ALICE First Physics Results

ALICE First Physics ResultsAndrea Dainese(INFN Padova)

on behalf of the ALICE Collaboration

Physics at LHC 2010, DESY-Hamburg, 09.06.10 Andrea Dainese 1

Outline Particle multiplicity

dNch/d and dN/dNch measurements at 0.9, 2.36, and 7 TeV

Momentum spectra Charged particle pt spectra at 0.9 TeV

Baryon production Anti-proton/proton ratio at 0.9 and 7 TeV

Particle emitting source Femtoscopy (HBT) at 0.9 TeV

Momentum spectra, outlook Identified charged pt spectra at 0.9 TeV

Strangeness at 0.9 and 7 TeV 0 and jet reconstruction at 7 TeV

Event topology Event shape and underlying event at 7 TeV

Heavy Flavour production Charm and J/ production at 7 TeV


published

preliminary

in preparation

ITS, TPC, TOF, HMPID, MUON, V0, T0, FMD, PMD, ZDC (100%) TRD (7/18) EMCAL (4/12) PHOS (3/5)

Detector configuration 2009/2010

3Physics at LHC 2010, DESY-Hamburg, 09.06.10 Andrea Dainese

J.Schukraft

3

“Minimum bias”, based on interaction trigger: SPD or V0-A or V0-C

o at least one charged particle in 8 units

read out all ALICE single-muon trigger:

forward muon in coincidence with Min Bias read out MUON, SPD, V0, FMD, ZDC

Activated in coincidence with the BPTX beam pickups

Trigger & Data samples

4Physics at LHC 2010, DESY-Hamburg, 09.06.10 Andrea Dainese 4

2009 (0.9 and 2.36 TeV)~10.3 µb-1 (5×105 min bias)

2010, to June 8th (0.9 and 7 TeV)~2.7 nb-1 (2×108 min bias)

Only 7 TeV data = 72 mb











published

preliminary

in preparation


Multiplicity measurements

Multiplicity measured using tracklets in the two pixel layers (R ~ 4 and 7 cm)

ALICE has published the pseudorapidity density and multiplicity distribution at 0.9, 2.36, and 7 TeV First LHC paper First 7 TeV paper All three papers accepted by

EPJC

arXiv:1004.3514

EPJ C: Vol. 65, 1 (2010)

P.Hristov

284 events, Nov 23rd 2009


Event Classes0.9 and 2.36 TeV

INEL and NSD Use measured cross sections

for diffractive processes Change MC generator fractions

(SD/INEL, DD/INEL) such that they match these fractions

Use Pythia and Phojet to assess effect of different kinematics of diffractive processes

7 TeV Diffraction is quite unknown Hadron-level definition of events

(similar to ATLAS first paper) All events that have at least one

charged primary particle in || < 1: “INEL>0”

Minimizes model dependence

INEL: MBOR (SPD OR VZEROA OR VZEROC) AND offline background suppression

NSD: MBAND (VZEROA AND VZEROC) AND offline background suppressionINEL>0: INEL AND at least one charged primary particle in || < 1

UA5: only stat. uncertainty shown

arXiv:1004.3034

arXiv:1004.3034

dNch/d vs. other experiments

Consistent with UA5 (only 900 GeV)

Consistent with CMS (only NSD) does not include charged

leptons ~1.5 % difference

0.9 TeV

2.36 TeV


dNch/d vs. Monte Carlo Pythia D6T and Perugia-0 match

neither INEL, NSD or INEL>0 at any energy

Pythia ATLAS-CSC and Phojet reasonably close with some deviations at 0.9 and 2.36 TeV

Only ATLAS-CSC close at 7 TeV


2.36 TeV

0.9 TeV 7 TeV

dNch/d vs. s

points at the same energy slightly shifted

arXiv:1004.3514

arXiv:1004.3514

Significantly larger increase from 0.9 to 7 TeV than in MCs

Increase in dNch/dh

in || < 1 for INEL > 0

arXiv:1004.3514

s ALICE (%) MCs (%)

