ITEP Physics Winter School, Moscow, 13-20/02/2010 1 D. Froidevaux, CERN Honorabilis et amplissimus...

ITEP Physics Winter School, Moscow, 13-20/02/20101D. Froidevaux, CERN

Honorabilis et amplissimus rector, laudati conlegae

Experimental prospects at the LHC


Particle detection and reconstruction at the LHC (and Tevatron)

Lecture 1 Introduction to ATLAS/CMS experiments at the LHC: historical perspective and physics goals Experimental environment and main design choices

Lecture 2 Expected performances of ATLAS and CMS detectors

Lecture 3 Experience collected with cosmics (2008-2009), first data with collisions

(end 2009) and expectations for early physics


Experimental particle physics: 1976 to 2010 Today we are able to ask questions we were not able to formulate 25-30 years ago when I was a student:

What is dark matter? How is it distributed in universe? What is the nature of dark energy? Is our understanding of general relativity correct at all scales? Will quantum mechanics fail at very short distances, in conscious systems, elsewhere? Origin of CP violation, of baryons, what about the proton lifetime? Role of string theory? Duality?

Some of these questions might well lead me towards astrophysics or astro-particle physics today if I would become a young student again!

The more we progress, the longer will be the gap between the reformulation of fundamental questions in our understanding of the universe and its complexity? This gap is already ~ equal to the useful professional lifetime of a human being? This poses real problems.


What next?Why this fear that experimental particle physics is an endangered species? The front-wave part of this field is becoming too big for easy continuity between the generations. I have been working on LHC for 25 years already. Most of the analysis will be done by young students and postdocs who have no idea what the 7000 tonnes of ATLAS is made of. More importantly, fewer and fewer people remember for example that initially most of the community did not believe tracking detectors would work at all at the LHC. The stakes are very high: one cannot afford unsuccessful experiments (shots in the dark) of large size, one cannot anymore approve the next machine before the current one has yielded some results and hopefully a path to follow Theory has not been challenged nor nourished by new experimental evidence for too longThis is why the challenge of the LHC and its experiments is so exhilarating! A major fraction of the future of our discipline hangs on the physics which will be harvested at this new energy frontier.How ordinary or extraordinary will this harvest be? Only nature knows.There is much more to experimental particle physics than its dinosaurs!


Interplay and Synergy

of different tools (accelerators – cosmic rays – reactors . . .)

of different facilities of different initial states

lepton collider (electron-positron) hadron collider (proton-proton) lepton-hadron collider at the energy frontier: high collision energy and intensity frontier: high reaction rate

Features of Particle Physics


Possible thanks to precision

measurements and to known higher-order

electroweak corrections)ln(,)( 2

W

h

W

t

M

M

M

M

LEP

Test of the SM at the Level of Quantum Fluctuations

Indirect determination of the top mass

Prediction of the rangefor the Higgs mass


Status Summer Conferences 2007

However . . .one piece missing within Standard

Model...

plus many open questions


Key Questions of Particle Physics

origin of mass/matter or origin of electroweak symmetry breaking

unification of forces

fundamental symmetry of forces and matter

unification of quantum physics and general relativity

number of space/time dimensions

what is dark matter

what is dark energy


LEP

Natural Width 0.01 1 10 100 GeV

At the LHC the search for the SM Higgs boson provides an excellent benchmark for detector performance

LEP200: mH > 114.4 GeV

Physics Requirements


mH > 114.4 GeV

Standard Model Higgs searches


1. Is there a Higgs boson?

2. What is the Higgs mass?

3. Is the Higgs a SM-like weak doublet?

4. Is the Higgs elementary or composite?

5. Is the stability of mW / mPlanck explained by a symmetry or dynamical principle?

6. Is supersymmetry effective at the weak scale?

7. Will we discover dark matter at the LHC?

8. Are there extra dimensions? Are there new strong forces?

9. Are there totally unexpected phenomena?

10. What is the mechanism of EW breaking?

Standard

Nearlystandard

Not at allstandard

Initial phase of LHC will t

ell

us

which way nature wants us to

go

Need fully commissioned machine and experiments to answer today’s questions about nature


Historical introduction Higgs boson has been with us for several decades as:1. a theoretical concept,

P.W. Higgs, Phys. Lett. 12 (1964) 132

Only unambiguous example of observed Higgs

(apologies to ALEPH collab.)

