Unstable particles (such as the vector mesons) have an important role to play in low mass dilepton production resulting from heavy ion collisions and this has been a subject of several investigations. Yet subtleties, such as the implications of the generalization of the Breit-Wigner formula for nonzero temperature and density, e.g., the question of collisional broadening, the role of Bose enhancement, etc., the possibility of the kinematic opening (or closing) of decay channels due to environmental effects, the problem of double counting through resonant and direct contributions, are often given insufficient emphasis. The present study attempts to point out these features using the $\rho$ and $\omega$ mesons as illustrative examples. The difference between the two versions of the vector meson dominance model in the present context is also presented. Effects of nonzero temperature and density, through vector meson masses and decay widths, on dilepton spectra are studied, for concreteness within the framework of a Walecka-type model, though most of the basic issues highlighted apply to other scenarios as well.

First order quark hadron phase transition is considered in scaling hydrodynamics including the Csernai-Kapusta model of nucleation of hadronic bubbles leading to supercooling. The effect of viscosity on the entropy production is studied in ideal as well as in viscous hydrodynamics with and without supercooling. It is found that the excess entropy produced due to supercooling depends weakly on viscosity.

We present a dynamical study of $e^{+}{e}^{-}$ production in ${\pi }^{-}A$ reactions at 1.3 and 1.7 GeV on the basis of the coupled-channel-BUU approach. The contributions from vector mesons $(\rho ,\omega )$ are calculated taking into account the collisional broadening effect and are compared to background sources in the dilepton spectrum from the Dalitz decays of $\omega$ and $\eta$ mesons produced in the reaction. Two possible scenarios for the medium modifications of the vector mesons are investigated, i.e., the “dropping mass” scheme for the $\rho$ and $\omega$ and a momentum-dependent $\rho$-meson spectral function that includes the polarization of the $\rho$-meson due to resonant $\rho -N$ scattering.

An improved nuclear model with derivative scalar couplings is proposed. It includes the freedoms of nucleons, σ mesons, ω mesons, and ρ mesons. Employing the thermofield dynamics, we have calculated the self-energy Feynman diagrams of ρ meson and found the temperature and density dependence of the ρ meson. We found that the density dependence of the ρ meson given by this model is in good agreement with recent experimental results.

Effect of scattering in the matter on particle decays at high energies is studied. The systematic method for the calculation of the decay probability of the particles undergoing multiple elastic collisions in the equilibrium medium is developed. The probability of the decay of such particles is obtained. The Dalitz decay of pion in the matter is studied.

A two-source emission picture is used to study the conditional moments of the charge distributions of projectile fragments in sulphur fragmentation at 200 $\mathrm{G}\mathrm{e}\mathrm{V}/\mathrm{n}\mathrm{u}\mathrm{c}\mathrm{l}\mathrm{e}\mathrm{o}\mathrm{n}$ by the Monte Carlo partition method. The distributions of the conditional moments are investigated and the logarithmic correlations between the conditional moments are found. The comparison of results of nuclear diffractive excitation, electromagnetic dissociation, and the Monte Carlo method are made. The calculated results are shown to be in agreement with the available experimental data.

We interpret hadronic particle abundances produced in S-Au/W/Pb $200A\hspace{0.5em}\mathrm{GeV}$ reactions in terms of the final state hadronic phase space model and determine by a data fit of the chemical hadron freeze-out parameters. Allowing for the flavor abundance nonequilibrium a highly significant fit to experimental particle abundance data emerges, which supports the possibility of strangeness distillation. We find under different strategies stable values for freeze-out temperature $T_f=143\mathrm{}\pm 3\hspace{0.5em}\mathrm{MeV},$ baryochemical potential ${\mu }_B=173\mathrm{}\pm 6\hspace{0.5em}\mathrm{MeV},$ ratio of strangeness $\left({\gamma }_s\right)$ and light quark $\left({\gamma }_q\right)$ phase space occupancies ${\gamma }_s/{\gamma }_q=0.60\mathrm{}\pm 0.02,$ and ${\gamma }_q=1.22\mathrm{}\pm 0.05$ without accounting for collective expansion (radial flow). When introducing flow effects which allow a consistent description of the transverse mass particle spectra, yielding $|v_c|=0.49\mathrm{}\pm 0.01\hspace{0.5em}c,$ we find ${\gamma }_s/{\gamma }_q=0.69\mathrm{}\pm 0.03,\hspace{0.5em}\hspace{0.5em}{\gamma }_q=1.41\mathrm{}\pm \mathrm{0.08.}$ The strange quark fugacity is fitted at ${\lambda }_s=1.00\mathrm{}\pm 0.02$ suggesting chemical freeze-out directly from the deconfined phase.

