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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