2. a scalar field linked to the vacuum,

3. the dark corner of the Standard Model,

4. an incarnation of the Communist Party, since it controls the masses (L. Alvarez-Gaumé in lectures for CERN summer school in Alushta),

5. a painful part of the first chapter of our Ph. D. thesis


1967: 1967: Electroweak unification, with W, Electroweak unification, with W, Z Z and H (Glashow, Weinberg, and H (Glashow, Weinberg, Salam)Salam)

1973: 1973: Discovery of neutral currents in Discovery of neutral currents in e scattering (Gargamelle, CERN)e scattering (Gargamelle, CERN)

1974: 1974: Complete formulation of the standardComplete formulation of the standard model with SU(2)model with SU(2)WWU(1)U(1)YY (Iliopoulos) (Iliopoulos)

1983: 1983: LEP and SLC construction LEP and SLC construction startsstarts W and Z discovery (UA1, UA2)W and Z discovery (UA1, UA2)

UA2UA2

One of the first Z-bosons detected in the worldOne of the first Z-bosons detected in the world

qq qq Z Z e e+ + ee- - --

Historical introduction1981: 1981: The CERN SpS becomes a The CERN SpS becomes a proton-proton- antiproton collider antiproton collider LEP and SLC are approved LEP and SLC are approved beforebefore W/Z boson discovery W/Z boson discovery

1964: 1964: First formulation of Higgs mechanismFirst formulation of Higgs mechanism (P.W.Higgs) (P.W.Higgs)


1984: 1984: Glimmerings of LHC and SSCGlimmerings of LHC and SSC

1987: 1987: First comparative studies of physics First comparative studies of physics potential of hadron colliders (LHC/SSC) potential of hadron colliders (LHC/SSC) and e and e++ee-- linear colliders (CLIC) linear colliders (CLIC)

1989: 1989: First collisions in LEP and SLCFirst collisions in LEP and SLC Precision tests of the SM and searchPrecision tests of the SM and search for the Higgs boson begin in earnest for the Higgs boson begin in earnest

R&D for LHC detectors beginsR&D for LHC detectors begins

1993:1993: Demise of the SSC Demise of the SSC

1994:1994: LHC machine is approved (start in 2005) LHC machine is approved (start in 2005)

1995: 1995: Discovery of the top quark at FermilabDiscovery of the top quark at Fermilab by CDF (and D0) by CDF (and D0) Precision tests of the SM and searchPrecision tests of the SM and search for the Higgs boson continue at LEP2 for the Higgs boson continue at LEP2 Approval of ATLAS and CMSApproval of ATLAS and CMS

Historical introduction2000:2000: End of LEP running End of LEP running

2001:2001: LHC schedule delayed by two more LHC schedule delayed by two more years years

During the last 15 years, During the last 15 years, three parallel activities three parallel activities have been ongoing, all have been ongoing, all with impressive results:with impressive results:

1)1) Physics at LEP with aPhysics at LEP with awonderful machinewonderful machine

2)2) Construction of the LHCConstruction of the LHCmachine machine

3)3) Construction of the LHC Construction of the LHC detectors after an initial detectors after an initial very long R&D periodvery long R&D period


Historical introductionWhat has been the evolution of our HEP culture over these past 30 years?

1. In the 70-80’s, the dogma was that e+e- physics was the only way to do clean and precise measurements and even discoveries (hadron physics were dirty).

2. With the advent of high-energy colliders, the 80-90’s have demonstrated that:

2. Most discoveries have occurred in hadronic machines

3. Unprecedented precision has been reached in electroweak measurements at LEP with state-of-the-art detectors remember the first time ALEPH announced that luminosity could be measured to 0.1%!

