Recent results from the ALICE experimenton p-p and Pb-Pb collisions

E. Scomparin (INFN Torino)for the ALICE Collaboration

• Introduction• The ALICE experiment• The first LHC Pb-Pb run• Selected pp highlights• Prospects and conclusions

Bormio, January 23-27, 201250th International Winter Meeting on Nuclear Physics

Introduction• ALICE (A Large Heavy-Ion Collision Experiment):

the dedicated heavy-ion experiment at the LHC

• Main focus on Pb-Pb collisions QGP studies

• p-p collisions studied too (luminosity limited to a few 1030 cm-2s-1) • Reference for heavy-ion collision studies• Genuine p-p physics

From the problem…. …to the solution

Size: 16 x 26 metersWeight: 10,000 tonsDetectors: 18

ALICE: specific features•ALICE peculiarities among the LHC experiments

•Focus on PID investigate chemical composition of the hot matter

•Push acceptance down to pT=0 (low material budget, low B) many QGP-related features become more evident at low pT

•Sustain very high hadronic multiplicities (up to dNch/d~8103)

PID performance: selected plots

TPC dE/dx ITS Silicon Drift/Strip dE/dx

Ω ΛΚ

TOF

Analyzed data samplesSystem Energy

(TeV)Trigger Analyzed

events∫Ldt

pp 7 MBMUON

300M130M

5 nb-1

16-100 nb-1

PbPb 2.76 MB 17M 1.7 mb-1

pp 2.76 MBMUON

65M~9M

1.1 nb-1

20 nb-1

2010

2011

TriggersMB: based on VZERO (A and C) and SPD SINGLE MUON: forward muon in coincidence with MB trigger

Focus on Pb-Pb

•Analysis of 2010 PbPb data well advanced. Results on:•Global event features•Collective expansion•Strangeness and chemical composition•Parton energy loss in the medium

•Light flavours•Heavy flavours

•Quarkonia dissociation/regeneration in the medium

•Centrality estimate: standard approach

PRL106 (2011) 032301

•Glauber model fits•Define classes corresponding to fractions of the inelastic Pb-Pb cross section

See ALICE talks by:C. Perez Lara (Tue pm)S. Beole (Thu pm)A. Mischke (Thu pm)

Charged multiplicity – Energy density

• dNch/d = 1584 76

• (dNch/d)/(Npart/2) = 8.3 0.4

• ≈ 2.1 x central AuAu at √sNN=0.2 TeV

• ≈ 1.9 x pp (NSD) at √s=2.36 TeV• Stronger rise with √s in AA w.r.t. pp • Stronger rise with √s in AA w.r.t. log

extrapolation from lower energies8

PRL105 (2010) 252301

• Very similar centrality dependence at LHC & RHIC, after scaling RHIC results (x 2.1) to the multiplicity of central collisions at the LHC

PRL106 (2011) 032301

c)GeV/(fm15 2 Bj

System size

9

• Spatial extent of the particle emitting source extracted from interferometry of identical bosons

• Two-particle momentum correlations in 3 orthogonal directions -> HBT radii (Rlong, Rside, Rout)

• Size: twice w.r.t. RHIC

• Lifetime: 40% higher w.r.t. RHIC

ALICE: PLB696 (2011) 328 ALICE: PLB696 (2011) 328

Identified hadron spectra

10

• Combined analysis (ITS, TPC and TOF)• Lines = blast-wave fits, extract

• Integrated yields

• Average pT

• Parameters of the system at the thermal freeze-out, Tfo

and (radial flow)

Radial expansion of the system

• Significant change in mean pT between √sNN=200 GeV and 2.76 TeV harder spectra

• For the same dN/dh higher mean pT than at RHIC

• Common blast-wave fit to , K and p

• Strong radial flow: b≈ 0.66 for most central collisions, 10% higher than at RHIC

• Freeze-out temperature below 100 MeV

Blast-wave fit parameters

Centrality

STAR pp √s=200 GeV

Hadrochemistry•Relative abundances of hadron species can be described by

statistical distributions (Tch, B)A.Andronic et al., Nucl.Phys.A772(2006)

J.Cleymans et al., Phys.Rev.C73(2006)034905

•Description still not satisfactory at LHC energy•Low Tch suggested by p spectra, but excluded by and • If p excluded, Tch =164 MeV Tch (LHC) ~ Tch (RHIC) ~ Tc

Elliptic flow

])[cos(21)(d

d

1RP

nn

RP

nvN

RPv 2cos2

Reactionplane

In-planeOut

-of-

plan

e

Y

XFlow

Flow

Reactionplane

In-planeOut

-of-

plan

e

Y

XFlow

Flow

Reactionplane

In-planeOut

-of-

plan

e

Y

XFlow

Flow

•v2 (LHC) ~ 1.3 v2 (RHIC) (pT integrated)

• Increase consistent with increased radial expansion (higher pT)•System at LHC energy still behaves as a near-perfect fluid, not gas!

