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Register a new userTwo components in charged particle production in heavy-ion collisions
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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.
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We study the charged particle and transverse energy production mechanism from AGS, SPS, RHIC to LHC energies in the framework of nucleon and quark participants. At RHIC and LHC energies, the number of nucleons-normalized charged particle and transverse energy density in pseudorapidity, which shows a monotonic rise with centrality, turns out to be an almost centrality independent scaling behaviour when normalized to the number of participant quarks. A universal function which is a combination of logarithmic and power-law, describes well the charged particle and transverse energy production both at nucleon and quark participant level for the whole range of collision energies. Energy dependent production mechanisms are discussed both for nucleonic and partonic level. Predictions are made for the pseudorapidity densities of transverse energy, charged particle multiplicity and their ratio (the barometric observable, $frac{dE_{rm{T}}/deta}{dN_{rm{ch}}/deta} ~equiv frac{E_{rm{T}}}{N_{rm{ch}}}$) at mid-rapidity for Pb+Pb collisions at $sqrt{s_{rm{NN}}}=5.5$ TeV. A comparison with models based on gluon saturation and statistical hadron gas is made for the energy dependence of $frac{E_{rm{T}}}{N_{rm{ch}}}$.
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.
In relativistic nucleus-nucleus collisions the transverse energy per charged particle, E_T/N_ch, increases rapidly with beam energy and remains approximately constant at about 800 MeV for beam energies from SPS to RHIC. It is shown that the hadron resonance gas model describes the energy dependence, as well as the lack of centrality dependence, qualitatively. The values of E_T/N_ch are related to the chemical freeze-out criterium E/N about 1 GeV valid for primordial hadrons.
Recent results related to the chemical equilibration of hadrons in the final state of p-p and heavy ion collisions are reviewed.
This paper investigates the electromagnetic production of lepton pairs with low transverse momentum in relativistic heavy ion collisions. We estimate the initial photons transverse momentum contributions by employing models where the average transverse momentum squared of the incoming photon can be calculated in the equivalent photon approximation. We further derive an all order QED resummation for the soft photon radiation, which gives an excellent description of the ATLAS data in ultra-peripheral collisions at the LHC. For peripheral and central collisions, additional $p_T$-broadening effects from multiple interaction with the medium and the magnetic field contributions from the quark-gluon plasma are also discussed.
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