4. Hadronic colliders can rival with the e+e- machines in certain areas of precision measurements remember the almost simultaneous publication of the Z-mass measurements from CDF and SLC with comparable precision (200 MeV!) even with Run I (100 pb-1), CDF has been able to compete with LEP in the field of B-physics


Historical introduction^

Ecm (GeV)^

Fij(Ecm) (GeV-1)

Parton luminosities^

Fij(Ecm)

where Ecm is the centre-of-mass energy of two “partons” i and j,

are useful to compare intrinsic potential of different machines

^

Important to note that:1. as centre-of-mass energy grows,

processes without beam-energy constraint such as vector-boson fusion become also important at e+e- machines;

2. Proton-proton collisions are equivalent to e+e- collisions for

spp 5 se+e-

a


Historical introduction

All particles in plot were discovered first at hadron machines with one notable exception:

the -lepton was (and could have been) observed only in vector-boson decays at the CERN proton-antiproton collider.


How huge are ATLAS and CMS? Size of detectors Volume: 20 000 m3 for ATLAS Weight: 12 500 tons for CMS 66 to 80 million pixel readout channels near vertex 200 m2 of active Silicon for CMS tracker 175 000 readout channels for ATLAS LAr EM calorimeter 1 million channels and 10 000 m2 area of muon chambers Very selective trigger/DAQ system (see lectures by A. Yagil) Large-scale offline software and worldwide computing (GRID)

Time-scale will have been about 25 years from first conceptual studies (Lausanne 1984) to solid physics results confirming that LHC will have taken over the high-energy frontier from Tevatron (early 2009?) Size of collaboration Number of meetings and Powerpoint slides to browse through


ATLAS Collaboration

(As of July 2006)

35 Countries 162 Institutions

1650 Scientific Authors(1300 with a PhD)

Albany, Alberta, NIKHEF Amsterdam, Ankara, LAPP Annecy, Argonne NL, Arizona, UT Arlington, Athens, NTU Athens, Baku, IFAE Barcelona, Belgrade, Bergen, Berkeley LBL and UC, Bern, Birmingham, Bologna, Bonn, Boston, Brandeis,

Bratislava/SAS Kosice, Brookhaven NL, Buenos Aires, Bucharest, Cambridge, Carleton, Casablanca/Rabat, CERN, Chinese Cluster, Chicago, Clermont-Ferrand, Columbia, NBI Copenhagen, Cosenza, AGH UST Cracow, IFJ PAN Cracow, DESY, Dortmund,

TU Dresden, JINR Dubna, Duke, Frascati, Freiburg, Geneva, Genoa, Giessen, Glasgow, LPSC Grenoble, Technion Haifa, Hampton, Harvard, Heidelberg, Hiroshima, Hiroshima IT, Humboldt U Berlin, Indiana, Innsbruck, Iowa SU, Irvine UC, Istanbul Bogazici, KEK,

Kobe, Kyoto, Kyoto UE, Lancaster, UN La Plata, Lecce, Lisbon LIP, Liverpool, Ljubljana, QMW London, RHBNC London, UC London, Lund, UA Madrid, Mainz, Manchester, Mannheim, CPPM Marseille, Massachusetts, MIT, Melbourne, Michigan, Michigan SU, Milano,

Minsk NAS, Minsk NCPHEP, Montreal, McGill Montreal, FIAN Moscow, ITEP Moscow, MEPhI Moscow, MSU Moscow, Munich LMU, MPI Munich, Nagasaki IAS, Naples, Naruto UE, New Mexico, New York U, Nijmegen, BINP Novosibirsk, Ohio SU, Okayama, Oklahoma, Oklahoma SU, Oregon, LAL Orsay, Osaka, Oslo, Oxford, Paris VI and VII, Pavia, Pennsylvania, Pisa, Pittsburgh, CAS Prague, CU Prague, TU Prague, IHEP Protvino, Ritsumeikan, UFRJ Rio de Janeiro, Rochester, Rome I, Rome II, Rome III, Rutherford Appleton

Laboratory, DAPNIA Saclay, Santa Cruz UC, Sheffield, Shinshu, Siegen, Simon Fraser Burnaby, Southern Methodist Dallas, NPI Petersburg, SLAC, Stockholm, KTH Stockholm, Stony Brook, Sydney, AS Taipei, Tbilisi, Tel Aviv, Thessaloniki, Tokyo ICEPP, Tokyo MU, Toronto, TRIUMF, Tsukuba, Tufts, Udine, Uppsala, Urbana UI, Valencia, UBC Vancouver,

Victoria, Washington, Weizmann Rehovot, Wisconsin, Wuppertal, Yale, Yerevan


ATLAS CMSOverall weight (tons) 7000 12500Diameter 22 m 15 mLength 46 m 22 mSolenoid field 2 T 4 T

ATLAS superimposed tothe 5 floors of building 40

CMS

ATLAS

How huge are ATLAS and CMS?


