-ray excess is not from dark matter

0

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, Gal. longitude (deg.)

b ,

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Gamma-ray map of an area (24° wide)around the galactic centre showing thewavelet transform of the Fermi data. This processing reveals many peaks circled inblack. The brightest ones match the sourcesdetected by Fermi (red circles), while the fainter ones suggest a hidden population of point sources. The colour scale encodes thesignal-to-noise ratio.

B a r t e l s e t a l . ( U n i v e r s i t y o f A m s t e r d a m )

E S O

A c k n o w l e d g e m e n t : V S T / O m e g a c a m L

o c a l G r o u p S u r v e y

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19


Breakthrough

Antonella Del Rosso, CERN.

On 11 February, the Laser Interferometer Gravitational-WaveObservatory (LIGO) and Virgo collaborations published a historicpaper in which they showed a gravitational signal emitted by themerger of two black holes. The signal has been observed with 5σ signicance and is the rst dire ct observation of gravitational waves.

This result comes after 20 years of hard work by a large collabo-ration of scientists operating the two LIGO observatories i n theUS. Barry Barish, Linde profess or of physics, emeritus, at the Cali-fornia Institute of Technology and former director of the GlobalDesign Effort for the International Linear Collider (ILC), led theLIGO endeavour from 1994 to 2005. On the day of the ofcial

announcement to the scientic community and the public, Barish

was at CERN to give a landmark seminar tha t captivated the wholeaudience gathered in the packed Main Auditorium.

The CERN Courier had the unique opportunity to interviewBarish just after the announcement.

Professor Barish, this achievement comes after 20 yearsof hard work, uncertainties and challenges. This is whatresearch is all about, but what was the greatest challenge youhad to overcome during this long period?It really was to do anything that takes 20 years a nd still be sup-ported and have the energy to reach completion. We started long

before that, but the project itself started in 1994. LIGO is an incred-ible technical achievement. The idea that you can take on high r iskin such a scientic endeavour requires a lot of support, diligence

and perseverance. In 1994, we convinced the US National ScienceFoundation to fund the project, which became the biggest pro-gramme to be funded. After that, it took us 10 years to build it andto make it work well, plus 10 years to improve the sensitivity andbring it to the point where we were able to detect the gravitationalwaves. And along the way no one had done this before.

Indeed, the experimental set-up we used to detect the

gravitational signal is an enormous extrapolation from anythingthat was done before. As a physicist, you learn that extrapolating afactor of two can be within reach, but a factor of 10 sounds alreadylike a dream. If you compare the rst 40 m interferometer we builton the CALTECH campus with the two 4000 m interferometers wehave now, you already have an idea of the enormous leap we hadto make. The leap of 100 in size a lso involved at least that in com-plexity and sophistication, eventually achieving more than 10,000times the sensitivity of the original 40 m prototype.

The experimental confirmation of the existence of thegravitational waves could have a profound impact on thefuture of astrophysics and gravitational physics. What do youthink are the most important consequences of the discovery?The discovery opens two new a reas of research for physics. Oneis on the general-relativity theory itself. Gravitational waves area powerful way of testing the heart of the theory by investigatingthe strong-eld realm of gravitational physics. Even with just this

rst event – the merging of two black holes – we have created a true

laboratory where you can study all of this, and understanding gen-eral relativity at an absolutely fund amental level is now opening up.

The second huge consequence of the discovery is that we cannow look at the universe wit h a completely new “telescope”. So far,we have used and built all kinds of telescopes: infrared, ultraviolet,radio, optical… And the idea of recent years has been to look at thesame things in different bandwidths.

However, no such previous instrument could have seen what wesaw with the LIGO interferometers. Nature has been so generouswith us that the very rst event we have seen is new astrophysics,

as astronomers had never seen stellar black holes of these masses.With just the rst glimpse at the universe with gravitational waves,

we now know that they exist in pairs and that they can merge. Thisis all new astrophysics. When we designed LIGO, we thought thatthe rst thing we would see gravitational waves emitted by was

neutron sta rs. It would still be a huge discovery, but it would not benew astrophysical information. We have been really lucky.

Over the next century, this eld will provide a completely new

way of doing an incredible amount of new science. And somehow

we had a glimpse of that with the rst single event.

