First measurements in Pb—Pb collisionsat sNN=2.76 TeV with ALICE at the LHC
M. Nicassio (University and INFN Bari)for the ALICE Collaboration
Rencontres de MoriondLa Thuile, Aosta valley (Italy)March 20-27, 2011
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Contents Introduction
ALICE experiment goal and apparatus data taking conditions and event selection centrality determination
First measurements multiplicity density of primary charged particles
Phys. Rev. Lett. 105, 252301 (2010), arXiv:1011.3916 Phys. Rev. Lett. 106, 032301 (2011), arXiv:1012.1657
elliptic flow of primary charged particles Phys. Rev. Lett. 105, 252302 (2010), arXiv:1011.3914
Bose—Einstein correlations Phys. Lett .B 696 (2011) 328-337, arXiv:1012.4035
pt spectra of primary charged particles and RAA
Phys. Lett .B 696 (2011) 30-39, arXiv:1012.1004
Summary and outlook
M. Nicassio
See Jochen talk
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The ALICE experiment Main goal
study the properties of Quark—Gluon Plasma formed in heavy-ion collisions
Main detectors used for first analyses Inner Tracking System
• Silicon Pixel Detector r=3.9 cm |h|<2.0 inner layer r=7.6 cm |h|<1.4 outer layer 10M pixels,1200 read-out chips
Time Projection Chamber• 85<r<247 cm, |h|<0.9
V0 scintillators• at 0.9 and 3.3 m on either side of IP• 2.8<|h|<5.1 and -3.7<|h|<-1.7
Zero Degree Calorimeters• at 114 m on either side of IP
ITS
TPC
ZDC
ZDC
V0A
V0C
M. Nicassio
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Trigger and event selection First runs with Pb ions @ sNN=2.76 TeV
4 or 66 bunches 1 to 7107 ions of 208Pb per bunch 0.5 to 81023 cm-2s-1 estimated luminosity interaction rate 50 Hz = 4Hz+45 Hz+1Hz luminous region: 5.9 cm in z, 50 mm in transverse direction
Triggering with SPD and both V0 arrays two out of three trigger signals
two SPD pixel chips hit in the outer layer a signal in V0A a signal in V0C
coincidence between both V0 arrays more efficient for electromagnetic interaction suppression
Background removal electromagnetic interactions: neutron ZDC minimal energy deposit beam-background: V0 timing information, correlation between TPC
tracks and SPD hits
Hadronic collisionsElectromagnetic interactionsBeam-background
M. Nicassio
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Centrality determination Two component model of particle production based on Glauber description of nuclear collisions
Nancestors=fNpart+(1-f)Ncoll
particle produced by each source described by NBD Assumption in Glauber model calculation
Woods-Saxon distribution for nuclear densityr = 6.620.06 fmskin depth a = 0.5460.010 fm
minimum inter-nucleon distance = 0.40.4 fm interaction distance given by sinel
NN = 645 mb at sNN=2.76 TeV (from interpolation of data)
Glauber fit of V0 amplitude distribution above an anchor pointcorresponding to 88% of the hadronic cross-section, where triggeris fully efficient and background negligible
M. Nicassio
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Centrality determination
Glauber fit of V0 amplitude distribution above an anchor pointcorresponding to 88% of the hadronic cross-section, where triggeris fully efficient and background negligible
M. Nicassio
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Results I: multiplicity density of primarycharged particles
Energy dependence
Increase relative to RHIC and pp factor 2
Analysis on 3600 events:- 5% most central - |zvtx|<7 cm- primary vertex and tracklets reconstructed in the SPD- TPC tracks as a cross check (dNch/dh)/(0.5<Npart>)= 8.3 0.4 (sys.)
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Analysis on 49k events:-|zvtx|<7 cm- primary vertex and tracklets reconstructed in the SPD- TPC tracks as a cross check
Centrality dependence
Results I: multiplicity density of primarycharged particles
M. Nicassio
Same trend as at RHIC Increase from peripheral to central factor 2
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Comparison with models two-component models (pQCD processes + soft interactions) saturation models
Results I: multiplicity density of primarycharged particles
M. Nicassio
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Results II: elliptic flow of primary charged particles
Transverse momentum dependence of v2
Analysis on 45k events:- centrality selection cutting on TPC track multiplicity- |zvtx|<10 cm- ITS and TPC tracks (TPC tracks as a cross check)- various analysis techniques
pt dependece in centrality classes similar to RHIC
M. Nicassio
No changes in v2(pt) compared to RHIC
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Centrality dependence of integrated elliptic flow
Comparison with RHIC results 30 % higher Hydrodynamic model predictions with viscous
corrections in agreement with observed increase
|h|<0.8 0.2<pt<5GeV/c
M. Nicassio
Results II: elliptic flow of primary charged particles
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Energy dependence of integrated elliptic flow
Increase in the magnitude of elliptic flow with energy
20-30% centrality class
M. Nicassio
Results II: elliptic flow of primary charged particles
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Results III: two-pion Bose—Einsteincorrelations
HBT radii and decoupling time multiplicity dependence
Volume of homogeneity region: linear dependence on dNch/dh, two times larger than at RHIC
Analysis on 16k events:- 5% most central collisions- |zvtx|<12 cm- TPC tracks
M. Nicassio Rencontres de Moriond - 23/03/2011
tf: linear dependence on (dNch/dh)1/3, 1.4 times larger than at RHIC
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Results IV: pt spectra of primary charged particles and RAA
pp reference at 2.76 TeV: main approach interpolation of ALICE measurements in pt bins (at 0.9 and 7 TeV),
assuming a power law for the increase of the yield with s
Analysis on 2.3M events:- |zvtx|<10 cm- ITS+TPC tracks
|h|<0.8 0.3<pt<20GeV/c
Power law
Exponential
M. Nicassio
Peripheral events and scaledpp reference have samept dependence
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|h|<0.8 0.3<pt<20GeV/c
Alternatives forpp reference:-CDF data in interpolation-NLO scaling of pp data at 0.9 TeV
M. Nicassio
Results IV: pt spectra of primary charged particles and RAA
Comparison with RHIC in central collisions: stronger suppression of high pt particles very dense medium formed at LHC
In central collisions strong suppression and clear change in shape In peripheral collisions no pt dependence above 2 GeV/c
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Summary and outlook First ALICE measurements in PbPb collisions
multiplicity a factor 2 higher than at RHIC, same centrality dependence
elliptic flow 30% higher than at RHIC HBT radii and decoupling time in central collisions
10-35% and 30% higher than at RHIC
Ongoing studies and outlook measurements for identified particles soft physics studies (resonances, baryons and strangeness) hard physics studies (open heavy flavours, energy loss, jets) looking forward at PbPb collisions at the end of 2011
• target for integrated luminosityfactor 3 (or more) higher• first injections of pA
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