An Aerial View of Point-1

(Across the street from the CERN main entrance)

How huge are ATLAS and CMS?


Executive Board

ATLAS management: SP, Deputy SP, RC, TCCollaboration Management, experiment execution, strategy, publications, resources, upgrades, etc.

PublicationCommittee,Speaker Committee

CB

Detector Operation (Run Coordinator)Detector operation during data taking, online data quality, …

Trigger (Trigger Coordinator)Trigger data quality,performance, menu tables, new triggers, ..

Data Preparation (Data Preparation Coordinator)Offline data quality, first reconstruction of physics objects, calibration, alignment (e.g. with Zll data)

Computing (Computing Coordinator)Core Software, operation of offline computing, …

Physics (Physics Coordinator)optimization of algorithms for physics objects, physics channels

(Sub)-systems:Responsible for operation and calibration of their sub-detector and for sub-system specific software

TMB

Operation Model (Organization for LHC Exploitation)(Details can be found at http://uimon.cern.ch/twiki//bin/view/Main/OperationModel )


Speakers age distribution

0

2

4

6

8

10

12

20 24 28 32 36 40 44 48 52 56 60 64 68 72 76

Age (years)

Entries / 2 years

About 100 talks,~ 22% women

ATLAS physics workshop in Rome (June 2005)

~ 450 participants


Generic features required of ATLAS and CMS Detectors must survive for 10 years or so of operation Radiation damage to materials and electronics components Problem pervades whole experimental area (neutrons): NEW!

Detectors must provide precise timing and be as fast as feasible 25 ns is the time interval to consider: NEW!

Detectors must have excellent spatial granularity Need to minimise pile-up effects: NEW!

Detectors must identify extremely rare events, mostly in real time Lepton identification above huge QCD backgrounds (e.g. e/jet ratio at the LHC is ~ 10-5, i.e. ~ 100 worse than at Tevatron) Signal X-sections as low as 10-14 of total X-section: NEW! Online rejection to be achieved is ~ 107: NEW! Store huge data volumes to disk/tape (~ 109 events of 1 Mbyte size per year: NEW!


Generic features required of ATLAS and CMS Detectors must measure and identify according to certain specs Tracking and vertexing: ttH with H bb Electromagnetic calorimetry: H and H ZZ eeee Muon spectrometer: H ZZ Missing transverse energy: supersymmetry, H

Detectors must please Collaboration: physics optimisation, technology choices Funding agencies: affordable cost (originally set to 475 MCHF per experiment by CERN Council and management) Young physicists who will provide the main thrust to the scientific output of the collaborations: how to minimise formal aspects? How to recognise individual contributions?

Review article on ATLAS and CMS as built (DF and P. Sphicas) athttp://arjournals.annualreviews.org/eprint/HMcWjWGjGZHCFNgVvabI/full/10.1146/annurev.nucl.54.070103.181209 (in ARNPS)


Muon chambers

Hadronic calorimeter

Electromagnetic calorimeter

Inner detector

µ

eK0,n

,p

Higgs at the LHC: the challenge


How to extract this… … from this …

+30 min. bias eventsHiggs 4Without knowing really where to look for!

Physics at the LHC: the challenge



Orders of magnitude of event rates for various physics channels:• Inelastic : 1010 Hz• W -> l : 103 Hz• tt production : 102 Hz• Higgs (m=100 GeV) : 1 Hz• Higgs (m=600 GeV) : 10-1 Hz(and include branching ratios: ~ 10-2)

Selection power for Higgs discovery 1014-15

i.e. 100 000 times better than achieved at Tevatron so far for high-pT leptons!

Small x-sections

need highest luminosity L= 1034-35 cm-2s-1


Process Events/s Events for 10 fb-1 Total statistics collected (one year) elsewhere by 2010 (?)

