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Register a new userForward-backward correlations in nucleus-nucleus collisions: baseline contributions from geometrical fluctuations
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We discuss the effects of initial collision geometry and centrality bin definition on correlation and fluctuation observables in nucleus-nucleus collisions. We focus on the forward-backward correlation coefficient recently measured by the STAR Collaboration in Au+Au collisions at RHIC. Our study is carried out within two models: the Glauber Monte Carlo code with a `toy wounded nucleon model and the hadron-string dynamics (HSD) transport approach. We show that strong correlations can arise due to averaging over events in one centrality bin. We, furthermore, argue that a study of the dependence of correlations on the centrality bin definition as well as the bin size may distinguish between these `trivial correlations and correlations arising from `new physics.
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Particle number fluctuations and correlations in nucleus-nucleus collisions at SPS and RHIC energies are studied within the statistical hadron-resonance gas model in different statistical ensembles and in the Hadron-String-Dynamics (HSD) transport approach. Event-by-event fluctuations of the proton to pion and kaon to proton number ratios are calculated in the HSD model for the samples of most central collision events and compared with the available experimental data. The role of the experimental acceptance and centrality selection is discussed.
The number of particles detected in a nucleus-nucleus collision strongly depends on the impact parameter of the collision. Therefore, multiplicity fluctuations, as well as rapidity correlations of multiplicities, are dominated by impact parameter fluctuations. We present a method based on Bayesian inference which allows for a robust reconstruction of fluctuations and correlations at fixed impact parameter. We apply the method to ATLAS data on the distribution of charged multiplicity and transverse energy. We argue that multiplicity fluctuations are smaller at large rapidity than around central rapidity. We suggest simple, new analyses, in order to confirm this effect.
We present an universal treatment for a substantial nuclear suppression representing a common feature of all known reactions on nuclear targets (forward production of high-pT hadrons, production of direct photons, the Drell-Yan process, heavy flavor production, etc.). Such a suppression at large Feynman xF, corresponding to region of minimal light-cone momentum fraction variable x2 in nuclei, is tempting to interpret as a manifestation of coherence or the Color Glass Condensate. We demonstrate, however, that it is actually a simple consequence of energy conservation and takes place even at low energies, where no effects of coherence are possible. We analyze this common suppression mechanism for several processes performing model predictions in the light-cone dipole approach. Our calculations agree with data.
Event-by-event fluctuations of the kaon to pion number ratio in nucleus-nucleus collisions are studied within the statistical hadron-resonance gas model (SM) for different statistical ensembles and in the Hadron-String-Dynamics (HSD) transport approach. We find that the HSD model can qualitatively reproduce the measured excitation function for the $K/pi$ ratio fluctuations in central Au+Au (or Pb+Pb) collisions from low SPS up to top RHIC energies. Substantial differences in the HSD and SM results are found for the fluctuations and correlations of the kaon and pion numbers. These predictions impose a challenge for future experiments.
The partition function of nonequilibrium distribution which we recently obtained [arXiv:0802.0259] in the framework of the maximum isotropization model (MIM) is exploited to extract physical information from experimental data on the proton rapidity and transverse mass distributions. We propose to partition all interacting nucleons into ensembles in accordance with the number of collisions. We analyze experimental rapidity distribution and get the number of particles in every collision ensemble. We argue that even a large number of effective nucleon collisions cannot lead to thermalization of nucleon system; the thermal source which describes the proton distribution in central rapidity region arises as a result of fast thermalization of the parton degrees of freedom. The obtained number of nucleons which corresponds to the thermal contribution is treated as a ``nucleon power of the created quark-gluon plasma in a particular experiment.
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