Identified particle v2

•Elliptic flow mass dependence due to large radial flow

•Magnitude and mass splitting predicted by viscous hydro in all centrality bins

•Radial flow too small from hydro, hadronic rescatterings play an important role in flow development

• and K v2(pT) well described, disagreement for p in central data

Charged hadron RAA

• RAA(pT) for charged particles : larger suppression wrt RHIC

• Suppression increases with increasing centrality

• Minimum for pT ~ 6-7 GeV/c in all centrality classes

• RAA increases in the region pT>10 GeV/c

• Hint of flattening above 30 GeV/c Tpp

TAA

AATAA dpd

dpdN

TpR

/

/1)(

Related to parton energy loss, in the BDMPS approach 2ˆ LqCE Rs

Identified particle RAA

•Mesons vs baryons: different RAA at intermediate pT

•Related to baryon enhancement, observed e.g. in /K ratio•At high pT (>8-10 GeV/c) RAA universality for light hadrons

•For hadrons containing heavy quarks, smaller suppression expected: dead cone effect, gluon radiation suppressed for <mq/Eq

ALICE pp data sample>100M muon triggers (MUS)(to get >2104 J/)>800M MB triggers (INT)

>25M high-multiplicitytriggers (SH1)

Interaction trigger reading all detectors: SPD (min bias) or V0-A or V0-Cat least one charged particle in 8 -unitsSingle-muon trigger reading MUON, SPD, V0, FMD, ZDC :single muon, low-pT threshold, in the muon arm in coincidence with interaction trigger High Multiplicity trigger

Open charm in ALICE

•Analysis strategy

• Invariant mass analysis of fully reconstructed decay topologies displaced from the primary vertex

•Feed down from B (10-15 % after cuts) subtracted using FONLL

•Plus in PbPb hypothesis on RAA of D from B

K p

D+K-p+p+

D-meson RAA

• pp reference from measured D0, D+ and D* pT differential cross-sections at 7 TeV scaled to 2.76 TeV with FONLL

• Suppression of prompt D mesons in central (0-20%) PbPb collisions by a factor 4-5 for pT>5 GeV/c

•Little shadowing at high pT suppression comes from hot matter

•Similar suppression for D mesons and pions

•Maybe a hint of RAAD > RAA

π at low pT

Electrons from heavy-flavour decays

• Cocktail method

• Inclusive electron pT spectrum

•Electron PID from TOF+TPC

•TRD used in pp

•Subtract cocktail of known background sources

e

• Impact parameter method (only pp for now)• Track impact parameter cut to select electrons from beauty

RAA of cocktail-subtracted electrons• pp reference from measured heavy flavour electrons pT differential

cross-sections at 7 TeV scaled to 2.76 TeV with FONLL

•Analysis of pp data at 2.76 TeV ongoing (direct reference)

• Suppression of cocktail-subtracted electrons

•Factor 1.5 - 4 for pT>3.5 GeV/c in the most central (0-10%) events

•Suppression increases with increasing centrality

Heavy-flavor decay muons

• Single muons at forward rapidity (-4<<-2.5)

• Background from primary /K decay not subtracted

•estimated with HIJING to be 9% in the most central class (0-10%) for pT>6 GeV/c

m

• RCP for inclusive muons in 6<pT<10 GeV/c

•suppression increases with increasing centrality

J/ suppression

• Inclusive J/y RAA

•pp reference from pp data set at 2.76 TeV

•Contribution from B feed-down not subtracted (very small effect)

• J/y are suppressed with respect to pp collisions

• J/y RAA almost independent of centrality

peripheral central

J/: comparison with RHIC

• Less suppression than at RHIC at forward rapidity:

•RAA(ALICE) > RAA(PHENIX, 1.2<y<2.2)• Similar suppression as at RHIC at midrapidity (not for

central!)