W e 30 108 104 LEP / 107 Tevatron

Z ee 3 107 106 LEP/Tevatron

Top 2 107 104 Tevatron

Beauty 106 1012 – 1013 109 Belle/BaBar

H (m=130 GeV) 0.04 105

Gluino 0.002 104

(m= 1 TeV)

Black holes 0.0002 103 m > 3 TeV

Expected event rates for representative (known and new) physics processes at “low” luminosity (L=1033 cm-2 s-1) in ATLAS/CMS

LHC is a “factory” for top, W/Z, Higgs, SUSY, black holes ...



What do we mean by particle reconstruction and identification at LHC?Elementary constituents interact as such in “hard processes”, namely: Quarks and leptons as matter particles, and

Electrons, neutrinos and photons are the only rigorously stable particles in the zooAt collider energies, muons can be considered as stable tooSome of the other particles are considered as long-lived (, c, b) meaning that their decay vertex may be measured by vertexing detector (requires excellent accuracy)All other particles can only be seen through their stable decay products

Leptons

e (0.0005) (0.105) (1.777)

e

Quarks u (< 0.005) c (~ 1.25) t (~ 175)

d (< 0.005) s (~ 0.1) b (~ 4.2) Gluons and EW bosons as gauge particles

Gluon(0)

Colour octet

Photon(0)

W+,W-

(80.42)

Z

(91.188)

Physics at the LHC: the environment

Allmassesin GeV


Physics at the LHC: the environmentWhich type of particles does one actually see in the final state?

LHC physics processes are dominated by strong interactions (QCD) : hard processes: quarks and gluons materialise as hadronic jets,

which consist mostly of charged and neutral hadrons (pions, kaons, and to a lesser extent protons and neutrons, which at these energies can be all considered as stable). Jets will be discussed in lecture 4.

soft processes: non-perturbative QCD processes with soft gluons materialising as almost uniform soup of charged and neutral pions, kaons, etc.

Heavy quarks with “long” lifetime are produced abundantly also High-pT (above ~ 10 GeV) leptons are produced mostly in c,b decays. High-pT isolated leptons may be found in fraction of J/ and decays For pT > 25 GeV, dominant source of high-pT leptons: W/Z/tt decays

Main challenge at Tevatron and LHC: find e,,,b amidst q/g soup


Physics at the LHC: the environmentWhat drives the luminosity at the LHC?

L(=0) = 1.07 10-4 1/t N2 E / e where:

is the crossing angle between the beams

t is the time between bunch crossings, t = 25 ns

N is the number of protons per bunch, N = 1011

E is the energy per beam, E = 7 TeV

e is the -function at the interaction point, e = 0.5 m

is the normalised emittance,m.rad


Extract number of inelastic collisions per bunch crossing

<n> = inel x L x t / bunch

LHC: <n> = 70 mb x 1034 cm-2s-1 x 25 ns / 0.8 = 23

Big change compared to recent and current machines:

LEP: t = 22 s and <n> << 1SppS: t = 3.3 s and <n> 3HERA: t = 96 ns and <n> << 1Tevatron: t = 0.4 s and <n> 2



Physics at the LHC: the environmentExperimental environment Machine performance x Physics

Event rates in detectors: number of charged tracks expected in inner tracking detectors energy expected to be deposited in calorimeters radiation doses expected (ionising and neutrons) event pile-up issues (pile-up in time and in space)

Need to know the cross-section for uninteresting pp inelastic events: simple trigger on these “minimum bias” trigger


Measurement of tot (pp) and inel = tot- el - diff

Curves are ~ (log s)


tot(pp)100 mb

10 102 103 104

Centre-of-mass energy (GeV)

At the LHC, inel 70 mb

Goal of TOTEM: ~ 1 % precision

Curves are ~ (log s)


p pT

d/dpTdy isLorentz-invariant

= y for m 0

Physics is ~ constantversus at fixed pT



<pT> ~ 500 MeV

Charged particle multiplicity and energy in pp inelastic events at s = 14 TeV

Charged particle multiplicities from different models

Present models extrapolated from Tevatron give sizeable differencesat the LHC



What do we expect roughly speaking at L = 1034 cm-2s-1 ?

