Hot Hadron Gas Research Articles

Low-energy thermal photons from meson–meson bremsstrahlung

Within an effective hadronic model including electromagnetic interactions via a U em ( 1 ) gauge, we re-investigate photon bremsstrahlung from a hot hadronic gas as expected to be formed in relativistic heavy-ion collisions at SPS energies. We calculate photon emission from the reactions π π → π π γ and π K → π K γ by an explicit (numerical) evaluation of the multi-dimensional phase space integral. This, in particular, allows to avoid the commonly employed soft photon approximation (SPA), as well as to incorporate final-state thermal enhancement factors. Both improvements are shown to result in an appreciable increase of the photon production rate over previous hadronic calculations. Upon convolution over a thermal fireball we find an improvement in the description of recent low transverse-momentum WA98 data at SPS. The influence of both Landau–Pomeranchuk–Migdal and in-medium effects on “ σ” and ρ-meson exchanges are briefly discussed.

Effect of geometrical size of the particles in a hot and dense hadron gas

Incorporation of the finite size of baryons into the equation of state (EOS) of a hot and dense hadron gas (HG) in a thermodynamically consistent manner has been a much studied problem. We first review its current status. Various models have been proposed in order to account for the repulsive force generated by the hard-core geometrical size of the baryons resulting in an excluded volume effect in the EOS. We examine the criterion of the thermodynamical consistency of these models and summarize their shortcomings. In order to remove the shortcomings, we propose a new model which incorporates the excluded volume effect in a thermodynamically consistent manner. We find that the new model works even for the cases of extremely large temperatures and densities where most of other approaches fail. Furthermore, the new expressions for thermodynamical variables resemble in form with those obtained from thermodynamically inconsistent models and thus a useful correction factor has been suggested here which converts inconsistent expressions into thermodynamically consistent ones. Finally we compare the predictions of new model with those obtained from various old models.

J/ψ strong couplings to vector mesons

We present a study of the cross sections J/Psi X --> D^(*) \bar D^(*) (X = rho, Phi) based on the calculation of the effective tri- and four-linear couplings J/Psi (X) D^(*) \bar D^(*) within a constituent quark model. In particular, the details of the calculation of the four-linear couplings J/Psi X D^(*)\bar D^(*) are given. The results obtained have been used in a recent analysis of J/Psi absorption by the hot hadron gas formed in peripheral heavy-ion collisions at SPS energies.

Single photons from Pb + Pb collisions at CERN SPS: quark–gluon plasma versus hadronic gas

In a hydrodynamic model, we have analysed the direct photon data obtained by the WA98 collaboration in 158 A GeV Pb + Pb collisions. The transverse expansion of the system has been taken into account. Two scenarios have been considered: (i) the formation of quark–gluon plasma and (ii) the formation of hot hadronic gas. Both scenarios describe the data equally well. However, the hadronic gas scenario requires a very high initial temperature (∼300 MeV) and it is difficult to conceive of the existence of hadron gas at that high temperature. If, initially, the hadronic fluid possesses small radial velocity (0.2–0.3 c), the data are well explained in the hadronic gas scenario, with reasonable initial temperatures.

The ϕ mean free path in hot hadronic matter

We calculate the ϕ meson collision rate and mean free path in a hot hadronic gas. The hidden local symmetry model is used to take into account interactions of ϕ mesons with pseudoscalar (π, K) and vector mesons (ρ, ω, K*, ϕ). In contrast to previous calculations we find a significantly small mean free path (around 1 fm at T = 200 MeV). This implies that ϕ mesons produced after hadronization in relativistic heavy-ion collisions will not leave the collision region without scattering. The consequences of these findings to the analysis of ϕ yields are discussed.