•RAA(ALICE) ≈ RAA(PHENIX, |y|<0.35)• Caveat: cold nuclear matter effects different at RHIC and LHC

needs pPb running24

ALICE, LHC, forward rapidityPHENIX, RHIC, mid-rapidityPHENIX, RHIC, forward rapidity

J/: comparison to models

•Parton transport model• J/ dissociation in QGP• J/ regeneration by charm

quark pair recombination•Feed-down from B-decays•Shadowing

R.Rapp, X.Zhao, NPA859(2011)114A.Andronic et al., arXiv:1106.6321P.Braun-Munzinger et al.,PLB490(2000) 196

•Statistical hadronization model•Screening by QGP of all J/•Charmonium production at phase boundary by statistical combination of uncorrelated c-quarks

A pp new result: J/ polarization

•ALICE focusses on pp results mainly as reference for PbPb

•On hard probes usually no competition with other LHC experiments due to smaller luminosity in ALICE

•Some notable exceptions, too J/ polarization (first LHC results from ALICE, arXiv:1111.1630)

• Important measurement to discriminate among the different theoretical models of J/ production

•Long-standing puzzle with CDF results

• J/ polarization measured via anisotropies in the angular distributions of J/ decay products (polarization parameters )

cos2sin2cossincos13

1, 22

W

>0 transverse polarization, <0 longitudinal polarization

J/ polarization resultsALICE Coll., arXiv:1111.1630,accepted by PRL

M.Butenschoen, A.Kniehl, arXiv:1201.3862

•First result: almost no polarization for the J/•First theoretical calculation (NLO NRQCD) compared to data: promising result, reasonable agreement with theory

Prospects, 2011 Pb-Pb data•2011 Pb-Pb data quite successful•Smooth running, much higher luminosity ~10 times more statistics (centrality and rare triggers) compared to 2010•New, exciting results expected soon!

Total 2011 statistics 40000 J/

A couple of performance plots

Triggering on EMCAL

Conclusions• 2011 has been (another) memorable year for ALICE !

•After an already excellent start in 2010, with plenty of pp results, focus in 2011 on the analysis of the first Pb-Pb run

•First results•Medium with >3 times higher energy density than at RHIC•Abundance of hard probes

• Soft observables•Smooth evolution of global event characteristics from RHIC to LHC energies better constraints for existing models•Move now towards precision differential measurements

•Pb-Pb 2011 running: success !•Work on experiment upgrades is starting

•Hard probes: novelties, surprises, challenges for theory•Strong suppression of high pT hadrons (factor 7 at pT=7 GeV/c)

•Light and heavy quarks RAA similar

• J/ is less suppressed than at RHIC

Backup

• ITS, TPC, TOF, HMPID, MUON, V0, To, FMD, PMD, ZDC (100%)• TRD (7/18)• EMCAL (4/10) • PHOS (3/5)• HLT (60%)

2010 data taking: detector configuration

Open symbols: ppbarClose symbols: pp

Identified particle spectra

More on strangenessInverse slope increaseswith masss do not follow this trend(limited statistics?)

<pT> has almost no increase over a factor 36 in √s(ISRLHC)

Still on HBT radii

Increase with multiplicityboth in p-p and A-A, but different features

35

• Analysis strategy– Require muon trigger signal to remove hadrons and low pt

secondary muons – Remove residual decay muons by subtracting MC dN/dpt

normalized to data at low pt

• Alternative method: use muon distance-of-closest-approach to primary vertex

• What is left are muons from charm and beauty– Apply efficiency corrections

36

D meson reconstruction

• Analysis strategy: invariant-mass analysis of fully-reconstructed topologies originating from displaced vertices– Build pairs/triplets/quadruplets of tracks with correct

combination of charge signs and large impact parameters

– Particle identification from TPC and TOF to reject background (at low pt)

– Calculate the vertex (DCA point) of the decay tracks– Require good pointing of reconstructed D momentum to

the primary vertex

D0 K-π+ D+ K-π+π+

D*+ D0π+

D0 K-π+π+π- Ds K-K+π+

Λc + pK-π+

37

D0 K-p+

• Signals from 108 events– 7 pt bins in the range 1<pt<12 GeV/c

• Selection based mainly on cosine of pointing angle and product of track impact parameters (d0

Kd0p)

PID (ITS, TPC, TOF)

MonteCarlo scoreboard

Centrality vs models

High pT elliptic flow

Due to path length dependence of parton energy loss

RAA – comparison with models