Assume detector with coverage over –3 < < 3 (= 5.7o) for tracks and –5 < < 5 (= 0.8o) for calorimetry:• Most of the energy is not seen! (300000 GeV down the beam

pipe)• ~ 900 charged tracks every 25 ns through inner tracking• ~ 1400 GeV transverse energy (~ 3000 particles) in calorimeters

every 25 ns

dncharged/d 7.5 per = 1

ncharged consists mostly of ± with <pT> 0.6 GeV

dnneutral/d 7.5, nneutral consists mostly of

from 0 decay with <n0> 4 and <pT> 0.3 GeV


… still much more complex than a LEP event


Radiation resistance of detectors

New aspect of detector R&D (from 1989 onwards) for once make use of military applications!

The ionising radiation doses and the slow neutron fluences are

almost entirely due to the beam-beam interactions and can therefore be predicted was not and is not the case in recent and current machines

Use complex computer code developed over the past 40 years or more for nuclear applications (in particular for reactors)


ATLAS neutron fluences


1. Damage caused by ionising radiation caused by the energy deposited by particles in the detector

material: 2 MeV g-1 cm-2 for a min. ion. particle also caused by photons created in electromagnetic showers the damage is proportional to the deposited energy or dose

measured in Gy (Gray):• 1 Gy = 1 Joule / kg = 100 rads• 1 Gy = 3 109 particles per cm2 of material with unit density

At LHC design luminosity, the ionising dose is: 2 106 Gy / rT

2 / year,where rT (cm) is the transverse distance to the beam



2. Damage caused by neutrons• the neutrons are created in hadronic showers in the

calorimeters and even more so in the forward shielding of the detectors and in the beam collimators themselves

• these neutrons (with energies in the 0.1 to 20 MeV range) bounce back and forth (like gas molecules) on the various nuclei and fill up the whole detector

• expected neutron fluence is about 3 1013 per cm2 per year in the innermost part of the detectors (inner tracking systems)

• these fluences are moderated by the presence of Hydrogen: (n,H) ~ 2 barns with elastic collisions mean free path of neutrons is ~ 5 cm in this energy range at each collision, neutron loses 50% of its energy

(this number would be e.g. only 2% for iron)



• the neutrons wreak havoc in semiconductors, independently of the deposited energy, because they modify directly the cristalline structure need radiation-hard electronics (military applications only in the early R&D days) off-the-shelf electronics usually dies out for doses

above 100 Gy and fluences above 1013 neutrons/cm2

rad-hard electronics (especially deep-submicron) can survive up to 105-106 Gy and 1015 neutrons/cm2

• most organic materials survive easily to 105-106 Gy (beware!)

Material validation and quality control during production areneeded at the same level as for spatial applications!!



Pile-up effects at high luminosity

Pile-up is the name given to the impact of the 23 uninteresting (usually) interactions occurring in the same bunch crossing as the hard-scattering process which generally triggers the apparatus

Minimising the impact of pile-up on the detector performance has been one of the driving requirements on the initial detector design:

• a precise (and if possible fast) detector response minimises pile-up in time very challenging for the electronics in particular typical response times achieved are 20-50 ns (!)• a highly granular detector minimises pile-up in space large number of channels (100 million pixels, 200,000 cells in electromagnetic calorimeter)



Pile-up effects at high luminosityPhysics at the LHC: the environment

ATLASPhoton converts at R = 40 cm and electron pair is visible in ATLAS TRT and EM calo


Weight:

7000 t44 m

22 m

Tim

e-o

f-fl

igh

t

Interactions every 25 ns …

In 25 ns particles travel 7.5 m

Cable length ~100 meters …

In 25 ns signals travel 5 m



Physics at the LHC: the challengeUnprecedented scope and timescales for simulations

Many examples from ATLAS/CMS (and also ALICE/LHCb): 30 million volumes simulated in GEANT

Tbytes (hundreds of millions) of simulated events over 10 years

Full reconstruction of all benchmark Higgs-boson decays

Unprecedented amount of material in Inner Detectors leads to significant losses of e.g. charged-pion tracks (up to 20% at 1 GeV) and to significant degradation of EM calo intrinsic performance (mostly for electrons but for photons too)

b-tagging at low luminosity, for e.g. H bb searches, yields performance similar to that which has been achieved at LEP


Main specific design choices of ATLAS/CMS Size of ATLAS/CMS directly related to energies of particles produced: need to absorb energy of 1 TeV electrons (30 X0 or 18 cm of Pb), of 1 TeV pions (11 or 2 m Fe) and to measure momenta of 1 TeV muons outside calorimeters (BL2 is key factor to optimise)

