Understanding the Properties of Chemical Freeze-Out

Understanding the Properties of Chemical Freeze-Out

Christoph BlumeUniversity of Frankfurt

NICA/JINR-FAIR Workshop“Matter at highest baryon densities in the laboratory and in space”April 2 – 4, 2012FIAS

Outline

Christoph Blume NICA/JINR-FAIR Workshop, FIAS Frankfurt 2

The QCD phase diagram

Chemical freeze-out Results from statistical model fitsRole of phase boundariesDeconfinement phase transition (low μB)Quarkyonic phase transition (high μB)

System size dependenceFreeze-out parameters vs. Npart

Indications for long lived hadronic phase ?Current experimental situation

Early decouplingRare particles with low hadronic cross sectionsMeasurements at low energies

The QCD Phase Diagram

Christoph Blume NICA/JINR-FAIR Workshop, FIAS Frankfurt

Broad experimental programPast: SPS (and AGS)Present: RHIC and SPSFuture: FAIR and NICA

What can be learned at low energies?

New exotic phases?Quarkyonic matter

Can phase boundaries be mapped by hadron yields ?

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RHIC

SPSFAIR

NICA

The QCD Phase DiagramExperimental Access


Control parameter: √sNN Allows to scan different regions of phase diagram

System freezes out at different positions along freeze-out curve

Trajectory might cross critical area

Variation of system sizeProgram ofNA61@SPS

H. Stöcker, E.L. Bratkovskaya, M. Bleicher, S. Soff, and X. Zhu, JPG31, S929 (2005)

3-fluid hydro

Y.B. Ivanov, V.N. Russkikh,V.D. Tonnev, PRC73, 044904 (2006)

Chemical Freeze-Out Statistical Model Fits

A. Andronic et al. Phys. Lett. B673, 142 (2009).

Assumption: Multiplicities are determined by statistical weights (chemical equilibrium)

Grand-canonical partition function:

Parameters:V, T, B, (s)

Allows in general excellent fits to measured multiplicities


Chemical Freeze-OutEnergy Dependence


L. Kumar, QM11

Chemical Freeze-OutWhere does Equilibration Happen?


Dynamical equilibration?QGP phase at high energies

How about lower energies?Hadron gas phase sufficient?

Other mechanisms?Phase space dominance?

Hadronization always leads to statistical equilibrium?

ExperimentAny evidences for long lived hadron gas phase after?

Systematic studies at low energies

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U. Heinz, Nucl. Phys. A661, 140 (1999).R. Stock, Phys. Lett. B456, 277 (1999).M. Gazdzicki and M. Gorenstein, Acta Phys. Polon. B30, 2705 (1999). J. Hormuzdiar et al., Int. J. Mod. Phys. E12, 649 (2003).V. Koch, Nucl. Phys. A715, 108 (2003).F. Becattini, arXiv:0901.3643.

Dynamical decouplingShould happen over certain time span and temperature range

Theoretical approachesHydro + hadronic transport (e.g. UrQMD)

Hydro + extended freeze-out condition

System size dependenceInterplay mean free path and system size

⇒ decrease of T with increasing size (?)

Hadronic cross sectionRare particles (e.g. Ω) with low cross section earlier freeze-out⇒

Chemical Freeze-OutContinuous Freeze-Out


H. van Hecke, H. Sorge, N. Xu, PRL81, 5764 (1998).

J. Knoll, NPA821, 235 (2009)

S. Bass and A. Dumitru, PRC61, 064909 (2000).

H. Petersen et al., PRC78, 044901 (2008)

Hydro-phase

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Chemical Freeze-OutRole of Phase Boundaries


A. Andronic et al., Nucl. Phys. A837, 65 (2010).

EquilibrationDriven by proximity of freeze-out to phase boundary

Deconfinement transition at low μB

How about high μB?

Quarkyonic matterProvides another phase boundary

Would determine position of chemical freeze-out points for higher μB

Consequence: no need for an extended hadronic phase

But: conjecture might be wrong ...

L. McLarren and R.D. Pisarski,Nucl. Phys. A796, 83 (2007).

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P. Braun-Munzinger, J. Stachel, and C. Wetterich,PLB596, 61 (2004).

