Hot Hadronic Matter Research Articles

Charmonium production from hot hadronic matter

Charmonium production from D D ̄ annihilation in the hadron gas formed in ultra-relativistic heavy-ion collisions is studied. With initial conditions taken from the HIJING parton model, we have estimated the number of J/ ψ produced from an expanding hadron gas, using the J/ ψ absorption and production cross sections obtained from either an effective Lagrangian or the quark-exchange model. We find that J/ ψ production is negligible in heavy-ion collisions at the Relativistic Heavy Ion Collider (RHIC) but may be important at the Large Hadron Collider (LHC), where more charm mesons are produced. Similar results are obtained for ψ′ production from the hadron gas at RHIC and LHC.

Quark-hadron phase transition and strangeness conservation constraints

The implications of the strangeness conservation in a hadronic resonance gas (HRG) on the expected phase transition to the quark gluon plasma (QGP) are investigated. It is assumed that under favourable conditions a first order hadron-quark matter phase transition may occur in the hot hadronic matter such as those produced in the ultra-relativistic heavy-ion collisions at CERN and BNL. It is however shown that the criteria of strict strangeness conservation in the HRG may not permit the occurrence of a strict first order equilibrium quark-hadron phase transition unlike a previous study. This emerges as a consequence of the application of a realistic equation of state (EOS) for the HRG and QGP phases, which account for the finite-size effect arising from the short range hard-core hadronic repulsion in the HRG phase and the perturbative QCD interactions in the QGP phase. For a first order hadron-quark matter phase transition to occur one will therefore require large fluctuations in the critical thermal parameters, which might arise due to superheating, supercooling or other nonequlibrium effects. We also discuss a scenario proposed earlier, leading to a possible strangeness separation process during hadronization.

Photons from hadronic matter at finite temperature

Temperature dependence of hadronic decay widths and masses are studied within the framework of an effective Lagrangian approach. At finite temperature the hadronic masses do not seem to follow a universal scaling law. Considering an exhaustive set of hadronic reactions and vector meson decays we have estimated the photon spectrum emitted from hot hadronic matter taking into account medium effects through thermal loop corrections on the hadronic decay widths and masses. An enhancement in photon emission rate is obtained when we use the in-medium masses of vector mesons in our calculations. It is observed that the effect of p decay width on the photon spectra is negligible.

Thermochemical equilibrium and strangeness production at freeze-out in S-S collisions at

In the framework of our recently proposed model for the hadronic gas, the CERN NA35 collaboration data from central S-S collisions at are analysed. The model which provides a thermodynamically consistent equation of state (EOS) of hot hadronic matter consisting of hadrons with hard-core repulsion, is used to analyse the relative and absolute abundance of strange (antistrange) particles in full phase space. The analysis yields information regarding the thermal parameters at freeze-out, namely the light-quark fugacity , the strange-quark fugacity , the temperature T and the strangeness phase-space saturation factor . For the sake of comparison a similar analysis is performed by making use of the thermodynamically inconsistent model EOS. These results are further compared with those of Sollfrank et al obtained earlier by ignoring the hard-core hadronic repulsion and treating all hadrons as point particles and also those of Braun-Munzinger et al for limited phase-space data. Our models are generalized to allow for non-equilibrium strange-particle abundances.

Collision broadening of the φ meson in baryon rich hadronic matter

Phi meson-baryon cross sections, estimated within a one-boson-exchange model, serve as input for a calculation of the collision rates in hot hadronic matter. We find that the width of the \ensuremath{\varphi} meson is modified through collisions with baryons by 1--10 MeV at 160 MeV temperature depending on the baryon fugacity. Thermalization of the \ensuremath{\varphi} in high energy heavy ion collisions is discussed.

Equation of state of hadronic matter and electromagnetic radiation from relativistic heavy ion collisions

We study the radiation of thermal photons and dileptons likely to be produced in relativistic heavy ion collisions. We find that the thermal photon multiplicity scales with the charged pion multiplicity as $dN_{ch}/dy)^\alpha$ with $\alpha\sim 1.2$ for a transversely expanding system, contrary to the general belief of a quadratic dependence. The scaling is shown to be valid, both for real and virtual photons. The coefficient of proportionality at a given energy may help us identify the appropriate equation of state of hot hadronic matter produced in such collisions.

