ALICE – Highlights di fisica e prospettive 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 ( )  LHC towards design energy  “Intermediate” future ( )  Pb-Pb (other systems ?) at  s NN for nuclear collisions > 4 TeV (5.5 TeV design energy)  Integrated luminosity  x10 2 b -1  Second shutdown (2018)  detector upgrades installation  2019 onwards  Pb-Pb at 50 kHz collision rate, ALICE goal L int ~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 p T thanks to the low material budget) Important also for upgrade-related considerations

Charged multiplicity – Energy density dN ch /d = 1584  76 (dN ch /d)/(N part /2) = 8.3  0.4 ≈ 2.1 x central AuAu at √s NN =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 energies 4 PRL105 (2010) 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)

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 (R long, R side, R out ) Size: twice w.r.t. RHIC Lifetime: 40% higher w.r.t. RHIC ALICE: PLB696 (2011) 328

Identified hadrons and radial flow Combined analysis (ITS, TPC and TOF) Significant change in mean p T between √s NN =200 GeV and 2.76 TeV  harder spectra For the same dN/d higher mean p T than at RHIC Common blast-wave fit to , K and p Strong radial flow: ≈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 (T ch,  B ) A.Andronic et al., Nucl.Phys.A772(2006) J.Cleymans et al., Phys.Rev.C73(2006) Description still not satisfactory at LHC energy Low T ch suggested by p spectra, but excluded by  and  If p excluded, T ch =164 MeV  T ch (LHC) ~ T ch (RHIC) ~ T c

Elliptic flow v 2 (LHC) ~ 1.3 v 2 (RHIC) (p T integrated) Increase consistent with increased radial expansion (higher p T ) System at LHC energy still behaves as a near-perfect fluid, not gas!

Identified particle v 2 Elliptic flow mass dependence due to large radial flow Magnitude and mass splitting predicted by viscous hydro in all centrality bins Observation of v 2 scaling with the number of constituent quarks not as good as at RHIC

Charged hadron R AA R AA (p T ) for charged particles : larger suppression wrt RHIC Suppression increases with increasing centrality Minimum for p T ~ 6-7 GeV/c in all centrality classes R AA increases in the region p T >10 GeV/c Hint of flattening above 30 GeV/c Model comparison Related to parton energy loss, in the BDMPS approach

Identified particle R AA Mesons vs baryons: different R AA at intermediate p T Related to baryon enhancement (coalescence), observed e.g. in /K ratio At high p T (>8-10 GeV/c) R AA universality for light-flavour hadrons For hadrons containing heavy quarks, smaller suppression expected: dead cone effect, gluon radiation suppressed for <m q /E q

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 R AA of D from B K  arXiv:

D-meson R AA pp reference from measured D 0, D + and D* p T 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 p T >5 GeV/c Little shadowing at high p T  suppression comes from hot matter Similar suppression for D mesons and pions Maybe a hint of R AA D > R AA π at low p T arXiv:

J/ suppression Inclusive J/ R AA pp reference from pp data set at 2.76 TeV Contribution from B feed- down not subtracted (very small effect) J/ are suppressed with respect to pp collisions J/ R AA almost independent of centrality peripheralcentral arXiv:

J/: comparison with RHIC Less suppression than at RHIC at forward rapidity: R AA (ALICE) > R AA (PHENIX, 1.2<y<2.2) Similar suppression as at RHIC at midrapidity (not for central!) R AA (ALICE) ≈ R AA (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-rapidity PHENIX, RHIC, forward rapidity arXiv:

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: n 2n 3n

Event background fluctuations and jet reconstruction  Low-p t 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 p T =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 p T,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: ) 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    )  >0  transverse polarization,  <0  longitudinal polarization

J/ polarization results ALICE Coll., arXiv: , accepted by PRL M.Butenschoen, A.Kniehl, arXiv: 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  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 R AA and elliptic flow for J/  Another example: D 0 and D + elliptic flow Confidential: still to be released!

