ALICE – Highlights di fisica eprospettive 2012 e oltre

E. Scomparin (INFN Torino)

Meeting referees – 9 maggio 2012

Present: first precision results in PbPb collisions at the LHC

“Next” future: p-Pb (Pb-p) 2012 cold nuclear matter

First shutdown (2013-2014)LHC towards design energy

“Intermediate” future (2015-2017) Pb-Pb (other systems ?) at sNN for nuclear collisions > 4 TeV (5.5 TeV design energy) Integrated luminosity x102 b-1

Second shutdown (2018) detector upgrades installation

2019 onwards Pb-Pb at 50 kHz collision rate, ALICE goal Lint ~10 nb-1

Papers from 2010 runs

Scientific production in terms of published papers very good

pp collisions: 14 published papers + 4 on arXiv

PbPb collisions: 8 published papers + 3 on arXiv

…plus 11 ready and circulating in the Collaboration …plus many more in preparation

Strong participation of Italian groups in the analysis/publication process : at least 1 Italian physicist in the Paper Committee for ~50% of the papers

Let’s now focus on PbPb collisions and review the main results…

Focus on Pb-Pb

Results from 2010 PbPb data for all the observables: Global event features (energy density) Collective expansion (flow) Strangeness and chemical composition (chemical freeze-out) Parton energy loss in the medium

Light flavours Heavy flavours

Quarkonia dissociation/regeneration (deconfinement) in the medium

Main advantage of ALICE with respect to other LHC experiments:

Excellent tracking in a very high multiplicity environment Particle identification over a large range of transverse momenta(down to very low pT thanks to the low material budget)

Important also for upgrade-related considerations

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 energies4

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

5

• 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 hadrons and radial flow

• Combined analysis (ITS, TPC and TOF)• 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

•Observation of v2 scaling with the number of constituent quarks not as good as at RHIC

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

• Model comparisonTpp

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 (coalescence), observed e.g. in /K ratio•At high pT (>8-10 GeV/c) RAA universality for light-flavour hadrons•For hadrons containing heavy quarks, smaller suppression expected: dead cone effect, gluon radiation suppressed for <mq/Eq

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

arXiv:1203.2160

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 3-4 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

arXiv:1203.2160

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

arXiv:1202.1383

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 running

15

ALICE, LHC, forward rapidity

PHENIX, RHIC, mid-rapidityPHENIX, RHIC, forward rapidity

arXiv:1202.1383

e.m. dissociation Measure e.m. dissociation cross section in Pb-Pb via neutron emission at very forward angles (ZDC)

… in good agreement with model predictions (RELDIS)

arXiv:1203.2436

1n

2n3n

Event background fluctuationsand jet reconstruction

Low-pt component of jets important for the measurement of medium modifications (jet quenching) Not accessible to ATLAS/CMS Region to region background fluctuations main source of jet momentum uncertainty, affect jet structure observables

For a pT=0.15 GeV/c cut-off fluct=10.98 0.01 GeV/c (R=0.4, 0-10% central PbPb) fluct decreases to 4.82 GeV for pT,min = 2 GeV/c (reduced region to region fluctuations) Asymmetric shape of fluctuations have a large impact on the jet yield up to 100 GeV/c

JHEP 03(2012) 053

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 on this issue, 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

Data analysis in 2012: 2011 Pb-Pb data•2011 Pb-Pb data very 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

Data analysis in 2012: first 2011 Pb-Pb results soon

Analysis is progressing fast: first results from 2011 Pb-Pb run will be released at the end of May (Hard Probes 2012, Cagliari)

Confidential: still to be released!

Examples: new results on differential RAA and elliptic flow for J/ Another example: D0 and D+ elliptic flow

Confidential: still to be released!

