FIAS Scientific Report 2011 - Frankfurt Institute for Advanced Studies ...

Research Projects of Prof. Reinhard Stock
Project 1: Investigation of hadronic fluctuations in relativistic nuclear collisions
Collaborators: Tim Schuster 1 , Thosten Kollegger 1 , Reinhard Stock 1 with the CERN NA49 Collaboration
1 Frankfurt Institute for Advanced Studies
We have investigated the event-by-event-fluctuation of hadrons produced in central collisions of Pb+Pb at five
energies at the CERN SPS. More precisely: the ratios of Kaon to Pion and Kaon to Proton eventwise fluctuation,
which are predicted to be sensitive to the existence of a “critical point” of QCD that was recently conjectured
in lattice QCD calculations at finite baryochemical potential. The CERN SPS energy interval covered by our
investigation coincides with the energy domain of critical point occurence, envisioned in the Lattice calculations.
The analyzed data were gathered by our NA49 experiment in 2002. We indeed observe small amplitude
fluctuations of these hadron ratios. For the Kaon to Pion event by event fluctuation this fluctuation could be
traced to finite number statistics effects inherent in the studied observable, However, the Kaon to Proton ratio
fluctuation exhibits a steep rise toward the low energy end of the studied interval, for which no such “trivial”
explanation could be found. The theoretical discussion is still going on.
Project 2: Hadronization, and the hadronic freeze-out curve in relativistic nuclear collisions
Collaborators: Marcus Bleicher 1 , Thosten Kollegger 1 , Tim Schuster 1 , Reinhard Stock 1 , Francesco Becattini 2 ,
Michael Mitrovski 3
1 Frankfurt Institute for Advanced Studies, 2 University of Florence, Italy, 3 Brookhaven National Laboratory, NY, USA
One of the outstanding open questions of relativistic nuclear collision research consists of the observation of
a global statistical equilibrium among all produced hadron/resonance species. This “hadro-chemical” equilibrium
is captured within the Grand Canonical Gibbs ensemble. Its principal parameters, temperature T
and baryochemical potential µB, as determined at a multitude of different incident energies, establish the
so-called “hadronic freeze-out curve”. Its most spectacular aspect: it merges, closely, with the confinement/deconfinement
line (between partons and hadrons) predicted by recent Lattice QCD, at temperature of
about 165 MeV. Have we thus located, semi-experimentally, this main quantity of QCD matter: the “critical
temperature”? We investigate the question of how such a hadronic species equilibrium can survive the final
expansion cascade of hadrons that is characteristic for a nucleus-nucleus-collision. To this end we employ the
microscopic hadron transport model UrQMD that was developed in the Frankfurt Theory Group. We propose
certain corrections to the freeze-out curve but confirm its relevance indicated above.
Figure: Sketch of the QCD phase diagram, showing the
parton-hadron coexistence boundary line, inferred from
lattice QCD at low µB, and from chiral restoration theory
at high µB and, second, the hadronic freeze-out curve of
the statistical model.
Related publications in 2011:
1) T. Schuster and NA49 Collaboration, New results on event-by-event ratio fluctuations in Pb+Pb collisions
at CERN SPS energies, J. Phys. G 38, 124096 (2011)
2) T. Anticic et al. (NA49 Collaboration), Energy dependence of kaon-to-proton ratio fluctuations in central
Pb+Pb collisions from √ sNN = 6.3 to 17.3 GeV, Phys. Rev. C 83, 061902 (2011)
3) F. Becattini, M. Bleicher, Th. Kollegger, M. Mitrovski, T. Schuster and R. Stock, Validity of the hadronic
freeze-out curve, J. Phys. G 38, 124075 (2011)
4) F. Becattini, M. Bleicher, Th. Kollegger, M. Mitrovski, T. Schuster and R. Stock, Hadronization and hadronic
freeze-out in relativistic nuclear collisions, submitted to Phys. Rev. C
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