S. Floerchinger and C. WettericharXiv:1202.1671

System Size DependenceKinetic Freeze-Out


Tkin decreases withincreasing system sizeDynamical decoupling: interplay mean free path and size of system

Hydro reproduces trendFreeze-out condition:

U. Heinz and G. Kestin, PoS(CPOD2006), 038

STAR: PRC83, 034910 (2011)

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System Size Dependence Chemical Freeze-Out at small μB


Different behaviour of Tkin and Tch

Tkin: clear centrality dependenceTch: no centrality dependence

Difficult to reconcile with hydro for normal HGRequires scattering rate proportional to Tn with n ≥ 20

At phase boundary: τΩ ∝ T-60P. Braun-Munzinger, J. Stachel, and C. Wetterich,PLB596, 61 (2004).

U. Heinz and G. Kestin, PoS(CPOD2006), 038

STAR: PRC83, 034910 (2011)

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System Size Dependence Chemical Freeze-Out at higher μB (SPS)


Tch changes with system sizeCentral events of different A

μB independent on system size

Different freeze-out behaviourat different energies ? Careful systematic study needed

Geometric effects (“core-corona”) might be important

F. Becattini et al.,PRC73, 044905 (2005)

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√sNN Tch(p+p) Tch(Au+Au/Pb+Pb)

17.3 GeV 181.5 MeV 157.5 MeV

200 GeV 170.1 MeV 168.5 MeV

F. Becattini et al., PRC73, 044905 (2006),EPJC66, 377 (2010).

System Size Dependence Geometrical Effects (“Core-Corona”)

Corona:Elementary N+N collisions

Core:Dense fireball

System size dependencies determined by ratio core/coronaf (NW) = fraction of nucleons that scatter more than once (Glauber model)

P. Bozek, Acta Phys. Polon. B36, 3071 (2005).F. Becattini and J. Manninen, J. Phys. G35, 104013 (2008)J. Aichelin and K. Werner, Phys. Rev. C79, 064907 (2009)

K. Werner


158A GeV

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C.B. J.Phys.Conf.Ser. 230, 012003 (2010)

System Size Dependence Chemical Freeze-Out at higher μB (RHIC)


Recent STAR dataBeam Energy Scan program

Also significant system size dependence observed

BUT:Tch increases with system size!Opposite trend than observed by NA49

Contrary to naive expectation ? ⟶ Contribution by D. Blaschke

pp data?

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L. Kumar, CPOD11

Early Decoupling Rare Particles



-/

= 1.5 (+ + -)

NA49: PRL94, 192301 (2005).

Particles with low hadronic cross sectionE.g. multi-strange baryons (Ω)

Might decouple earlier if there was a long lived hadronic phase

Deviations from statistical model fits?

No evidence seen at RHIC and SPSMore precise data on multi-strange particles (low energies) will be helpful

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Early DecouplingStrange Baryon to Pion Ratios


X. Zhu, SQM11

Good agreement between SPS and RHICClose to statistical model curve

Low energy data scarce FAIR and NICA⟶

Λ/π

Λ/π_

Ξ-/π

Ξ+/π_

Early Decoupling Statistical Model Fits at Low √sNN


HADES data: Ar+KCl at √sNN = 2.61 GeVGood agreement with statistical model fit

Except: Ξ-! Off by factor 25Sub-threshold production

Also deviations for η (TAPS)

Freeze-out parameter fit into known systematics

How about system size dependence?

M. Lorenz, CPOD11

Early Decoupling Hydro + Cascade Predictions


No real effect for ΩsNo effect expected if Ω enters hadronic phase with equilibrium yield

Strong effect on antibaryonsCascade removes p, Ξ+

⇒ Reduction from chemical equilibrium

Caveat: multi-meson fusion processes not included

First implementation in transport: HSD(anti-protons)

_ _

R. Rapp and E. Shuyak, PRL86, 2980 (2001).

S. Bass and A. Dumitru,PRC61, 064909 (2000).

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Early Decoupling Antibaryons


Effect on statistical model fitsFits with and w/o antibaryonsStudies ongoing

System size dependence of pTends to decrease towards central

Indication for absorption in hadronic phase or just evolution of baryon number transfer ?