0.9 2.36 TeV 23.3 ± 0.4 -0.7+1.1 15 – 18

0.9 7 TeV 57.6 ± 0.4 -1.8+3.6 33 – 48

Power law dependence fits well


fits ~ s0.1

Multiplicity Distributions Distributions in three -regions Consistent with UA5 (||<0.5) Fits with one negative binomial work well in limited -regions

clan-based model of production production Difference between INEL and NSD in low-multiplicity region

arXiv:1004.3034arXiv:1004.3034


0.9 TeV: ALICE vs. UA5 0.9 TeV: INEL vs. NSD

11

dN/dNch: vs. Monte Carlo Phojet

provides a good description at 900 GeV fails at 2.36 and 7 TeV

Pythia Atlas CSC fails at 0.9 TeV reasonably close at 2.36 and 7 TeV but deviations around 10-20

Pythia D6T and Perugia-0 far from the distribution at all energies

0.9 TeV 7 TeV2.36 TeVarXiv:1004.3034 arXiv:1004.3514

1212Physics at the LHC 2010, DESY-Hamburg, 09.06.10 Andrea Dainese 12

arXiv:1004.3034












published

preliminary

in preparation

dNch/dpt at 0.9 TeV


Paper ready for submission: dNch/dpt and <pt> vs. Nch

n

T

T

ch

T nT

E

dpd

Nd

p

1

1 2

Not described by MCs

PHOJET (at 900 GeV) and PYTHIAATLAS-CSC tune (at 2.36 and 7 TeV)gave best description of the multiplicitydistributions

nT

T

TT

TT

pnT

E

T

E

nT

E

mpmE

:~

exp:~

1

1

22

14

FIT

dNch/dpt at 0.9 TeV


Paper ready for submission: dNch/dpt and <pt> vs. Nch

ALICE measures harder spectrum than CMS, ATLAS, UA1 (narrower window at central rapidity)

15


“Power-law”-like tail becomes harder at high multiplicity

<pt> vs multiplicity (0.9 TeV)

<pt> vs Nch from fit function, with 3 pt cutoffs (> 0, 150, 500 MeV/c)

16

<pt> vs multiplicity (0.9 TeV)


Perugia-0 (which fails for multiplicity) describes well <pt>, but only for pt>500 MeV/c (ATLAS found agreement for pt > 500 MeV/c)

Phojet (which describes multiplicity) fails for <pt>

17

Comparison to MCs: summary


MC << data MC >> data MC ≈ data

MC/TUNE

D6T Perugia0

CSC PHOJET

-20% -17% +3% -2%

NchNch>10 Nch>5 Nch>15 Nch>10

ptpt>4Ge

vpt>1GeV pt>1Ge

V

<pt>

-24% -21% -2% -8%

NchNch>10 Nch>5 Nch >20 Nch>15

-27% -24% -4% -17%

NchNch >30

√s =

0.9

TeV

√s =

2.3

6 T

eV√s

= 7

TeV

0.9 TeV: PHOJET better for Nch, Perugia-0 for pt

2.36, 7 TeV: CSC better for Nch











published

preliminary

in preparation

p/p measurement at mid-rapidity Proton identification with TPC dE/dx

Special care for secondaries and absorption corrections pbar/p at |y| < 0.5 and 0.45 < pt < 1.05 GeV/c

Baryon-stopping at low

y=ybeam-yCM

Vanishes at high LHC energy


M.Broz

p/p measurement vs. MCs


Baryon number transport is usually explained by a String Junction transfer (gluon field)

What is the intercept of the corresponding Regge trajectory

(SJ = ½ or 1?)

