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ALICE – Highlights di fisica e prospettive 2012 e oltre. 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

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alice highlights di fisica e prospettive 2012 e oltre
ALICE – Highlights di fisica eprospettive 2012 e oltre
  • Present: first precision results in PbPbcollisions 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  x102 b-1
  • Second shutdown (2018)  detector upgrades installation
  • 2019 onwards  Pb-Pb at 50 kHz collision rate,
  • ALICE goal Lint ~10 nb-1

E. Scomparin (INFN Torino)

Meeting referees – 9 maggio 2012

papers from 2010 runs
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
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 pTthanks to the low material budget)

Important also for upgrade-related considerations

charged multiplicity energy density
Charged multiplicity – Energy density

PRL106 (2011) 032301

PRL105 (2010) 252301

  • dNch/d = 1584  76
  • (dNch/d)/(Npart/2) = 8.3  0.4
      • ≈ 2.1 x central AuAuat √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 energies
  • Very similar centrality dependence at LHC & RHIC, after scaling RHIC results (x 2.1) to the multiplicity of central collisions at the LHC
system size
System size
  • 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
Identified hadrons and radial flow


STAR pp √s=200 GeV

Blast-wave fit parameters

  • 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
  • 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/dhhigher mean pT than at RHIC
  • Relative abundances of hadron species can be described by statistical distributions (Tch, B)

A.Andronic et al., Nucl.Phys.A772(2006)

  • 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

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

elliptic flow
Elliptic flow
  • v2 (LHC) ~ 1.3 v2 (RHIC) (pTintegrated)
  • Increase consistent with increased radial expansion (higher pT)
  • System at LHC energy still behaves as a near-perfect fluid, not gas!
identified particle v 2
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 r aa
Charged hadron RAA

Related to parton energy loss, in the BDMPS approach

  • 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
    • RAAincreases in the region pT>10 GeV/c
    • Hint of flattening above 30 GeV/c
    • Model comparison
identified particle r aa
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
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




d meson r aa
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 > RAAπ at low pT
j suppression
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 RAAalmost independent of centrality


j comparison with rhic
J/: comparison with RHIC


ALICE, LHC, forward rapidity

PHENIX, RHIC, mid-rapidity

PHENIX, RHIC, forward rapidity

  • 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
e m dissociation
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)


event background fluctuations and jet reconstruction
Event background fluctuationsand jet reconstruction
  • Low-ptcomponent 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

JHEP 03(2012) 053

  • For a pT=0.15 GeV/ccut-off
  •  fluct=10.98  0.01 GeV/c
  • (R=0.4, 0-10% central PbPb)
  • fluctdecreases 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
a pp new result j polarization
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
  • distributionsof J/ decay products
  • (polarization parameters    )

>0  transverse polarization, <0  longitudinal polarization

j polarization results
J/ polarization results

ALICE 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
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!

A couple of performance plots

Triggering on EMCAL

Total 2011 statistics  40000 J/

data analysis in 2012 first 2011 pb pb results soon
Data analysis in 2012: first 2011 Pb-Pbresults 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!

Confidential: still to be released!

  • Examples: new results on differential RAA and elliptic flow for J/
  • Another example: D0 and D+ elliptic flow
analysis prospects for 2012 2013
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 run
    • essential to distinguish hot/cold nuclear matter effects on
    • QGP-related observables
    • essential to evaluate initial state effects (partonshadowing),
    • 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
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 TeVper 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 pTreach
  • Examples
    • J/ physics: final determination of regeneration vs screening
    • Heavy flavor correlations, jet tagging
upgrade planning
Upgrade planning
  • Strong detector/physics efforts in view of the LHC upgrade
  • 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 pt J/psi and
    • low-mass di-muons at forward y
    • VHMPID: New high momentum PID
    • capabilities
    • FOCAL: Low-x physics with identified g/p0
  • ITS upgrade presented to LHCC (March 20)
  • Technical details on detector developments to be discussed in
  • other presentations  shortly review physics aspects,
  • in particular on hard and electromagnetic probes
its upgrade
ITS upgrade
  • Current problems to be overcome
    • charmdifficult for pt0 (background is too large);
    • resolution not sufficient for charmed baryons
    • (Lcct=1/2 D0=1/5 D+);
    • physics results on Lcimpossible in Pb-Pbcollisions (only hints of a signal), difficult in pp (only high pt)
    • Lbimpossible in Pb-Pb collisions (insufficient statistics and resolution)
    • B/D separation difficult, especially at low pt (e PID + vertexing)
its upgrade1
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 upgrade2
ITS upgrade

Estimate of statistical uncertainties

for RAADfromb/RAADfromc

Estimate of statistical


for /D0 ratio, 0-20%

muon upgrade mft
MUON upgrade - MFT
  • Low-mass dileptonphysics
  • 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
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

  • 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 RAAsimilar
    • 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)
2010 data taking detector configuration
2010 data taking: detector configuration
  • ITS, TPC, TOF, HMPID, MUON, V0, To, FMD, PMD, ZDC (100%)
  • TRD (7/18)
  • EMCAL (4/10)
  • PHOS (3/5)
  • HLT (60%)
identified particle spectra
Identified particle spectra

Open symbols: ppbar

Close symbols: pp

more on strangeness
More on strangeness

Inverse slope increases

with mass

s do not follow this trend

(limited statistics?)

<pT> has almost no increase

over a factor 36 in √s


still on hbt radii
Still on HBT radii

Increase with multiplicity

both in p-p and A-A, but different



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


d meson reconstruction
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+


d 0 k p
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 (d0Kd0p)


high p t elliptic flow
High pTelliptic flow

Due to path length dependence of parton energy loss

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

the dedicated heavy-ion experiment at the LHC

  • Main focus on Pb-Pb collisions  QGPstudies

From the problem….

…to the solution

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

Size: 16 x 26 meters

Weight: 10,000 tons


alice specific features
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~8103)
pid performance selected plots
PID performance: selected plots

ITS Silicon Drift/Strip dE/dx

TPC dE/dx



analyzed data samples
Analyzed data samples



  • Triggers
    • MB: based on VZERO
    • (A and C) and SPD
    • SINGLE MUON: forward muon in coincidence with MB trigger
identified hadron spectra
Identified hadron spectra
  • Combined analysis

(ITS, TPC and TOF)

  • Lines = blast-wave fits, extract
    • Integrated yields
    • Average pT
    • Parameters of the system at the thermal freeze-out, Tfoand  (radial flow)
heavy flavor decay muons
Heavy-flavor decay muons


  • Single muonsat 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
  • RCP for inclusive muonsin 6<pT<10 GeV/c
    • suppression increases with increasing centrality
j comparison to models
J/: comparison to models

A.Andronic et al., arXiv:1106.6321

P.Braun-Munzinger et al.,PLB490(2000) 196

R.Rapp, X.Zhao, NPA859(2011)114

  • Parton transport model
    • J/ dissociation in QGP
    • J/ regeneration by charm
  • quark pair recombination
    • Feed-down from B-decays
    • Shadowing
  • Statistical hadronizationmodel
    • Screening by QGP of all J/
    • Charmonium production at
    • phase boundary by statistical
    • combination of uncorrelated
    • c-quarks
electrons from heavy flavour decays
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
  • Impact parameter method (only ppfor now)
    • Track impact parameter cut to
    • select electrons from beauty
r aa of cocktail subtracted electrons
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 abs j is so relevant


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




J//Ncoll/nucl. Abs.

Anomalous suppression!



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