A Study of the Thermodynamic Properties of a Hot, Dense Hadron Gas Using an Event Generator: Hadro-Molecular-Dynamical Calculation

We investigate the equilibration and the equation of state of a hot hadron gas at finite baryon density using an event generator which is designed to approximately satisfy detailed balance at finite temperatures and finite baryon densities of the hadronic scale (80 MeV <T <170 MeV and 0.157 fm −3 <n B < 0.315 fm −3 ). Molecular-dynamical simulations were performed for a system of hadrons in a box with periodic boundary conditions. Starting from an initial state composed of only nucleons with a uniform phase-space distribution, the evolution takes place through interactions such as collisions, productions and absorptions. The system approaches a stationary state of baryons, mesons and their resonances, which is characterized by the value of the exponent in the energy distribution common to the different particles, i.e., the temperature. After the equilibration, thermodynamic quantities, such as the energy density, particle density, entropy and pressure are calculated. Above T ∼ mπ, the obtained equation of state exhibits a significant deviation from the naive mixed free gas model. Large values of the entropy per baryon are also notable. In our system, the excitation of heavy baryon resonances and meson production are enhanced simultaneously, and the increase of the temperature becomes moderate, but a Hagedorn-type artificial temperature saturation does not occur. The pressure exhibits a linear dependence on the energy density.

Hard thermal photon production in relativistic heavy ion collisions

The recent status of hard thermal photon production in relativistic heavy ion collisions is reviewed and the current rates are presented with emphasis on corrected bremsstrahlung processes in the quark–gluon plasma (QGP) and quark–hadron duality. Employing Bjorken hydrodynamics with an EOS supporting the phase transition from QGP to hot hadron gas (HHG), thermal photon spectra are computed. For SPS 158 GeV Pb+Pb collisions, comparison with other theoretical results and the WA98 direct photon data indicates significant contributions due to prompt photons. Extrapolating the presented approach to RHIC and LHC experiments, predictions of the thermal photon spectrum show a QGP outshining the HHG in the high-pT-region.

Causality in an excluded volume model

We have earlier proposed a thermodynamically consistent equation of state (EOS) incorporating the Van der Waal's type of excluded volume effect for a hot and dense hadron gas consisting of many species of finite-size baryons. We demonstrate here that our excluded volume approach does not violate causality.

On colour non-singlet representations of the quark–gluon system at finite temperature

We use a group theoretical technique to project out the partition function for a system of quarks, antiquarks and gluons onto a particular representation of the internal symmetry group SU(3): the colour singlet, colour octet and colour 27-plet, at finite temperature. We do this to calculate the thermodynamic quantities for those representations. We also calculate the change in free energy of the plasma droplet formed from the hot hadronic gas. We find that the size of the droplet in the colour-octet representation is smaller than that in the colour-singlet representations at different temperatures in the vicinity of the critical temperatures of the phase transitions.

Effect of Viscosity on One Dimensional Hydrodynamic Flow and Direct Photons from 200 AGeV S+Au Collisions at CERN SPS

We have analyzed the direct photon data obtained by the WA80 collaboration in 200 A GeV S+Au collision at CERN SPS, in a one dimensional hydrodynamical model. Two scenarios were considered: (i) formation of quark-gluon plasma and (ii) formation of hot hadronic gas. For both the scenarios, ideal as well as extremely viscous fluid was considered. It was found that direct photon yield from QGP is not affected much whether the fluid is treated as ideal or extremely viscous. The yield however differ substantially if hadron gas is produced. None of the scenarios gives satisfactory description of the data.

Unique signatures for QGP in strangeness sector

We suggest that the variations of certain strange particle ratios either with the energy density or with the baryon density constitute a significant signal for identification of the QGP formation in ultra-relativistic nucleus–nucleus collisions. We use realistic equations of state (EOS) for the QGP as well as for dense, hot hadron gas (HG) scenarios. We suggest that a direct comparison of the ratios obtained in the QGP and HG scenarios will be immensely helpful in identifying the QGP formation.