Relativistic statistical mechanics and particle spectroscopy

The formulation of manifestly covariant relativistic statistical mechanics as the description of an ensemble of events in spacetime parametrized by an invariant proper-time τ is reviewed. The linear and cubic mass spectra, which result from this formulation (the latter with the inclusion of anti-events) as the actual spectra of an individual hadronic multiplet and hot hadronic matter, respectively, are discussed. These spectra allow one to predict the masses of particles nucleated to quasi-levels in such an ensemble. As an example, the masses of the ground-state mesons and baryons are considered; the results are in excellent agreement with the measured hadron masses. Additivity of inverse Regge slopes is established and shown to be consistent with available experimental data on the D meson and Λc baryon production.

QCD Sum Rules and J/Ψ Suppression in Hot Hadronic Matter and Quark-Gluon Plasma**The project supported in part by National Natural Science Foundation of China

A consistent treatment based on the QCD sum rules at finite temperature has been established to calculate the J/Ψ suppression in hot hadronic matter (HM) and quark-gluon plasma (QGP) respectively. We have found that the J/Ψ suppression occurs in both hadronic matter and quark-gluon plasma, but a J/Ψ mass shift of about 0.49 GeV exists in QGP background only. The results obtained by QCD sum rules and by potential model are compared.

Excess of pions with chiral symmetry restoration

We study the effect of the chiral phase transition on pion production in hot hadronic matter. The phase of restored chiral symmetry is characterized by the appearance of the scalar σ-meson as a chiral partner of the pion as well as by the degeneracy of the vector and axial-vector mesons. We find rapid thermal and chemical equilibration of these degrees of freedom in the symmetric phase. Provided that the chiral transition temperature is not considerably high, the presence of a chirally symmetric phase will result in ∼ 1.6 times more thermal pions in the final state.

Chemical relaxation time of pions in hot hadronic matter

We calculate characteristic time scales for the chemical equilibration of pions in hot hadronic matter using an effective chiral Lagrangian. We find that inelastic processes involving vector and axial vector mesons reduce the chemical equilibration time by a factor of {approximately}10 compared to the result previously calculated in chiral perturbation theory. For a temperature of T{approximately}160 MeV we obtain a chemical relaxation time of {tau}{sub ch}{approx_equal}7 fm/c, which is comparable with typical time scales for a hadronic system generated in SPS-energy heavy-ion collisions. The effect of baryons is also estimated and found to be negligible for SPS energies but might be important for AGS energies. We predict that chemical freeze-out should take place at considerably higher temperatures {Delta}T{approx_equal}40 MeV than thermal freeze-out and that the hadronic phase would not sustain a pion chemical potential larger than 50 MeV. {copyright} {ital 1997} {ital The American Physical Society}

Rho meson propagation and dilepton enhancement in hot hadronic matter

A realistic model for the free rho meson with coupling to two-pion states is employed to calculate the rho propagator in a hot and dense hadron gas. The medium modifications are based on hadronic rescattering processes: intermediate two-pion states are renormalized through interactions with surrounding nucleons and deltas, and rho meson scattering is considered off nucleons, deltas, pions and kaons. Constraints from gauge invariance as well as the full off-shell dynamics of the interactions are accounted for. Within the vector dominance model we apply the resulting in-medium rho spectral function to compute e +e − production rates from π + π − annihilation. The calculation of corresponding e +e − spectra as recently measured in central collisions of heavy-ions at CERN/SpS energies gives reasonable agreement with the experimental data.

Hyperon production in proton-sulphur collisions at 200 GeV/ c

The WA94 experiment uses the production of strange particles and antiparticles to investigate the properties of hot hadronic matter created in heavy-ion interactions. Λ, Λ , Ξ − and Ξ + particle yields and transverse mass spectra are presented for pS interactions. These results are compared with those from SS interactions. Our results are also compared with those from pW and SW interactions of the WA85 experiment.