Choice of magnet system has shaped the experiments in a major way Magnet required to measure momenta and directions of charged particles near vertex (solenoid provides bend in plane transverse to beams) Magnet also required to measure muon momenta (muons are the only charged particles not absorbed in calorimeter absorbers) ATLAS choice: separate magnet systems (“small” 2 T solenoid for tracker and huge toroids with large BL2 for muon spectrometer) Pros: large acceptance in polar angle for muons and excellent muon momentum resolution without using inner tracker Cons: very expensive and large-scale toroid magnet system CMS choice: one large 4 T solenoid with instrumented return yoke Pros: excellent momentum resolution using inner tracker and more compact experiment Cons: limited performance for stand-alone muon measurements (and trigger) and limited space for calorimeter inside coil


Main specific design choices of ATLAS/CMS At the LHC, which is essentially a gluon-gluon collider, the unambiguous identification and precise measurement of leptons is the key to many areas of physics:

electrons are relatively easy to measure precisely in EM calorimeters but very hard to identify (imagine jet leading - with - leading 0 very early in shower) muons in contrast are relatively easy to identify behind calorimeters but very hard to measure accurately at high energies

This has also shaped to a large extent the global design and technology choices of the two experiments EM calorimetry of ATLAS and CMS is based on very different technologies

ATLAS uses LAr sampling calorimeter with good energy resolution and excellent lateral and longitudinal segmentation (e/ identification) CMS use PbWO4 scintillating crystals with excellent energy resolution and lateral segmentation but no longitudinal segmentation Broadly speaking, signals from H or H ZZ* 4e should appear as narrow peaks (intrinsically much narrower in CMS) above essentially pure background from same final state (intrinsically background from fakes smaller in ATLAS)


ATLAS/CMS: from design to reality

All magnets have reached their design currents: a major technical milestone!

20.5 kA



Active sensors and mechanics account each only for ~ 10% of material budget Need to bring 70 kW power into tracker and to remove similar amount of heat Very distributed set of heat sources and power-hungry electronics inside volume: this has led to complex layout of services, most of which were not at all understood at the time of the TDRs

Amount of material in ATLAS and CMS inner trackers

LEP detectors

Weight: 4.5 tons Weight: 3.7 tons



Material increased by ~ factor 2-2.5 from 1994 (approval) to now (end constr.) Electrons lose between 25% and 70% of their energy before reaching EM calo Between 20% and 65% of photons convert into e+e- pair before EM calo Need to know material to ~ 1% X0 for precision measurement of mW (< 10 MeV)!

1.90



Performance of CMS tracker is undoubtedly superior to that of ATLAS in terms of momentum resolution. Vertexing and b-tagging performances are similar. However, impact of material and B-field already visible on efficiencies.


• All modules and services integrated and tested• 80 million channels ! • 10%-scale system test with cosmics done at CERN• Inst. in ATLAS: June 2007

ATLAS pixels, September 2006

CMS Tracker Inner Barrel, November 2006

CMS silicon strips• 200 m2 Si, 9.6 million channels• 99.8% fully operational• Signal/noise ~ 25/1• 20% cosmics test under way• Inst. in CMS: August 2007

Remember that tracking at the LHC is a risky business!


Remember that tracking at the LHC is a risky business! ATLAS pixel beam tests:

intrinsic resolution in bending plane before and after irradiation to a fluence

of 1015 neutronsequ per cm2

Pixel size is 50 m x 400 m in R x z

CMS pixel beam tests in 3T field: extrapolate by simulation to expected

behaviour versus incidence angle, voltage bias and total neutron fluence

collected in 4T field

Pixel size is 150 m x 150 m in R x z

But ATLAS/CMS tracking specs do not marry well with detailed particle-ID


What are the limitations of ATLAS/CMS tracking detectors in terms of particle-ID? Look at ALICE/LHCb

ALICE TPC (Time Projection Chamber)• Measure many samples of dE/dx per track (need >> 25 ns!!)• At low momenta, non-relativistic particles can be separated from each other through precise dE/dx measurements:Bethe-Bloch: -<dE/dx> = k 1/2 ( 0.5 Log(2mec222Tmax/I2) - 2-/2)



