Neutral pions and η mesons as probes of the hadronic fireball in nucleus-nucleus collisions around1AGeV

Abstract

Chemical and thermal freeze-out of the hadronic fireball formed in symmetric collisions of light, intermediate-mass, and heavy nuclei at beam energies between 0.8A GeV and 2.0A GeV are discussed in terms of an equilibrated, isospin-symmetric ideal hadron gas with grand-canonical baryon-number conservation. For each collision system the baryochemical potential µ B and the chemical freeze-out temperature T c are deduced from the inclusive π 0 and η yields which are augmented by interpolated data on deuteron production.With increasing beam energy µ B drops from 800 MeV to 650 MeV, while T c rises from 55 MeV to 90 MeV. For given beam energy µ B grows with system size, whereas T c remains constant. The centrality dependence of the freezeout parameters is weak as exemplified by the system Au + Au at 0.8A GeV.For the highest beam energies the fraction of nucleons excited to resonance states reaches freeze-out values of nearly 15%, suggesting resonance densities close to normal nuclear density at maximum compression. In contrast to the particle yields, which convey the status at chemical freeze-out, the shapes of the related transverse-mass spectra do reflect thermal freeze-out. The observed thermal freeze-out temperatures T th are equal to or slightly lower than 1 T c , indicative of nearly simultaneous chemical and thermal freeze-out.PACS numbers: 24.10. Pa, 25.75.Dw

Comparison of chemical freeze-out criteria in heavy-ion collisions

“…From the lowest at the GSI Schwerionen synchrotron (SIS) up to the highest energies at the BNL Relativistic Heavy Ion Collider (RHIC), all results on particle multiplicities are consistent with the assumption of chemical equilibrium in the final-state fireball produced after heavy-ion impact [1]. The particle yields are found to be described, with remarkable precision, by a thermal-statistical model that assumes approximate chemical equilibrium [1][2][3][4][5][6][7][8][9][10][11][12][13][14]17]. For a given collision energy, the thermal-statistical model with only two parameters, the temperature (T ) and baryon chemical potential (µ B ), provides a very systematic description of particle yields.…”

“…In order to have a consistent presentation, we have made some choices which are biased by our own experience: the code used in this paper is available for inspection [35]. Our analysis relies on the most recent results obtained in statistical-thermal model fits to Au + Au and Pb + Pb systems, performed by numerous groups over a wide range of energies [5][6][7][8][9][10][11][12][13][14]17]. These results are summarized in Table I and are also included in Fig.…”

Hadron production in central nucleus–nucleus collisions at chemical freeze-out

“…The following hadrons are included in the calculations: i) mesons: non-strange (37), strange (28), charm (15), bottom (16); ii) baryons: non-strange (30), strange (33), charm (10); iii) "composites" (nuclei up to 4 He and K − -clusters [28], 18). The corresponding anti-particles are of course also included.…”

“…The thermal model was initially used for the AGS and SPS data [2] and was subsequently employed to describe data at SIS [3,4], SPS [5] and more recently at RHIC [6,7,8,9]. An analysis of the energy dependence of the thermal parameters extracted from fits of the experimental data, temperature (T ) and baryo-chemical potential (µ b ), established the "line of chemical freeze-out" [10].…”

Effects of nuclear symmetry energy on η meson production and its rare decay to the dark U-boson in heavy-ion reactions

“…The corresponding threshold beam energy in free nucleon-nucleon collisions is E η th = 1.2 GeV and E K th = 1.6 GeV, respectively. For a comparison, the experimental K + /π + and η/π 0 ratios in the most central Au+Au reactions at 1 GeV/nucleon is approximately 2×10 −3 [76] and 1.4×10 −2 [77], respectively. It is thus interesting to know if the η meson might be useful for exploring the E sym (ρ) .…”