Strangeness Dynamics and Transverse Pressure in HIC

Marcus Bleicher, CCAST- Workshop 2004

Strangeness Dynamics andTransverse Pressure in HIC

Marcus Bleicher

Institut für Theoretische Physik

Goethe Universität Frankfurt

Germany


Contents

• Chemical equilibrium stage:

The ‘horn’, the ‘step’

• Kinetic decoupling stage:

Resonance (non-) suppression

• Summary


The tool: UrQMD

• Non-equilibrium transport model

• Hadrons and resonances

• String excitation and fragmentation

• Cross sections are parametrizedvia AQM or calculated by detailed balance

• Generates full space-time dynamics of hadrons and strings


Part I: Evidence for the tri-critical point?

B

• 1st Order phase transition at high

•No P.T. at low

• Search for irregularities around Ebeam = 10-40 GeV:

Flow, strangeness, E-by-E

B


Collision Spectrum

• Initial stage scattering before 1.5 fm/c:Baryon stopping, meson production, may be QGP formation

• Thermalization stage (1.5 – 6 fm/c):Cooking QCD matter

• Hadronic freeze-out stage (6 – 10 fm/c):Elastic and pseudo-elastic hadron scatterings

Pb+Pb @ 160 AGeV


Excitation functions

4 and midrapidity abundancies: OK

Energy dependence: OK

Hadron-string models work well


Excitation functions: ratios

• ‘Horn’ in the ratio not reproduced

• well reproduced

• relative strange baryon enhancement reproduced

/K

/K

/


Ratio excitation functions: Summary

• Yields are well reproduced

• Most ratios can be understood in transport models

• Model K/pi ratio does not reproduce the strong peak observed in data


Proton-Proton

• PP works well

• pQCD needed at RHIC

• PYTHIA included in

UrQMD 2.x and HSD


Proton-Nucleus

• pA is well under control

• What about AA?


Transverse mass spectra

• Standard UrQMD and HSD underestimate the data

• Additional resonances from 2-3 GeV do improve the description (UrQMD 2.1)


Inverse slope systematics

• Standard transport models fail

• What is missing?

• Maybe high mass resonanceslike in UrQMD 2.1

• Maybe QGP pressure…


Phase diagram• QGP might be reachedalready at 11 GeV!

• Tricritical point at 10-40 GeV

• No phase transition at RHIC

• Necessary to explore low energy region to study the phase transition


Flow excitation functions: Summary

• Standard transport models can not describe radial flow

• Inclusion of high mass resonances leads to additional flow

• Assumption of an early QGP phase might create additional flow


Part II: Probing the late stage of the reaction: Resonances

• Is there a (long living) hadronic rescattering stage at SPS and RHIC?

• Lifetime of the hadronic stage is measured by resonance absorption/re-feeding

• Use different resonances to explore this stage: e.g. mesons: baryons:

• Are resonances dissolved in matter?

,,),892( 0* fK

)1385(),1520(),1232(


Hadronic vs leptonic channel

ρ0

ρ0

+

+

-

-

π- π+

ρ0

-

+

ρ0 ρ0π-

π+

π+

π-

Hot and dense medium

Au+Au

Particle yields

Particle spectra

time

pK

pK


Statistical model fitting

• Particle ratios well reproduced

• Resonance ratios not reproduced(Braun-Munzinger, QM 2004)

• too low

• K*/K too high

Braun-Munzinger et al.

p/


Yields and scaling in AABaryon resonances: Meson resonances:

All ratios a smooth, no sudden resonance disappearance


Baryon resonances at RHICAll decaying resonances Finally observed resonances

Signal loss due to rescattering of daughters


Meson resonances at RHICAll decaying resonances Finally observed resonances

Note that yields information about all decays (l.h.s), while yields information about r.h.s.

ee


What about the centrality dependence?

• Where is the suppression?

• K*/K deceases!

• stays constant!p/


Data vs. models Thermal model

[1]:T = 177 MeVB = 29 MeV

[1] P. Braun-Munzinger et.al., PLB 518(2001) 41 D.Magestro, private communication[2] Marcus Bleicher and Jörg Aichelin Phys. Lett. B530 (2002) 81. M. Bleicher and Horst Stöcker .Phys.G30 (2004) 111.

UrQMD [2]

Life time [fm/c] :(1020) = 40 (1520) = 13 K(892) = 4 ++ = 1.7

Rescattering and refeeding are needed rather long living hadron stage


How can we understand these differences?

Tch freeze-out

Tkin freeze-out

• Strong decrease in kinetic freeze-out temperature from peripheral to central

• Kinetic freeze-out as low as 80 - 90 MeV

• Consequences for resonance re-feeding


Estimate of re-feeding prob.

• can re-created until end of the reaction• re-creation is only possible near chemical freeze-out

Tmms

2

3221

Estimate of available energy for re-feeding at different reaction stages


Decay time analysis

• In the model rho mesons are not re-created after T=120 MeV.Check with mass shift.Check hadronic vs leptonicCheck centrality dependence

• Deltas can be re-created until T=80-90 MeV.No centrality dependence


Simple mass shift

• Breit-Wiegner distr. for the rho mass

• Fold with thermal distribution

Temperature dependent mass

• For T=150-100 MeV the mass is shifted by 10-20 MeV downwards


Rho mass shift predictions

• Mass drops towards central reactions

• Mass drops towards low pt


Summary

• K/ shows a peak around 20 GeV:-Peak can not be reproduced by transport models-Maybe sign of tri-critical point

• Observed transverse flow is large:-Strength of flow can not be described in standard transport models-May be sign of early QGP formation-Maybe sign of high mass resonances

• Resonances have been observed-Apparently no resonance dissolution-Statistical models fail to describe data-Strong re-feeding and absorption effects


Thanks

• Elena Bratkovskaya

• Henning Weber

• Christina Markert

• Patricia Fachini

• Manuel Reiter

Documents

Strangeness Dynamics and Transverse Pressure in HIC