Transverse-mass effective temperature in heavy-ion collisions from AGS to SPS

Abstract

Abstract. Transverse-mass spectra in Au + Au and Pb + Pb collisions in the incident energy range from 2A to 160A GeV are analyzed within the model of 3-fluid dynamics (3FD). It is shown that the dynamical description of freeze-out, accepted in this model, naturally explains the incident-energy behavior of the inverse-slope parameters of these spectra observed in experiment. The simultaneous reproduction of the inverse slopes of all considered particles (p, π and K) suggests that these particles belong to the same hydrodynamic flow at the instant of their freeze-out. PACS. 24.10.Nz Hydrodynamic models -25.75.-q Relativistic heavy-ion collisionsExperimental data on transverse-mass spectra of kaons produced in central Au + Au [1] or Pb + Pb [2] collisions reveal peculiar dependence on the incident energy. The inverse-slope parameter (the so-called effective temperature T ) of these spectra at mid rapidity increases with incident energy in the energy domain of BNL Alternating Gradient Synchrotron (AGS) and then saturates at the energies of CERN Super Proton Synchrotron (SPS). In refs. [3,4] it was assumed that this saturation is associated with the deconfinement phase transition. This assumption was indirectly confirmed by the fact that microscopic transport models, based on hadronic degrees of freedom, failed to reproduce the observed behavior of the kaon inverse slope [5,6]. Hydrodynamic simulations of ref. [7] succeeded to describe this behavior. However, in order to reproduce it these hydrodynamic simulations required incident-energy dependence of the freeze-out temperature which almost repeated the shape of the corresponding kaon effective temperature. This happened even in spite of using the equation of state (EoS) involving the phase transition into quark-gluon plasma (QGP). This way, the puzzle of kaon effective temperatures was just translated into a puzzle of freeze-out temperatures. Moreover, the results of ref. [7] imply that the peculiar incident-energy dependence of the kaon effective temperature may be associated with the dynamics of freeze-out. In this paper we would like to present calculations of effective temperatures within the 3FD model [8][9][10][11] which is suitable for simulating heavy-ion collisions in the range from AGS to SPS energies. We perform our simulations [8,10,11] with a simple, hadronic EoS [12], which involves only a density-dependent mean field providing saturation of cold nuclear matter at normal nuclear density and with the proper binding energy. The 3FD model with the intermediate EoS turned out to be able to reasonably reproduce a great body of experimental data [8] in a wide energy range from AGS to SPS. In particular, transversemass spectra of protons were reproduced. This was done with the unique set of model parameters summarized in ref. [8]. Problems were met only in the description of the transverse flow [10]. The directed flow requires a softer EoS at top AGS and SPS energies (in particular, this desired softening may signal the occurrence of the phase transition int...

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“…The step-like structure in the energy dependence of the inverse slope parameter of the transverse mass spectra was obtained within the hydrodynamical model by introduction of a rapid change of the freeze-out conditions at low SPS energies [69]. However, this assumption does not explain the increase of the inverse slope parameter suggested by the RHIC results.…”

Alternative scenarios of relativistic heavy-ion collisions. III. Transverse momentum spectra

“…In Refs. [20,21] it was shown that dynamical description of the freeze-out [22,23], accepted in the 3FD model, naturally explains the step-like behavior of inverse-slope parameters even without any deconfinement transition in the EoS. This freeze-out dynamics, effectively resulting in a pattern similar to that of the dynamic liquid-gas transition, differs from conventionally used freeze-out schemes.…”

The ⟨mT⟩ excitation function: freeze-out and equation of state dependence

“…If there is a phase transition the mean transverse mass excitation function should show a flattening because of the mixed phase region where the temperature of the system stays constant. However, recently, it has been suggested in a 3-fluid model with a hadron gas equation of state that the characteristic steplike-behaviour of the mean transverse mass in dependence of the incident beam energy might be a relic of the freeze-out treatment in a hydrodynamic calculation and may not be a unique sign of the transition to the QGP [11].…”