On the thermodynamics of hot hadronic matter

The equation of state of hot hadronic matter is obtained, by taking into account the contribution of the massive states, with the help of the resonance spectrum τ( m) ∼ m 3 justified by the authors in previous papers. This equation of state is in agreement with that provided by the low-temperature expansion for the pion interacting gas. The formulas for the temperature dependence of the non-strange and strange quark condensates are derived by taking into account the contribution of the massive resonances. The critical temperature of the chiral symmetry restoration transition is established to be 190 MeV if only meson resonances are considered, and 180 MeV if both meson and baryon resonances are taken into account. The phase transition of chiral symmetry restoration is modelled by the restriction of the number of the effective degrees of freedom in the hadron phase to that of the microscopic degrees of freedom in the quark-gluon phase, through the corresponding truncation of the hadronic resonance spectrum, and the decrease of the effective hadron masses with temperature predicted by Brown and Rho. The results are in agreement with lattice gauge data and show a smooth crossover in the thermodynamic variables in a temperature range ∼ 50 MeV.

What is the Role of Higher Baryonic Resonances inAACollisions at AGS Energies?

Thermal evolution of hadronic resonance matter in ultrarelativistic nucleus-nucleus col­ lisions is studied in the framework of cascade models. We investigate the role of higher baryonic resonances during the time evolution in (p, T) diagram of hot and dense hadronic matter at AGS energies. Although final hadronic spectra do not strongly depend on the number of higher resonances included in the calculation, it is shown that they prevent the temperature from going beyond 200 MeV.

Pion electromagnetic form factor at finite temperature.

Temperature effects on the electromagnetic couplings of pions in hot hadronic matter are studied with an effective chiral Lagrangian. We show that the Ward-Takahashi identity is satisfied at nonzero temperature in the soft pion limit. The in-medium electromagnetic form factor of the pion is obtained in the timelike region and shown to be reduced in magnitude, especially near the vector-meson resonance region. Finally, we discuss the consequences of this medium effect on dilepton production from hot hadronic matter. {copyright} {ital 1996 The American Physical Society.}

Partial restoration of chiral symmetry of vector mesons in hot hadronic matter: a possible signature of quark-gluon plasma

A φ meson is an excellent candidate to see the dynamical and critical phenomena at finite temperature of hadronic and quark matter which can be produced through relativistic nucleus-nucleus collisions.

Strangeness production in p+W and S+W Interactions at 200 A GeV/c

Strange and multistrange baryon and antibaryon production is a useful probe into the dynamics of the hot hadronic matter created in central heavy ion interactions. Relative production yields and transverse mass spectra are presented and compared for pW and SW interactions.

Hyperon production in proton-sulphur interaction at 200 GeV/c

The WA94 collaboration uses the production of strange and anti-strange particles to investigate the properties of hot hadronic matter created in heavy-ion interactions. Λ,\(\bar \Lambda \), Ξ− and\(\bar \Xi ^ + \) particle yields and transverse mass spectra are presented for pS interactions and are compared with those for SS interactions and for the pW and SW interactions of WA85.

Effective mass of phi mesons at finite temperature

The effective mass of phi meson at non-zero temperature is re-examined with an effective chiral Lagrangian. We find that the phi mass decreases with temperature but the effect is small compared to the result obtained from calculations using QCD sum rules. The leading contributions come from kaon loop corrections but vector meson contributions are also important as temperature increases. We discuss consequences of these changes to the phenomena of chiral phase transition in hot hadronic matter.

Restoration of chiral symmetry and dilepton production at finite temperature

Vector-axial-vector mixing at finite temperature is reexamined from the viewpoint of a chiral rotation on the chiral SU(2) L × SU(2) R SU(2) V manifold. Using this idea, we discuss the possibility of observing a temperature dependent mass-shift in dilepton production from hot hadronic matter. It is shown that a new peak or bump corresponding to the axial-vector meson appears in addition to the ordinary ones due to vector mesons.