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Marcus Bleicher, ICPAQGP 2005

Strangeness and Pressure in HIC:

Do we Observe the Phase Transition at 30 AGeV?

Marcus Bleicher

Institut für Theoretische Physik

Goethe Universität Frankfurt

Germany

Marcus Bleicher, ICPAQGP 2005

For further reading…


E. L. Bratkovskaya et al, ``Strangeness dynamics and
transverse pressure in relativistic nucleus nucleus
collisions,''

Phys. Rev. C 69, 054907 (2004)


E. L. Bratkovskaya et al. , ``Strangeness dynamics in
relativistic nucleus nucleus collisions,''

Prog. Part. Nucl. Phys. 53, 225 (2004)

Marcus Bleicher, ICPAQGP 2005

Contents


Kinetic observables:


longitudinal pressure (Landau or Bjorken?)


transverse pressure (radial flow & the ‘step’)



Chemical observables:


Strangeness enhancement & the ‘horn’,


Fluctuations



Conclusions

Marcus Bleicher, ICPAQGP 2005

The tool: UrQMD
v2.2



Non
-
equilibrium transport model



Hadrons and resonances



String excitation and fragmentation



Cross sections are parameterized



via AQM or calculated by detailed balance



Hard scattering at high energies




Generates full space
-
time dynamics of hadrons
and strings

Marcus Bleicher, ICPAQGP 2005



1
st

Order phase transition at
high



No P.T. at low




Search for irregularities


around Ebeam = 10
-
40 GeV:


Flow, strangeness, E
-
by
-
E

Where do we expect

interesting effects?

B

B

Plot adapted from L. Bravina

Marcus Bleicher, ICPAQGP 2005

Longitudinal pressure:

Is it Landau or Bjorken?



Bjorken
: Boost invariant flow


(transparency of projectile and


target)


Rapidity density plateau!



Landau
: instant thermalization


followed by expansion


Rapidity density is Gaussian!


(with )

p
m
s
2
/
ln
2


Marcus Bleicher, ICPAQGP 2005

Particle dependent rapidity widths



Rapidity widths of all particles are different

Au+Au (Pb+Pb), central

particles

anti
-
particles

Marcus Bleicher, ICPAQGP 2005

Landau or Bjorken?

None of both!


Rapidity width decreases


with mass


No universal scaling of


the rapidity width


Maximal |y
-
ycm| of the


‘Bjorken plateau’ is 0.75 units

Marcus Bleicher, ICPAQGP 2005

Transverse Pressure:

Proton
-
Proton



PP works well




pQCD needed at RHIC




PYTHIA included in


UrQMD 2.x and HSD

Marcus Bleicher, ICPAQGP 2005

Proton
-
Nucleus



pA is well under control



CC and SiSi are also under control



What about AA?

Marcus Bleicher, ICPAQGP 2005

Transverse
mass spectra



Standard UrQMD and HSD


underestimate the data




Additional resonances of 2
-
3 GeV


mass may improve the description

Marcus Bleicher, ICPAQGP 2005

Transverse mass spectra:

Problems and Solutions

Standard transport approaches do not yield enough transverse flow!




Test two possible solutions:



Cronin effect (medium PT enhancement)



Meson
-
baryon scattering with


might be more resonance like than string like




GeV
s
GeV
MB
3
2


Marcus Bleicher, ICPAQGP 2005

Inverse slope systematics




High mass resonances improve


the description at low and high


energies




Cronin effect at high energies


improves RHIC results




How can we test those scenarios?

Marcus Bleicher, ICPAQGP 2005

Hints from elliptic flow



High mass resonances


can not explain


scaled v2 above 40 AGeV




Data shows saturation of scaled v2




Strong hint for large pressure


and short mean free paths in the


early stage of the reaction


already from 30 AGeV on !

Data for h
-

Marcus Bleicher, ICPAQGP 2005

Multiplicities: pp




PP works nicely in


both models



Pythia used for hard


scatterings above 50 GeV

Marcus Bleicher, ICPAQGP 2005

AA Excitation
functions




4 and mid
-
y abundancies: OK




Energy dependence: OK




Hadron
-
string models work well


Marcus Bleicher, ICPAQGP 2005

Check for

strangeness enhancement


Strangeness enhancement is


strongest at low energies



Apparent Lambda enhancement


from stopping



Disappearance of


canonical suppression

Marcus Bleicher, ICPAQGP 2005

Excitation
functions: ratios



‘Horn’ in the ratio


not reproduced




well reproduced




relative strange baryon


enhancement reproduced






/
K



/
K

/

Marcus Bleicher, ICPAQGP 2005


K/


fluctuations increase towards lower beam energy


Significant enhancement over hadronic cascade model


p/


fluctuations are negative


indicates a strong contribution from resonance decays

NA49 Preliminary

NA49 Preliminary

Fluctuation studies

Taken from Christoph Roland

Marcus Bleicher, ICPAQGP 2005

Conclusion


Scaled elliptic flow


saturates


K/pi ratio enhanced


Large dynamical


fluctuations

Most interesting energy
range, because:

Marcus Bleicher, ICPAQGP 2005

Thanks to



Elena Bratkovskaya



Henning Weber



Manuel Reiter



Sascha Vogel



Xianglei Zhu



Hannah Petersen



Diana Schumacher