global event features
DESCRIPTION
Global Event Features. Charged multiplicity (central collisions). Quantitative Difference from RHIC dN ch /d h ~ 1600 ± 76 ( syst ) on high side of expectations growth with √s faster in AA than pp : (√s -‘nuclear amplification’ - PowerPoint PPT PresentationTRANSCRIPT
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GLOBAL EVENT FEATURES
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Charged multiplicity (central collisions)
• Quantitative Difference from RHIC– dNch/dh ~ 1600 ± 76 (syst)
• on high side of expectations • growth with √s faster in AA than pp : (√s -‘nuclear
amplification’– Energy density ≈ 3 x RHIC (fixed t)
• lower limit, likely t0(LHC) < t0(RHIC)
tmdydN
AVE
0
1)(t
t
PRL105 (2010) 252301PRL105 (2010) 252301
15 GeV/fm3 …….. or more
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Bjorken’s formulaA simple way to estimate the energy density from charged
multiplicity or transverse energy measurements
HP: particle are produced at formation time tf (tf = 1 fm/c or less)
Consider a slice of longitudinal thickness Dz and section A
This slice contains the particles with speed
And such a number can be expressed as:where we have used b ≈ y for b0
With some further calculations we get:
Charged multiplicity: centrality dependence
• dNch/dh as function of centrality (normalised to ‘overlap volume’ ~ Nparticipants)
– DPMJET MC• fails to describe the data
– HIJING MC• strong centr. dependent
gluon shadowing– Others
• saturation models: Color Glass Condensate,‘geometrical scaling’ fromHERA/ photonuclear react. DPMJET
HIJING
Important constraint for modelssensitive to details of saturation
Saturation Models
Published on PRL
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Interferometry - I• Experimentally, the expansion rate and the spatial extent at decoupling are
accessible via intensity interferometry, a technique which exploits the Bose–Einstein enhancement of identical bosons emitted close by in phasespace. This approach, known as Hanbury Brown–Twiss analysis
p
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Interferometry - II
The three-dimensional correlation functions can be fitted with the following expression, accounting for the Bose-Einstein enhancement and for the Coulomb interaction between the two particles:
l= correlation strenght, k(qinv)= squared Coulomb wawefuntion
PLB 696 (2011)
Time at decoupling: t R ̴ out
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System Size from pion interferometry
Spatial extent of the particle emitting source extracted from interferometry of identical bosonsTwo-particle momentum correlations in 3 orth. directions -> HBT radii (Rlong, Rside, Rout) Size: twice w.r.t. RHIC Lifetime: 40% higher w.r.t. RHIC
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Kaon interferometry
Kaon interferometry: complementary to pion due to different mT
Results consisten with those with pions
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Interferometry: pp vs Pb-Pb
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Low-pT particle production
arXiv:1208.1974 [hep-ex]
(low) pT spectra : superposition of collective motion of particles on top of thermal motionCollective motion is due to high pressure arising from compression and heating .
“Blast-Wave” fit to pT spectra [1]: Radial flow velocity <b> ≈ 0.65 (10 % larger than at RHIC)Kinetic freezout temp. TK ≈ 95 MeV (same as RHIC within errors)
Central collisions: radial flow
[1] E. Schnedermann, et al.; Phys. Rev. C48, 2462 (1993)
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Particle yields and ratiosAssuming that the medium created in the collision reaches thermal equilibrium, one can compute particle yield and ratios with thermal models.
Grand canonical ensamble:
Where b =1/T and mi is the chemical potential
Thermal models have been (and are being ) used to fit the measured particles yields (at different c.m. energies) T and the baryochemical potential mb are free parameters to be obtained by the fit.
N.B. T is the chemical freezout temperature……..
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Yield and ratios at RHIC
!
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Particle yields and ratios at LHC - Extracted from pT- integrated identified particle spectra. - Comparison /Fit with Thermal/ Statistical models work well at RHIC info on chemical freezout temperature and baryochemical potential
Predicted temperature T=164 MeVA.Andronic, P.Braun-Munzinger, J.Stachel NP A772 167Thermal fit (w/o res.): T=152 MeV (c2/ndf = 40/9)X and W significantly higher than statistical model
p/p and L/p ratios at LHC lower than RHIC Hadronic re-interactions ?F.Becattini et al. 1201.6349 J.Steinheimer et al. 1203.5302
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ANISOTROPIC FLOW
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Anisotropic flow: basic idea
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Elliptic flow
Px
Py Pz
Reac
tion plane
X
Z
Y
f
Nch
yie
ld
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v2: selected ALICE results
v2 for non-identified particles:
v2 for identified particles:
Large elliptic flow oberved at RHIC consistent with strongly coupled medium with low shear viscosity (ideal fluid)
Stronger mass dependence of the elliptic flow as compared to RHIC: Due to the larger radial flow? Some deviation from hydrodynamic predictions for (anti) protons in close-to-central collisions: rescattering?
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V2 scaling at RHIC
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More on anisotropic flow
v2 and v3 over extended * h interval
V3 sensitive to the fluctuations of the initial nucleon distribution
*Results already published in the central h region: v2 Phys. Rev. Lett. 105, 252302 (2010),v3 Phys. Rev. Lett. 107, 032301 (2011)
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HIGH-PT AND JETS
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Particle spectra at high pT
Parton energy loss:A parton passing through the QCD medium undergoes energy loss which results in the suppression of high-pT
hadron yields
Related observable:nuclear modification factor RAA
Reference: pp collisions
Pb-Pb at different centralities
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RAA for identified particles
• First measurement of (anti-)proton, K and p at high pT (>7 GeV/c) :– The RAA indicates strong suppression, confirming the indications from previous measurements
for non-identified particles– The RAA for (anti-)protons, charged pions and K are compatible above 7 GeV/c ̴ this
suggests that the medium does not affect the fragmentation.
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Charged jet: RAA and RCP
Strong jet suppression observed for jets reconstructed with charged particles– RAA (jet) is smaller than inclusive hadron RAA(h±) at similar parton pT
– data are reasonably well described by JEWEL model K.Zapp, I.Krauss, U.Wiedemann, arXiv:1111.6838
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Near-side (jet-like) structure
N.Armesto et al., PRL 93, 242301
Isolation of near-side peak:Dh–D correlation with triggerLong-range (large Dh) correlation used as proxy for backgroundsh
s
Evolution of near-side-peaksh and s with centrality:Strong sh increase for centralcollisions
Interesting: AMPT describesthe data very well
Influence of flowing medium?