Fluctuations and correlations in nucleus–nucleus collisions within transport models

Abstract

Abstract. Particle number fluctuations and correlations in nucleus-nucleus collisions at SPS and RHIC energies are studied within microscopic transport approaches. In this review we focus on the Hadron-String-Dynamics (HSD) and Ultra-relativisticQuantum-Molecular-Dynamics (UrQMD) models The obtained results are compared with the available experimental data as well as with the statistical models and the model of independent sources. In particular the role of the experimental centrality selection and acceptance is discussed in detail for a variety of experimental fluctuations and correlation observables with the aim to extract information on the critical point in the (T, µ B ) plane of strongly interacting matter.

Strongly intensive measures for multiplicity fluctuations

“…In this case, the hadron multiplicities n K and n π , scaled variances ω * K and ω * π , and correlation ρ * Kπ should be found as the corresponding quantities at fixed volume (i.e., at fixed N part ). The transport model calculations demonstrate [3] that the e-by-e fluctuations of the number of participants become negligible in the sample of most central A+A collisions, e.g., one finds ω part ≪ 1 in Pb+Pb (or Au+Au) collisions with impact parameter equal to zero, b = 0. Therefore, one may define the parameters of MIS as:…”

“…For example, the fluctuations of nucleon participants strongly contribute to the scaled variances of charged particles [5], pions and kaons [3]. To avoid these unnecessary contributions one needs to make a very rigid centrality selection.…”

Considerations concerning the fluctuations of the ratios of two observables

“…Yet some examples can be quoted. Here we want to refer to a recent review paper [17], summarizing the PhD thesis of V. Konchakovski. In particular we mention Sections 6 and 8.…”

“…an event-by-event analysis, of high energy heavy ion collision data has been ongoing for several years [1][2][3][4][5][6][7][8][9][10][11][12][13][14]. Recently, this discussion was refreshed [15][16][17][18][19][20][21] in the view that (event-by-event) fluctuations of multiplicity, or of ratios of multiplicities, might serve as an observable to pin down the critical behavior of the collective, and potentially at some point of its evolution thermalized, system created when two heavy ions collide. Thus, the measurement of ratio fluctuations might make it possible to fix the values of temperature and baryon chemical potential of a critical point (see e.g.…”

System size dependence of the non-monotonous pion freeze-out volume excitation function

“…In fact, the combination of theoretically predicted signals and corresponding measurement data, has led to the emergence of a complete phase diagram of strongly interacting matter [1][2][3][4][5][6][7][8][9]. Unfortunately, uncertainties still exist and the present data does not seem to allow for firm conclusions, especially when it comes to the location and observation of the critical point 1 [6,7,10]. To reduce these uncertainties and to draw final conclusions about the location of the critical endpoint and the boundary of the first order transition, work into creating a careful beam-energy scan of these signals for the onset of deconfinement is currently ongoing, with further work planned for the future.…”