Analysis prospects for  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 run  essential to distinguish hot/cold nuclear matter effects on QGP-related observables  essential 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 p T 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 dielectrons MFT: b-tagging for low p t J/psi and low-mass di-muons at forward y VHMPID: New high momentum PID capabilities FOCAL: Low-x physics with identified / 0  ITS upgrade presented to LHCC (March 20)

ITS upgrade  Current problems to be overcome  charm difficult for p t  0 (background is too large);  resolution not sufficient for charmed baryons ( c c=1/2 D 0 =1/5 D + );  physics results on  c impossible in Pb-Pb collisions (only hints of a signal), difficult in pp (only high p t )   b impossible in Pb-Pb collisions (insufficient statistics and resolution)  B/D separation difficult, especially at low p t (e PID + vertexing)

ITS upgrade  D-meson detection: factor 5 improvement in S/B  Assuming ~ 10 9 central events  Significance >100 in all p t bins   c -baryon detection  Assuming ~ 1.7 x 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 uncertainties for /D 0 ratio, 0-20% Estimate of statistical uncertainties for R AA Dfrom b / R AA Dfrom 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 function  Thermal dileptons  mass resolution: very strong improvement Bck rejection

HMPID upgrade - VHMPID  PID in jets, for p, , K in 5<p T <25 GeV/c  Identify strange particle and baryon components in jet fragmentation  strongly affected by the medium! PID performance at p T = 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 p T hadrons (factor 7 at p T =7 GeV/c) Light and heavy quarks R AA similar J/ is less suppressed than at lower energies : fully “booked” by the analysis of 2011 (Pb-Pb) and 2012 (pPb) runs : high-energy “campaign”, more physics ahead x: 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: ppbar Close symbols: pp Identified particle spectra

More on strangeness Inverse slope increases with mass s do not follow this trend (limited statistics?) has almost no increase over a factor 36 in √s (ISR  LHC)

Still on HBT radii Increase with multiplicity both in p-p and A-A, but different features

36 Analysis strategy –Require muon trigger signal to remove hadrons and low p t secondary muons –Remove residual decay muons by subtracting MC dN/dp t normalized to data at low p t 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 D 0  K - π + D +  K - π + π + D* +  D 0 π + D 0  K - π + π + π - D s  K - K + π + Λ c +  pK - π +

38 D0 K-+D0 K-+D0 K-+D0 K-+ Signals from 10 8 events –7 p t bins in the range 1<p t <12 GeV/c Selection based mainly on cosine of pointing angle and product of track impact parameters (d 0 K d 0  )

PID (ITS, TPC, TOF)

MonteCarlo scoreboard

Centrality vs models

High p T elliptic flow Due to path length dependence of parton energy loss

R AA – 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 cm -2 s -1 ) Reference for heavy-ion collision studies Genuine p-p physics From the problem…. …to the solution

Size: 16 x 26 meters Weight: 10,000 tons Detectors: 18

ALICE: specific features ALICE peculiarities among the LHC experiments Focus on PID  investigate chemical composition of the hot matter Push acceptance down to p T =0 (low material budget, low B)  many QGP-related features become more evident at low p T Sustain very high hadronic multiplicities (up to dN ch /d~810 3 )

PID performance: selected plots TPC dE/dx ITS Silicon Drift/Strip dE/dx Ω  ΛΚ TOF

Analyzed data samples SystemEnergy (TeV) TriggerAnalyzed events ∫Ldt pp7MB MUON 300M 130M 5 nb nb -1 PbPb2.76MB17M1.7 mb -1 pp2.76MB MUON 65M ~9M 1.1 nb nb Triggers MB: 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 p T Parameters of the system at the thermal freeze-out, T fo 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 p T >6 GeV/c  R CP for inclusive muons in 6<p T <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)114 A.Andronic et al., arXiv: P.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 p T 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

R AA of cocktail-subtracted electrons pp reference from measured heavy flavour electrons p T 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 for p T >3.5 GeV/c in the most central (0-10%) events Suppression increases with increasing centrality

Why  abs J/ 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 /nucl. Abs. 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