Analysis prospects for 2012-2013 Analysis effort on 2011 PbPb data will continue during 2012 and (at least) half 2013 (complete analysis, submit papers)

We are also expecting very important results from 2012 pPb runessential to distinguish hot/cold nuclear matter effects on QGP-related observablesessential to evaluate initial state effects (parton shadowing), very poorly known at LHC energy (only extrapolations by now)

An example from RHIC: back-to-back angular correlations

Only by looking at d-Au the observed effect can be ascribed to final state effects

Analysis efforts after 2013 (before upgrade)

Data analysis for p-Pb/Pb-p collisions (plus more involved analysis on Pb2011 data) expected to last at least to the end of 2014

2015: physics in the new high-energy range Precise running conditions still not known: for Pb-Pb running a higher luminosity and c.m.s. energies > 4 TeV per nucleon pair are expected

Physics prospects for ALICE pp physics topics accessible to the experiment Pb-Pb collision studies very relevant for QGP physics (excitation functions) In addition: larger luminosity higher pT reach

Examples J/ physics: final determination of regeneration vs screening Heavy flavor correlations, jet tagging

Upgrade planning Strong detector/physics efforts in view of the LHC upgrade

Technical details on detector developments to be discussed in other presentations shortly review physics aspects, in particular on hard and electromagnetic probes

Upgrade experiment to be able to run with 50 kHz Pb-Pb collision rates, several nb-1 per run (2 MHz proton-proton)

Various new detectors being proposed (stregthen ALICE uniqueness at LHC)

ITS: B/D separation, heavy baryons, low-mass dielectronsMFT: b-tagging for low pt J/psi and low-mass di-muons at forward yVHMPID: New high momentum PID capabilitiesFOCAL: Low-x physics with identified g/p0

ITS upgrade presented to LHCC (March 20)

ITS upgrade Current problems to be overcome

charm difficult for pt0 (background is too large);

resolution not sufficient for charmed baryons

(Lc ct=1/2 D0=1/5 D+);

physics results on Lc impossible in Pb-Pb collisions (only hints of a signal), difficult in pp (only high pt)

Lb impossible in Pb-Pb collisions (insufficient statistics and resolution)

B/D separation difficult, especially at low pt (e PID + vertexing)

ITS upgrade

D-meson detection: factor 5 improvement in S/B

Assuming ~ 109 central events Significance >100 in all pt bins

c-baryon detection

Assuming ~ 1.7 x 1010 central

events (10 nb-1) in 0-20%

Significance:

7 for 2<pt<4 GeV/c

>50 for 6<pt<8 GeV/c

ITS upgrade

Estimate of statistical uncertaintiesfor /D0 ratio, 0-20%

Estimate of statistical uncertaintiesfor RAA

Dfrom b/ RAADfrom c

MUON upgrade - MFT

Low-mass dilepton physics practically still untouched at LHC energy

Excellent thermometer of the medium (see NA60, PHENIX, STAR)

Modification of spectral functionThermal dileptons

mass resolution: very strong improvement Bck rejection

HMPID upgrade - VHMPID PID in jets, for p, , K in 5<pT<25 GeV/c

Identify strange particle and baryon components in jet fragmentation strongly affected by the medium!

PID performance at pT = 20 GeV/c

Conclusions•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 complete set of results at the LHC available•Medium with >3 times higher energy density than at RHIC• Soft observables

•Smooth evolution of global event characteristics from RHIC to LHC energies better constraints for existing models

•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 lower energies

•2012-2014: fully “booked” by the analysis of 2011 (Pb-Pb) and 2012 (pPb) runs

•2015-2017: high-energy “campaign”, more physics ahead

•2019-202x: physics with upgraded ALICE set-up (pp, PbPb, ArAr)

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

36

• 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

37

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+

38

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

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

Identified hadron spectra

51

• 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)

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/: 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

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

Why absJ/ is so relevant ?

• The cold nuclear matter effects present in pA collisions are of course present also in AA and can mask genuine QGP effects

L

J//N

coll

L

J//N

coll/

nu

cl.

Ab

s.

1

Anomalous suppression!

pA

AA

• It is very important to measure cold nuclear matter effects before any claim of an “anomalous” suppression in AA collisions