What were your feelings upon seeing the event on your screen?We initially thought that it could be some instrumental crazything. We had to worry about many possible instrumental glitches,including whether someone had purposely injected a fake eventinto our data stream. To carefully check the origin of the signal,we tracked back the forma tion of the event data from the two inter-ferometers, and we could see that the signal was recorded withinseven milliseconds – exactly the time we expect for the same

LIGO: a strong belief

Twenty years of designing, building and testing

innovative technologies, with the strong belief

that the endeavour would lead to a historic

breakthrough, have come to fruition in the first

direct detection of gravitational waves. Former

LIGO director Barry Barish shares his feelings

on this momentous occasion.

▲

18


Breakthrough

Two black holes about to merge. On 11 February, the LIGO interferometers in the US recorded the gravitational signal coming from

the event. It marked the discovery of the gravitational waves.

Image credit: The SXS (Simulating eXtreme Spacetimes) Project.

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21


Medical accelerators

Saverio Braccini, University of Bern, and Paola Scampoli, University of

Napoli Federico II and University of B ern.

Particle accelerators are fundamental instruments in modernmedicine, where they are used to study the human body and todetect and cure its diseases. Instrumentation issued by fundamen-tal research in physics is very common in hospitals. This includespositron emission tomography (PET) and cancer hadrontherapy.

To match the needs of a continuously evolving eld and to full

the stringent requirements of hospital-based installations, specic

particle accelerators have been developed in recent years. In pa r-ticular, modern medical cyclotrons devoted to proton cancer treat-ments and to the production of rad ioisotopes for diagnostics andtherapy are compact, user-friendly, affordable and able to ensurevery high performance.

Medical PET cyclotrons usually run during the night or early inthe morning, for the production of radiotracers that will be usedfor imaging. Their beam s, featuring about 20 MeV energy and cur-rents of the order of 100μA, are in principle available for other pur-poses during the daytime. This represents an opportunity to exploitthe science potential of these accelerators well beyond medical-

imaging applications. In par-ticular, they can be optimisedto produce beams in the pico-

ampere and nanoampere range,opening the way to nuclear anddetector physics, material sci-ence, radiation biophysics, andradiation-protection research.

On the other hand, to per-form cutting-edge multidis-ciplinary research, beams ofvariable shape and intensitymust be available, together with

the possibility to access the beam area. This cannot be realisedin standard medical PET cyclotron set-ups, where severe accesslimitations occur due to radiation-protection issues. Furthermore,the targets for the production of PET radioisotopes are directlymounted on the cyclotron right after extraction, with consequentlimitations in the use of the beams. To overcome these problems,medical PET cyclotrons can be equipped with a transp ort line lead-ing the beam to a second bunker, which can always be accessiblefor scientic activities.

The Bern cyclotron laboratoryThe Bern medical PET cyclotron laboratory was conceived to usethe accelerator for scientic purposes in parallel with radioisotope

production. It is situated in the campus of the Inselspital, the Bern

University hospital, and has been in operat ion since 2013. The heartof the facility consists of an 18 MeV cyclotron providing single ordual beams of H – ions. A maximum extracted current of 150 μAis obtained by stripping the negative ions. Targets can be locatedin eight different out-ports. Four of them are used for fluorine-18

production, one is equipped with a solid target station, and one isconnected to a 6 m-long beam transfer line (BTL). The acceleratoris located inside a bunker, while a second bunker with independentaccess hosts the BTL and is fully dedicated to research. The beamoptics of the BTL is realised by one horizontal and one vertical

Science with a medicalPET cyclotron

Beyond routine radioisotope production for

medical imaging, compact PET cyclotrons can

be at the heart of multidisciplinary research

facilities. The cyclotron laboratory in Bern

(Switzerland) sets an example.

The acceleration chamber of the PET cyclotron, showing the

out-port connected to the research beam transfer line.

▲

U n i v e r s i t y o f B e r n

To performcutting-edgemultidisciplinaryresearch, beams ofvariable shape andintensity must beavailable.

Magnetsand coilswww.scanditronix-magnet.se


Breakthrough

20

event to appear on the second inter ferometer. The two signals wereperfectly consistent, and this gave us total trust in our d ata.

I must admit that I was personally worried as, in physics, it isalways very dangerous to claim anything with only one event.However, we proceeded to perform the analysis in the most rigor-ous way and, indeed, we followed the normal publication path,namely the submission of the paper to the referees. They conrmed

that what we submitted was scienti cally well-justied. In this way,

we had the green light to announcing the discovery to the public.

