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Register a new userCommon Features of Particle Multiplicities in Heavy Ion Collisions
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Results of a systematic study of fully integrated particle multiplicities in central Au-Au and Pb-Pb collisions at beam momenta 1.7 A GeV, 11.6 A GeV (Au-Au) and 158 A GeV (Pb-Pb) using a statistical-thermal model are presented. The close similarity of the colliding systems makes it possible to study heavy ion collisions under definite initial conditions over a range of centre-of-mass energies covering more than one order of magnitude. We conclude that a thermal model description of particle multiplicities, with additional strangeness suppression, is possible for each energy. The degree of chemical equilibrium of strange particles and the relative production of strange quarks with respect to u and d quarks are higher than in e+e-, pp and pp(bar) collisions at comparable and even at lower energies. The average energy per hadron in the comoving frame is always close to 1 GeV per hadron despite the fact that the energy varies more than 10-fold.
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Transverse momentum spectra of charged particle production in heavy-ion collisions are considered in terms of a recently introduced Two Component parameterization combining exponential (soft) and power-law (hard) functional forms. The charged hadron densities calculated separately for them are plotted versus number of participating nucleons, $N_{part}$. The obtained dependences are discussed and the possible link between the two component parameterization introduced by the authors and the two component model historically used for the case of heavy-ion collisions is established. Next, the variations of the parameters of the introduced approach with the center of mass energy and centrality are studied using the available data from RHIC and LHC experiments. The spectra shapes are found to show universal dependences on $N_{part}$ for all investigated collision energies.
The shapes of invariant differential cross section for charged particle production as function of transverse momentum measured in heavy-ion collisions are analyzed. The data measured at RHIC and LHC are treated as function of energy density according to a recent theoretical approach. The Boltzmann-like statistical distribution is extracted from the whole statistical ensemble of produced hadrons using the introduced model. Variation of the temperature, characterizing this exponential distribution, is studied as function of energy density.
Recent results related to the chemical equilibration of hadrons in the final state of p-p and heavy ion collisions are reviewed.
Heavy flavor supplies a chance to constrain and improve the hadronization mechanism. We have established a sequential coalescence model with charm conservation and applied it to the charmed hadron production in heavy ion collisions. The charm conservation enhances the earlier hadron production and suppresses the later production. This relative enhancement (suppression) changes significantly the ratios between charmed hadrons in heavy ion collisions.
A study of the horn in the particle ratio $K^+/pi^+$ for central heavy-ion collisions as a function of the collision energy $sqrt{s}$ is presented. We analyse two different interpretations: the onset of deconfinement and the transition from a baryon- to a meson-dominated hadron gas. We use a realistic equation of state (EOS), which includes both hadron and quark degrees-of-freedom. The Taub-adiabate procedure is followed to determine the system at the early stage. Our results do not support an explanation of the horn as due to the onset of deconfinement. Using only hadronic EOS we reproduced the energy dependence of the $K^+/pi^+$ and $Lambda/pi^-$ ratios employing an experimental parametrisation of the freeze-out curve. We observe a transition between a baryon- and a meson-dominated regime; however, the reproduction of the $K^+/pi^+$ and $Lambda/pi^-$ ratios as a function of $sqrt{s}$ is not completely satisfying. We finally propose a new idea for the interpretation of the data, the roll-over scheme, in which the scalar meson field $sigma$ has not reached the thermal equilibrium at freeze-out. The rool-over scheme for the equilibration of the $sigma$-field is based on the inflation mechanism. The non-equilibrium evolution of the scalar field influences the particle production, e.g. $K^+/pi^+$, however, the fixing of the free parameters in this model is still an open issue.
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