Right now inconclusiveHigh precision data required

R. Stock et al., arXiv:0911.5705

STAR: PRC83, 034910 (2011)

NA49: PRC83, 014901 (2011)

_

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Early Decoupling Antibaryons


Antibaryons removed by hadron gas phase ?Simulation with hydro model + hadronic afterburner (UrQMD)

Afterburner removes antibaryons (p, Λ, Ξ) Reduction from chemical ⇒equilibrium value

Caveat: multi-meson fusion processes not simulated

No final conclusion from data yetHigh precision data needed

R. Stock et al.,arXiv:0911.5705

H. Petersen et al., PRC78, 044901 (2008)

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Early Decoupling Particle Ratios at LHC


M. Floris, QM11

K-/π- M. Floris, QM11

p/π- and K-/π- ratiosp/π-(LHC) = K-/π-(BRAHMS+PHENIX) (STAR: not feed down corrected!)

K-/π-(LHC) = K-/π-(RHIC)

_p/π-_

_

Statistical model predictions

Note: fit to larger set of particles (π, K, p, Λ, Ξ, Ω) results in expected Tch, problem only with p, p

_

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Conclusions


Freeze-out properties at low energiesIndications for presence of phase boundary to quarkyonic matter?

⟶ drives particle yields to equilibrium

If yes, no need for extended hadron gas phase

Indicators:No system size dependence of freeze-out parameterNo modification of rare particle yields (Ω, antiprotons)

Experimental situation unclearNo system size dependence at low μB

Results at high μB not in agreement (NA49, STAR)Data on rare particles scarce at low energies

Many opportunities for experiments at NICA and FAIR !

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Backup


Chemical Freeze-Out System Size Dependence: Data


LHCALICE: Pb—Pb at √sNN = 2.76 TeV

RHICSTAR: beam energy scan

SPSNA61, NA49

L. Kumar, QM11

p/π-_ K-/π-

M. Floris, QM11

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X. Zhu, SQM11

IntroductionThe QCD Phase Diagram


Variation of √sNN → Different regions of phase diagram

→ Map of chemical and kinetic freeze-out points

Limit μB → 0:RHIC experiments

New: ALICE (LHC)

High μB:SPS + AGS

New: STAR Beam Energy Scan (BES) NA61 (SPS)

Future: CBM (FAIR) + MPD (NICA)

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Kinetic Freeze-OutEarly Decoupling


STAR: PRL92, 182301 (2004).

Particles with low hadronic cross sectionE.g. multi-strange baryons (Ξ, Ω)

Should decouple earlier

Less affected by transverse expansion of hadronic phase

Effect visible in BW fits at RHIC and LHCHigher Tkin and lower 〈 βT 〉 for Ξ and Ω than for lighter hadrons (π, K, p)

NA49: PRL94, 192301 (2005).

(A) Tkin = 90 MeV 〈 βT 〉 = 0.5(B) Tkin = 170 MeV 〈 βT 〉 = 0.2

N. Xu and M. Kaneta, NPA698, 306 (2002).

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Energy Dependence/π- and /π-Ratios

NA49 dataPhys. Rev. C78, 034918 (2008) Transport models

OK for

Too low for Statistical models

Generally good description at allenergies

|y| < 0.4

|y| < 0.5

SHM(B): A. Andronic et al. Nucl. Phys. A 772, 167 (2006).UrQMD: M. Bleicher et al., J. Phys. G 25, 1856 (1999) and private communicationHSD: E. Bratkovskaya et al., Phys. Rev. C69, 054907 (2004)

/

/−

-/ +/− = 1.5 (+ + -)


Energy Dependenceϕ Meson: Total Yields


At SPS yields not fullydescribed by models

UrQMD1.3 underestimates ϕ/π-ratiobut good description of ϕ yieldat lower energies

Statistical model (γs = 1) above ϕ/π-ratioHGM: P. Braun-Munzinger et al., Nucl. Phys. A 687, 902 (2002).

NA49 dataPhys. Rev. C78, 044907 (2008)

Chemical Freeze-OutLimit μB → 0 and LHC Expectation


Freeze-out points coincide with phase boundary at RHIC and top SPS energiesChiral phase transition from LQCD

P. Braun-Munzinger and J. Stachel,arXiv:1101.3167

O. Kaczmarek et al., PRD83, 014504 (2011)

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Chemical Freeze-Out Energy Dependence of Freeze-Out Parameters



Parameterizations of Tch and μB as function of √sNN

Freeze-out curveJ. Cleymans et al., PRC73, 034905 (2006)

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Understanding the Properties of Chemical Freeze-Out

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