At the seminar you were welcomed very warmly by theaudience. It was a great honour for the CERN audience tohave you give the talk in person, just after your colleagues’announcement in the US. What are you bri nging back fromthis experience?I was very happy to be presenting this important achievement inthe temple of science. The thing t hat made me feel that we madethe case well was that people were inte rested in what we have doneand are doing. In the packed audience, nobody seemed to questionour methodology, analysis or the validity of our result. We haveone single event, but this was good enough to convince me and alsomy colleagues that it was a true discovery. I enjoyed receiving allof the science questions from the audience – it was really a greatmoment for me.

● The LIGO and Virgo collaborations are cur rently working onanalysing the rest of the data from the run that ended on 12 Janu-ary. New information is expected to be published in the comingmonths. In the meantime, the discovery event is available in opendata (see https://losc.ligo.org) for anyone who wants to analyse it.

Résumé LIGO : l’aboutissement d’un long chemin

Vingt ans de travail acharné porté par la conviction que ces effortsconduiraient un jour à une découverte historique : la première

détection directe d’ondes gravitationnelles signe l’aboutissement

de ce parcours. La découverte ouvre la voie à une méthode

d’observation entièrement nouvelle pour l’astrophysique et la

science gravitationnelle. Le jour de l’annonce ofcielle, Barry

Barish, ancien directeur de l’expérience LIGO, était au CERN pour

présenter un séminaire qui fera date, devant un auditoire nombreux

rassemblé dans l’amphithéâtre principal. En cette occasion

historique, il fait part de ses impressions au Courier.

One of LIGO’s mirrors is inspected by a technician.

M a t t H e i n t z e / C a l t e c h / M I T / L I G O

L a b

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23


Detectors

Harald Appelshäuser, Johann Wolfgang Goethe-Universität Frankfurt am

Main (Germany).

The ALICE experiment is devoted to the study of strongly intera ct-ing matter, where temperatures are sufciently high to overcome

hadronic connement, and the effective degrees of freedom are

governed by quasi-free quarks and g luons. This type of matter,known as quark–gluon plasma (QGP), has been produced in col-lisions of lead ions at the LHC since 2010. The detectors of theALICE central barrel aim to provide a complete reconstructionof the nal state of Pb–Pb collisions, including charged-particle

tracking and particle identication (PID). The latter is done by

measuring the specic ionisation energy loss, d E/dx.

The main tracking and PID device is the ALICE time projec-tion chamber (TPC). With an active volume of almost 90 m 3, theALICE TPC is the largest detector of its type ever built. During theLHC’s Runs 1 and 2, the TPC reached or even exceeded its designspecications in terms of track reconstruction, momentum resolu-

tion and PID capabilities.ALICE is planning a substantial detector upgrade during the

LHC’s second long shutdown, including a new inner trackingsystem and an upgrade of the TPC. Th is upgrade will allow theexperiment to overcome the TP C’s essential limitation, which isthe intrinsic dead time imposed by an active ion-gating scheme. Inessence, the event rate with the upgraded TPC in LHC Run 3 willexceed the present one by about a factor of 100.

The rate limitation of the current ALICE TPC arises from the

use of a so-called gating grid (GG) – a plane of wires installed inthe MWPC-based read-out chambers. The GG is switched by anexternal pulser system from opaque to t ransparent mode and back.In the presence of an event trigger, the GG opens for a time win-dow of 100 μs, which allows all ionisation electrons from the driftvolume to enter the amplication region. On the other hand, slow-

moving ions produced in the avalanche process head back into thedrift volume. Therefore, af ter each event, the GG has to stay closedfor 300–500 μs to keep the drift volume free of large space-chargeaccumulations, which would create massive drift-eld distortions.

This leads to an intrinsic read-out rate limitation of a few kHz forthe current TPC. However, it should be noted that the read-out ratein Pb–Pb collisions is currently limited by the bandwidth of theTPC read-out electronics to a few hundred Hz.

In Run 3, the LHC is expected to deliver Pb–Pb collision ratesof about 50 kHz, implying average pile-up of about ve collision

events within the drift time window of the TPC. Moreover, manyof the key physics observables are on low-transverse-momentumscales, implying small signal-over-background ratios, whichmake conventional triggering schemes inappropriate. Hence, theupgrade of the TPC aims at a triggerless, continuous read-out ofall collision events. Operating the TPC in such a video-like modemakes it necessary to exchange the present MWPC-based read-outchambers for a different technology, which eliminates the neces-sity of active ion gating, also including complete repla cement of thefront-end electronics and read-out system.

ALICE selects gas electronmultipliers for its

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