Overall particle-ID in ALICE for heavy-ion physics


LHC-b RICH detectors C4F10 3 GeV 30 GeV

(pion)

(Cerenkov)

0.9989

0.160 rad

0.999989

0.0526 rad

(kaon)

(Cerenkov)

0.9864

0.020 rad

0.99986

0.0502rad

RICH1:larger solid angle,lower part of momentum spectrum•Aerogel (hygroscopic…)

-n=1.03 (=1)=242 mrad-thickness=5 cm-nb detected photons=~7/ring (=1)

• C4F10 p=1013 mb at –1.9C-n=1.0014 /260 nm (=1)=53 mrad-thickness=85cm-nb photons=~30/ring

RICH2:•CF4 -n=1.0005 /260 nm (=1)=32 mrad

-thickness=180cm-nb photons=~30/ring




LHC-b RICH detectors

With RICH



Stand-alone performance measured in beams with electrons from 10 to 250 GeV

R&D and construction for 15 years excellent EM calo intrinsic performance


ATLAS/CMS: from design to realityActual performance expected in real detector quite different!!

Photons at 100 GeV ATLAS: 1-1.5%

energy resol. (all ) CMS: 0.8% energy resol.

( ~ 70%)

Electrons at 50 GeV ATLAS: 1.5-2.5%

energy resol. (use EM calo only)

CMS: ~ 2.0% energy resol.

(combine EM calo and tracker)

ATLAS

ATLASAll



Huge effort in test-beams to measure performance of overall calorimetry with single particles and tune MC tools: not completed!


One word about neutrinos in hadron colliders: since most of the energy of the colliding protons escapes down the beam pipe, one can only use the energy-momentum balance in the transverse plane concepts such as ET

miss, missing transverse momentum and mass are often used (only missing component is Ez

miss)

reconstruct “fully” certain topologies with neutrinos, e.g. W l and even better H ll h the detector must therefore be quite hermetic transverse energy flow fully measured with reasonable accuracy no neutrino escapes undetected no human enters without major effort (fast access to some parts of ATLAS/CMS quite difficult)



11

CMS



11

ATLAS



ATLAS


For an integrated luminosity of ~ 100 pb-1, expect a few events like this? This is apparent ET

miss occurring in fiducial region of detector!


ATLAS/CMS: from design to realityBiggest difference in performance perhaps for hadronic calo

ETmiss at ET = 2000 GeV

ATLAS: ~ 25 GeV CMS: ~ 40 GeV

This may be important for high mass H/A to

Curve: 0.57 √ET

Jets at 1000 GeV ATLAS ~ 3%

energy resolution CMS ~ 5%

energy resolution, (but expect sizable

improvement using tracks at lower

energies)

ATLAS

ATLAS

Curve: 0.55 √ET


ATLAS/CMS: from design to realityBiggest difference in performance perhaps for hadronic calo: how much can be recovered using energy-flow algorithms?Jets in 20-100 GeV range are

particularly important for searches (e.g. H bb)

For ET ~ 50 GeV in barrel:ATLAS: ~ 10% energy resolution CMS: ~ 19% energy resolution

(with calo only), ~ 14% energy resolution

(with calo + tracks)Some words of caution though: danger from hadronic interactions in tracker material non-Gaussian tails in response gains smaller at large (material) and at high energy linearity of response at low energy!

CMS


CMS muon spectrometer• Superior combined momentum resolution in central region• Limited stand-alone resolution and trigger (at very high luminosities) due to multiple scattering in iron• Degraded overall resolution in the forward regions (|| > 2.0) where solenoid bending power becomes insufficient

CMS



ATLAS muon spectrometer• Excellent stand-alone capabilities and coverage in open geometry• Complicated geometry and field configuration (large fluctuations in acceptance and performance over full potential x coverage (|| < 2.7)

ATLAS




CMS muon performance driven by tracker: better than ATLAS at ~ 0ATLAS muon stand-alone performance excellent over whole range

2(+4)

ITEP Physics Winter School, Moscow, 13-20/02/2010 1 D. Froidevaux, CERN Honorabilis et amplissimus...

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Transcript of ITEP Physics Winter School, Moscow, 13-20/02/2010 1 D. Froidevaux, CERN Honorabilis et amplissimus...