Chemical fluctuations in high-energy nuclear collisions

Abstract

Fluctuations of the chemical composition of the hadronic system produced in nuclear collisions are discussed using the Φ−measure which has been earlier applied to study the transverse momentum fluctuations. The measure is expressed through the moments of the multiplicity distribution and then the properties of Φ are discussed within a few models of multiparticle production. A special attention is paid to the fluctuations in the equilibrium ideal quantum gas. The system of kaons and pions, which is particularly interesting from the experimental point of view, is discussed in detail. PACS: 25.75.+r, 24.60.Ky,24.60.-k Keywords: Relativistic heavy-ion collisions; Fluctuations; Thermal model There are many sorts of hadrons produced in high energy collisions. The ratios of multiplicities of particles of given species to the total particle number characterize the chemical composition of the collision final state. The composition is expected to reflect the collisions dynamics. Generation of the quark-gluon plasma in heavy-ion collisions was argued long ago [1] to enhance the strange particle production. While the significant strangeness yield enhancement has been experimentally observed in the central nucleus-nucleus collisions at CERN SPS (see the data compilation [2] and the recent review [3]), it is a matter of hot debate whether the observation can be indeed treated as a plasma signal. The strange hadron abundance is naturally described within the models assuming the plasma occurrence at the early collision stage (see e.g. [4,5]) but the models, which neglect such a possibility (see e.g. [6,7] or the review [8]), can be also tuned to agree with the experimental data. Thus, it would be desirable to go beyond the average particle numbers and see whether the strangeness enhancement in the central heavy-ion collisions is accompanied with the qualitative change of the strangeness yield fluctuations. The equilibrium quark-gluon plasma scenario is obviously expected to lead to the smaller fluctuations than the nonequilibrium cascade-like hadron models but the specific calculations are needed to quantify such a prediction. Anyhow, it seems to be really interesting to study the strangeness yield fluctuations on the event-by-event basis. However, we immediately face the difficulty how to quantitatively measure the fluctuations in the events of very different multiplicity. The problem appears to be of more general nature.There are several interesting proposals to use fluctuations as a potential source of valuable information on the collision dynamics. If the hadronic system produced in the collision is in the thermodynamical equilibrium, the temperature and multiplicity fluctuations have been argued to determine, respectively, the heat capacity [9,10] and compressibility [11] of the hadronic matter at freeze-out. An extensive discussion of the equilibrium fluctuations can be found in [12]. In the experimental realization of such ideas one has to disentangle however the 'dynamical' fluctuations of interest from the 'trivial' ge...

K/πFluctuations at Relativistic Energies

“…We present results for the collision centrality dependence of ν dyn,Kπ for Au+Au collisions at 62.4 and 200 GeV. To gain insight concerning the origins of these K/π fluctuations [12], we also present the collision centrality dependence of ν dyn,Kπ…”

Event-by-event analysis of baryon-strangeness correlations: Pinning down the critical temperature and volume of quark-gluon-plasma formation

“…Fluctuation probes might be more adequate for the exploration of heavy ion reactions, because the distributions of energy density or initial temperature, isospin and particle density have strong fluctuations from event to event [2,3,4]. On the theoretical side event-by-event fluctuations where suggested to study• kinetic and chemical equilibration in nuclear collisions [5,6,7,8,9,10,11,12,13,14],• the onset of the deconfinement phase [15,16,17,18,19,20,21] • the location of the tri-critical end-point of the QCD phase transition [22,23,24] or• the formation of exotic states, like DCCs [25].On the experimental side, progress has been made by many experiments to extract momentum and particle number ratio fluctuations from heavy ion reaction: Eby-E fluctuations are actively studied in the SPS energy regime (starting from 20 AGeV on) by the NA49 group [26,27,28,29,30,31,32,33,34] and the CERES collaboration [35,36,37,38]. At RHIC energies the PHENIX [39,40,41] and STAR [42,43,44] experiments are addressing the field of single event physics....…”

Event-by-event pseudo-rapidity fluctuations in high energy nucleus–nucleus interactions

“…So, measuring the fluctuations we can learn about effective degrees of freedom of the system and their interactions. Event-by-event fluctuations may provide us information about the heat capacity [15,[17][18][19], possible equilibration of the system [20][21][22][23][24][25][26][27][28] or about the phase transition [19,29]. The NA49 Collaboration has presented a prototypical event-by-event analysis of fluctuations in central Pb + Pb collisions at 158 GeV/n [30,31].…”

“…For this purpose we have chosen heavy (AgBr) and light (CNO) groups of targets present in the nuclear emulsion. The projectiles used for this study include 16 O (at an incident momentum of 4.5 A GeV/c), 22 Ne (at an incident momentum of 4.1 A GeV/c), 28 Si (at an incident momentum of 4.5 A GeV/c) and 32 S (at an incident momentum of 4.5 A GeV/c). We have compared our experimental results with the results obtained from the analysis of the data generated by the modified FRITIOF model.…”