Data described well by PYTHIA tunes

Other models (HIJING-B, QGSM with αSJ ~ 1) underestimate the data

Data show suppression baryon transport over large rapidity gaps in pp collisions

0.9 TeV

7 TeV

No pt dependence

du

u

SJ











published

preliminary

in preparation

Femtoscopy:particle emitting source

Assess the space-time evolution of the system that emits particles in pp collisions

Measure the Bose-Einstein enhancement for pairs of pions (identical bosons) at low momentum difference qinv=|p1-p2|, vs. event multiplicity and pair kt = |pt1+pt2|/2

Fit with a Gaussian


0.9 TeV

D.Miskowiec

Femtoscopy results at 0.9 TeV Radius grows with

dNch/dconsistent with other data and expectations (larger correlation volume at large multiplicities)


No visible kt dependence











published

preliminary

in preparation

Identified spectra:one of ALICE’s specials


ITS

TPC

TOF

Identified spectra at 0.9 TeV

Analysis in progress (spectra not fully corrected yet) Good agreement between the 3 detectors (ITS, TPC, TOF) Shows that detectors’ calibration/understanding is OK


positives negatives

Strangeness at 0.9 TeV


A.Maire

The three ways to kaons K± TPC+TOF PID K0

S vertex reconstruction

K±± kink reconstruction

Spectra not fully corrected Good internal constency


and K*0 at 0.9 and 7 TeV

0.7 < pt < 1.0 GeV/c


TPC+TOF PID

K+K- K*0

K+K-

TOF PID

K*0

TOF PID

pp 0.9 TeV pp 0.9 TeV

pp 7 TeV pp 7 TeV

Multi-strange baryons at 7 TeV


A.Maire

Prospects for 0: conversions

Electron ID in TPC TRD to join soon

Conversion reconstruction in TPC+ITS also very important for material

budget scan For 0 and : double conversion


0.9 TeV

7 TeV

backgroundfrom mixed events

backgroundfrom mixed events

Prospects for 0: conversions

Electron ID in TPC TRD to join soon

Conversion reconstruction in TPC+ITS also very important for material

budget scan For 0 and : double conversion


ALICE Performance31/05/2010

Raw 0 dN/dpt

Prospects for 0: EM calorimeters

Two EM calorimeters (back-to-back): EMCAL, PHOS Calibration ongoing, but already nice 0 signals


EMCAL PHOS

1 GeV/c1 GeV/c

5 GeV/c5 GeV/c

2 GeV/c2 GeV/c

5 GeV/c5 GeV/c

Jet reconstruction Charged-track jets raw

spectra 0.9 and 7 TeV ||<0.5 Four jets algos compared uncorrected


7 TeV0.9 TeV

uncorrected

uncorrected











published

preliminary

in preparation

Event shape studies Event shapes are sensitive to underlying event properties

multiple interactions mechanism tuning of MC generators

Can be used to classify events as “soft” or “hard” Look for unusual topologies Transverse thrust (hard scattering frame moves longitudinally)


measures the transverse sphericity of the event

= 1-2/

Event shape: Thrust


( = <1-T> ) vs. multiplicity at 0.9 and 7 TeV Thrust spectrum is unfolded based on MC (2 minimization)

data more “spherical” (less back-to-back-ish) than MCs

not fully corrected not fully corrected

Event structure: Underlying Event

Event-by-event analysis: identify leading hadron define transverse regions pt in the two regions

Region with larger energy (MAX) sensitive to QCD final-state radiation

Region wth smaller energy (MIN) sensitive to soft component (multiple interactions)


Underlying Event 7 TeV: first look at


Start by looking at inclusive correlations wrt leading track Data not corrected, compared to MCs (geant + recon) Leading pt<10 GeV/c data less back-to-back-ish than MCs

0.5 GeV/c < pT,lt < 2.5 GeV/c

2.5 GeV/c < pT,lt < 5 GeV/c

5 GeV/c < pT,lt < 10 GeV/c



ALICE Performance











published

preliminary

in preparation

J/ee, ||<0.9 e PID from TPC

TRD and EMCAL calib. ongoing


100M 7 TeV events

acceptance to pt=0

+- e+e-

Forward J/


J/, -4<<-2.5

acceptance to pt=0

+- e+e-

31 M events

177±30 J/

Charm: D meson reconstruction

Main selection: displaced-vertex topology Example: D0K-+

good pointing of reconstructed D momentum to the primary vertex ppair of opposite-charge tracks with large impact parameters

K ID in TPC+TOF helps rejecting background at low pt


Impact parameter resolution close to target

M.Lunardon

Charm: D0, D+, D*+

1.25×108 pp at 7 TeV Signals in 4 pt bins in 2-10 GeV/c

expect to cover 0.5-15 GeV/c with 109

compare to pQCD (FONLL) at 7 TeV


Heavy flavour from single leptons c and b production in semi-leptonic channels in preparation