Thermodynamical Properties of Hot and Dense Hadron Gas by Numerical Simulation

Thermodynamical Properties of Hot and Dense Hadron Gas by Numerical Simulation

Thermal photons from S+Au collisions at 200A GeV: A hadron gas picture.

We have analyzed the preliminary results for the single photon spectrum, obtained by the WA80 Collaboration for 200{ital A} GeV S+Au collisions in a conventional hot hadronic gas model. It was seen that photon spectra depend sensitively on the value of the thermalization time. It is also affected greatly by the dissipative effects like viscosity. Experimental data are well described by the viscous hadron gas model with initial time of 5 fm. If one takes into account that experimental pion multiplicity may be uncertain by 15%, then hadron gas with viscosity gives excellent description of the data for initial time of 3 fm.

Dilepton emission from a resonance gas

We consider the dilepton emission from a dense and hot hadron gas near a critical point. The hadron gas is described as a resonance one. The dilepton spectrum generated by vector resonances is shown to be equivalent to the one generated by quarks.

Chiral dynamics, deconfinement and thermal dileptons

We show that in a hot hadron gas, chiral symmetry restoration and deconfinement lead to a strong suppression of dilepton production by pion annihilation in the continuum above the ϱ-ω-gf peak. The suppression due to deconfinement persists also for annihilation reactions involving heavier mesons.

High-energy photons from expanding quark-gluon plasma and hot hadronic matter.

The emission of high-energy photons from an expanding quark-gluon plasma (QGP), which undergoes a first-order phase transition to a hot hadronic gas before freeze-out, is evaluated. Compton and annihilation processes in the quark-gluon plasma and an exhaustive array of {ital hh}{r arrow}{ital h}{gamma} reactions as wel l as decays in hot hadronic matter are considered. We find that, if the initial temperature of the QGP is more than twice the critical temperature, then, beyond a transverse momentum of about 2--3 GeV, the photons from the QGP outshine those having their origin in hadronic matter. We further note that the increase in degrees of freedom in the hot hadronic matter reduces the lifetime of the mixed phase.

High-energy photons from quark-gluon plasma versus hot hadronic gas.

Photons in the energy range of about one-half to several GeV have been proposed as a signal of the formation of a quark-gluon plasma in high-energy collisions. To lowest order the thermal emission rate is infrared divergent for massless quarks, but we regulate this divergence using the resummation technique of Braaten and Pisarski. Photons can also be produced in the hadron phase. We find that the dominant contribution comes from the reactions $\ensuremath{\pi}\ensuremath{\pi}\ensuremath{\rightarrow}\ensuremath{\rho}\ensuremath{\gamma}$ and $\ensuremath{\pi}\ensuremath{\rho}\ensuremath{\rightarrow}\ensuremath{\pi}\ensuremath{\gamma}$; the decays $\ensuremath{\omega}\ensuremath{\rightarrow}\ensuremath{\pi}\ensuremath{\gamma}$ and $\ensuremath{\rho}\ensuremath{\rightarrow}\ensuremath{\pi}\ensuremath{\pi}\ensuremath{\gamma}$ are also significant. Comparing the thermal emission rates at a temperature $T=200$ MeV we conclude that the hadron gas shines just as brightly as the quark-gluon plasma.

Decay of ▭ in hot matter

We investigate how the presence of hadronic matter affects the width and the decay pattern of ▭ as a function of temperature and density and what this means qualitatively for the experiments studying ▭ production in relativistic nuclear collisions. We determine quantitatively in two models how the natural hadronic width of ▭ increases in a hot hadronic gas.

Formula omitted] interactions with hot hadronic matter

We consider the possibility that the suppression of the J ψ signal observed in oxygen-uranium collisions at CERN is due to interactions with a hot hadron gas. Assuming a longitudinally expanding meson gas, we obtain an overall suppression compatible with the CERN data. This suppression, however, varies only slowly with the J ψ transverse momentum. We also discuss the possibility of J ψ regeneration from the more abundantly produced χ and η c states, an effect that might become important at RHIC energies.