1. Dipartimento Interateneo di Fisica, Bari (Italy)
Soft Physics in ALICE
Annalisa Mastroserio
Dipartimento Interateneo di Fisica, Bari (Italy)
INFN-Bari (Italy)
Outline
Relativistic heavy ion collisions
Some observables of the Soft Physics sector in ALICE:
Particle Abundances at LHC
Momentum spectra at LHC
Resonances at LHC
Conclusions
Cracow July 2006
A. Mastroserio

2. (Hadrons →) partons → Hadrons
Summary on RHIC
relativistic Heavy Ion Collision
Thermalization
τ < 1 fm/c
Expansion
1 fm/c < τ < 10 fm/c
(Hadrons →) partons → Hadrons
(QGP phase )
q, g no more confined in hadrons.
q, g interact in a bigger volume
The particles are detected
AFTER this phase !!
Freeze-out
10 ~ 15 fm/c < τ
Cracow July 2006
A. Mastroserio

3. What’s LHC ?
Higher initial √sNN = 200 GeV → = 5.5 TeV
energy density
Higher initial MeV → 600 MeV (?)
Temperature
Longer lifetime ~ 6 fm/c → (?)
of the QGP phase
Hard processes contribute significantly to global observables,
in contrast to RHIC (ALICE PPR vol I)
Will the theoretical assumptions for RHIC data be still valid at LHC ?
- Particle ratios via a statistical approach : Will the equilibrium be reached?
-The expansion phase via hydrodynamics : At which level could it describe
momentum spectra and the bulk flow?
A good identification of particles is very important!!
Cracow July 2006
A. Mastroserio

4. ALICE experiment @ LHC p/K e /p TPC + ITS K/p (dE/dx) TOF HMPID (RICH)
A unique capability to identify particles :
- Larger acceptance
- efficient PID in large pt domain (especially in the low pt region)
p (GeV/c)
TPC + ITS
(dE/dx)
p/K
K/p
e /p
HMPID
(RICH)
TOF
p (GeV/c)
TRD e /p
Cracow July 2006
A. Mastroserio

5. Particle abundances
Cracow July 2006
A. Mastroserio

6. Particle abundances @ RHIC
…many models but a common hypothesis : Grand Canonical ensemble
- Chemical equilibrium
- Strangeness and charge conservation
parameters : T and μB
nucl-th/ v1
Cracow July 2006
A. Mastroserio

7. Particle abundances
…what if deviations from chemical equilibrium exist?
-> new parameters : s q (q = u, d)
[Chemical equilibrium means s = q = 1 ]
s = q = 1
q = 1 , s ≠ 1
q > 1 , s ≠ 1
nucl-th/ v3
Cracow July 2006
A. Mastroserio

8. Particle abundances @ LHC
results for
chemical
equilibrium
Predictions for chemical equilibrium will provide new insights in the chemical properties of the plasma
“Soft hadron ratios at the LHC” Eur. Phys. J. C 45, (2006)
Cracow July 2006
A. Mastroserio

9. Hadronic dynamics : Pt spectra elliptic flow (hydrodynamical picture)
Cracow July 2006
A. Mastroserio

10. mt spectrum Hadrons show a collective behaviour
Hydrodynamical approach : non viscous cells of fluid in
thermal equilibrium nucl-th/
(microscopic time scales << macroscopic time scales)
hep-ph/ U. Heinz
Hydrodynamical calculations
(sudden freeze out) show :
mT-scaling (valid in p+p) is
broken by the transverse
flow of the fireball :
if pt << m0
Tslope = Tdec + m<ßT>2
Cracow July 2006
A. Mastroserio

11. mt spectrum
At LHC a stronger flow should develop so there should be a flattening of the
pt spectrum
Tslope = Tdec + m<ßT>2
LHC
(hep-ph/ v1)
ALICE can clarify the interplay between soft processes and hard processes
“The main ingredient of hydrodynamic models is the equation of state (EoS) for thermalized matter in heavy ion collisions” see : nucl-th/ v1
Cracow July 2006
A. Mastroserio

12. Elliptic flow : v2
The spatial anisotropy and the pressure gradient lead to an anisotropic spatial distribution of the momenta in the transverse plane
At LHC the contribution of hydrodynamical expansion of the QGP (with respect to hadronic cascades) should be much larger than at RHIC -> Better sensitivity to EoS
Cracow July 2006
A. Mastroserio

13. Resonances
Cracow July 2006
A. Mastroserio

14. Physical motivations
A good tool to understand QGP, rHIC dynamics and the hadronization process
-> Modification of the mass values and widths e.g. ρ(770)
-> Interplay between recombination and rescattering e.g. K*/K
-> Hadronization : fragmentation, coalescence, … ? e.g. quark content/mass
Cracow July 2006
A. Mastroserio

15. Many preliminary studies in ALICE for the resonance identification
Resonances
Many preliminary studies in ALICE for the resonance identification
(simulation results)
r0(770) → p+p-
106 central Pb-Pb
Mass resolution
~ 2-3 MeV
K*(892)0 → K p
15000 central Pb-Pb
f (1020) → K+K-
Mass resolution
~ 1.2 MeV
CERN-ALICE-INT
Leptonic decay channel do not suffer the
multiple hadronic interactions in the final
state - good probes of the plasma phase –
see hep-ph/ v1
…Further studies for
Φ→e+e- identification
Eur. Phys. J. C 45, Number 3 , (2006)
Cracow July 2006
A. Mastroserio

16. Chemical freeze out vs thermal freeze out
lifetime is compatible with the lifetime
of the fireball (~15 fm/c)
Life times
Δ++ = 1.3 fm/c  too short?
Φ = 44 fm/c (small cross section for
scattering with non-strange
hadrons)
K*(892) = 4 fm/c suppressed
Λ(1520) = 13 fm/c suppressed
nucl-ex/ v1
Suppression due to the rescattering  Δτ > 5 fm/c
Cracow July 2006
A. Mastroserio

17. Hadronization : mass or a quark content effect ?
Eur.Phys.J.C43: ,2005
Phys.Rev.Lett. 91, (2003)
Φ(1020) spectrum can be described by the same parameters as the protons (->relevance of the mass!)
Φ(1020) meson is suppressed with respect to protons as pions (-> relevance of quark content!)
Cracow July 2006
A. Mastroserio

18. ALICE is getting ready to give answers.
Conclusions
At LHC a new regime should be found and this will affect particle production.
Among the questions :
What about chemical equilibrum scenario at LHC?
What about the hydro limit at LHC ?
ALICE can reach succesfully the intermediate pt region (2-5 GeV/c)
What is the interplay between soft and hard processes ?
ALICE is getting ready to give answers.
See you in September 2008 !
Cracow July 2006
A. Mastroserio

19. The End
Cracow July 2006
A. Mastroserio

20. arXiv:hep-ph/
Cracow July 2006
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21. Particle abundances
…many models but a common hypotesis : Grand Canonical ensemble
nucl-th/ v1
Cracow July 2006
A. Mastroserio