Electrons ||<0.9:TPC dE/dx after K and p rejection with TOF. TRD and EMCAL will join soon

e±

±

raw spectrum

+ displacement selection beauty

Heavy flavour from single leptons c and b production in semi-leptonic channels in preparation


Muons -4<<-2.5: Light quark contribution subtracted with PYTHIA (normalized to data at low pt).Not corrected.

c (low pt) & b to be separated by fitting based on pQCD shapes, in progress ...

First Physics Results from ALICE

Particle multiplicity increase from 0.9 to 7 TeV significantly larger (>20%) than predicted

Momentum spectra <pt> VS Nch not described by any of the MCs

Anti-proton/proton ratio at midrapidity p/p goes to 1 at 7 TeV baryon number transfer suppressed over

large y Femtoscopic measurement at 0.9 TeV

particle emitting source “size” increases with multiplicity Event topology

lower “jettiness” than expected in LHC collisions Promising performance for ID spectra, strangeness, charm, charmonium

ALICE is ready to deliver many more Physics Results


Charged particle pt distribution

Baryon-antibaryon asymmetry Bose-Einstein correlations Strangeness production (K0, Λ, Ξ, Φ) Identified particles pT (π, K, p)

Outlook


dN/dpt at 7 TeV

π0, η

c and b production

J/ production

high multiplicity

jet correlations

event shape

underlying events

reconstructed jets

b-tagged jets

…

Papers in the pipeline: Analyses in progress:

November 2010: Pb-Pb collisions in ALICE!H.Torii

EXTRA SLIDES


First LHC physics paper

dNch/dη for |η| < 0.5 dNch/dη vs η

K. Aamodt et al. (ALICE), Eur. Phys. J C 65 (2010) 111

data collected 23 Nov, paper submitted 28 Nov 284 events (~ 3.7 authors per event)



Negative Binomial Distributions

Bernoulli experiment Probability for n failures and k

success in any order, but the last trial is a success

Physical interpretation Cascade production (clan model,

Giovannini, Z. Phys. C30 391 (1986))o Ancestor particle are produced

independently (Poisson)o Existing particle can produce

additional one with some probability p

ISR, 30 GeV

UA5, 900 GeVSuccessful for NSD events•up to 540 GeV in full phase space (ISR, UA5)•central intervals up to 1.8 TeV (UA5, CDF)

arXiv:0912.0023

arXiv:0912.0023

Physics at LHC 2010, DESY-Hamburg, 09.06.10 Andrea Dainese

Data Sample

Analysis based on 900 GeV: 186k collision events

out of ~430k total 2.36 TeV: 41k collision events 7 TeV: 240 k collision events

MC adapted to mean vertex position and spread

Eve

nts

z vertex distribution (cm)

53


Basically if the detector was perfect

But… there is Detector acceptance, tracking efficiency Decay, conversions, stopping, etc. Vertex reconstruction efficiency/bias,

trigger efficiency/bias Low momentum cut-off

Three corrections needed Track-to-particle correction (VS , zVtx) Vertex reconstruction correction (VS mult, zVtx) Trigger bias correction (VS mult, zVtx) all events

novertex

notrigger

all events

novertex novertex

notrigger

all events

novertex novertex

notrigger

notrigger

dNch/d Measurement

Events

Tracks

ddNch

Primary particles = charged particles produced in the collision and their decay products excluding weak decays from strange particles

Average correction factors (900 GeV)Tracking: 1.53Events (INEL): 1.05Events (NSD): 1.02 [+8.3% (NSD) – 6.8% (SD)]Subtraction of SD tracks for NSD: 3%


Systematic Uncertainties

Influence of systematic effects on result evaluated

E.g. fractions ND/DD/SD Change SD/DD by

measurement error E.g. SD (15.3 ± 2.3)%

Larger uncertainty for NSD than for INEL

Systematic uncertainties at 900 GeV in %

INEL NSD

Fractions ND/DD/SD 0.7 2.8

MC dependence +1.7 -0.5

Detector efficiency 1.5

Particle composition* 0.5 - 1.0

Material budget negl.

pT spectrum 0.5

SPD triggering efficiency negl.

V0 triggering efficiency negl. 0.5

Background negl.

Total -1.8

+2.5

-3.3

+3.3* -dependence


Systematic Uncertainties dNch/d

Systematic uncertainties in % 900 GeV 2.36 TeV 7 TeV

Fractions ND/DD/SD* 0.5 0.3 1.0

MC dependence +0.8 +1.5 +2.8

Detector efficiency ±1.5

Particle composition** ±(0.5 - 1.0)

Material budget negl.

pT spectrum ±0.5

SPD triggering efficiency negl.

V0 triggering efficiency negl.

Background negl.

* Fractions changed at 0.9 and 2.36 TeV like in paper 2; at 7 TeV by 50% ** -dependence


Regularizations

Unfolding using 2-Minimization

One free parameter per bin for unfolded spectrum Ut

Regularization Prefer constant locally Prefer linear function locally

Weight parameter needs to be tuned 2/ndf not larger than 1 Keep bias low

t

1-tt

t

't

tU

– U U

U

U a

t

1tt1t

t

''t

tU

U2UU

U

Ua

βR(U) URM

(U)χ

2

mm

t tmtm2

e t

2t )a( R(U)

V. Blobel, Yellow report, 1984


Why INEL>0 at 7 TeV?

7 TeV Pythia Perugia-0 Phojet

INEL=0 INEL>0 INEL=0 INEL>0

All 18.3% 81.7% 10% 90%

Vertex with |z| < 5.5

1 tracklet in || < 1

0.19% 77.3% 0.14% 87%

•A correction for the events in the INEL=0 bin is not needed (by definition)•Contamination is very low•Only the events that are lost in the INEL>0 bin are corrected by MC

minimal MC dependence•The INEL>0 requirement selects 80% of the MB1 events in 7 TeV data


X² - constant regularizationX² - linear regularizationBayesian unfolding

stat. + syst error > 50%

ALICE Performancework in progress

A structure?

Independent of Regularization scheme Unfolding method Data sample

Unfolded distribution changed to exponential Residuals show structure Slope change already in raw

data But… errors in unfolded

spectrum correlated Moving all points by 2

structure disappears Might just be a fluctuation

ALICE preliminary

measured multiplicity

|| < 1E

ven

tsN

orm

aliz

edre

sid

ua

ls

Unidentified pt spectrum Track reconstruction in TPC (≤160 hits) +ITS (≤6 hits)

pt measurement from TPC only (ITS-TPC alignment not final)

o (pt)/pt ≈ (0.7 + 0.7 pt)%

Track selection: pt > 150 MeV/c, || < 0.8

nhitsTPC > 70, 2/hits<4 in TPC

at least 2 matching hits in ITSo at least 1 in SPDo 4.7 on average

cut on transverse impact

parameter (7)

From MC, cross-checked

with data: Efficiency 50-80% Secondary cont. 9-1%


PythiaPerugia-0

PythiaPerugia-0

Baryon number at midrapidity? Basic question: who is the carrier of the baryon number nB?

Valence quarks: Rossi and Veneziano, NPB123 (1977) 507 Gluonic field: Kopeliovich and Zakharov, ZPC43 (1989) 241

QGSM considers the baryon as a bound quark-diquark state. nB transport implies breaking the diquark pair.

Gluonic mechanism aka String Junction (SJ), as in Pythia. nB transport implies the stopping of the SJ.


u

u

d u

u

d

Conventional approach - QGSM

Within QGSM one expects an asymmetry ~0 at LHC energies No BN transported at mid-rapidity from the fragmentation region

du

u

SJ

String Junction

€

P(Δy) ~ e−a J Δy

€

P(Δy) = const

BN transport even at large rapidity gaps (large energies). Veneziano: Probability exponentially suppressed (aJ: SJ intercept – model dependent) Kopeliovich: Probability constant with rapidity

ALICE First Physics Results

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