The relevance of the dipole configurations of quarks in forming nucleus structure functions is discussed. It is shown that radiation generated by dipole configurations while moving relativistically along their axes is described by distributions which are finite and infrared stable in a low transverse momentum region. It is argued that there is an exponential transition to the perturbativelike behavior at large transverse momenta and its rate is defined by the distance between the dipole charges in its rest frame.

The quark meson coupling model which describes nuclear matter as a collection of nonoverlapping MIT bags interacting by the self-consistent exchange of scalar and vector mesons is used to study nuclear matter at finite temperature. In its modified version, the density dependence of the bag constant is introduced by a direct coupling between the bag constant and the scalar mean field. In the present work, the coupling of the scalar mean field with the quarks is considered exactly through the solution of the Dirac equation. Our results show that a phase transition takes place at a critical temperature around 200 MeV in which the scalar mean field takes a nonzero value at zero baryon density. Furthermore, it is found that the bag constant decreases significantly when the temperature increases above this critical temperature indicating the onset of quark deconfinement.

We study hot nuclear matter in an explicit quark model based on a mean-field description of nonoverlapping nucleon bags bound by the self-consistent exchange of scalar and vector mesons as well as the glueball field. The glueball exchange as well as a realization of the broken scale invariance of quantum chromodynamics is achieved through the introduction of a dilaton field. The calculations also take into account the medium dependence of the bag constant. The effective potential with dilatons is applied to nuclear matter. The nucleon properties at finite temperature as calculated here are found to be appreciably different from cold nuclear matter. The introduction of the dilaton potential improves the shape of the saturation curve at $T=0\mathrm{}$ and is found to affect hot nuclear matter significantly.

Bound $q\overline{q}$ systems are studied in the framework of different three-dimensional relativistic equations derived from the Bethe-Salpeter equation with the instantaneous kernel in the momentum space. Except the Salpeter equation, all these equations have a correct one-body limit when one of the constituent quark masses tends to infinity. The spin structure of the confining $\mathrm{qq}$ interaction potential is taken in the form $x{\gamma }_1^0{\gamma }_2^0+(1\mathrm{}-{x)I}_1{I}_2,$ with $0<~x<~1.$ At first stage, the one-gluon-exchange potential is neglected and the confining potential is taken in the oscillator form. For the systems $\left(u\overline{s}\right),\left(cū\right),\left(c\overline{s}\right)$ and $\left(uū\right),\left(s\overline{s}\right)$ a comparative qualitative analysis of these equations is carried out for different values of the mixing parameter x and the confining potential strength parameter. We investigate (1) the existence/nonexistence of stable solutions of these equations, (2) the parameter dependence of the general structure of the meson mass spectum and leptonic decay constants of pseudoscalar and vector mesons. It is demonstrated that none of the three-dimensional equations considered in the present paper simultaneously describes even general qualitative features of the whole mass spectrum of $q\overline{q}$ systems. At the same time, these versions give an acceptable description of the meson leptonic decay characteristics.

We investigate the two-point correlation function for the vector current. Gluons provide dressings for both the quark self-energy as well as the vector vertex function, which are described consistently by the rainbow Dyson-Schwinger equation and the inhomogeneous ladder Bethe-Salpeter equation. The form of the gluon propagator at low momenta is modeled by a two-parameter ansatz fitting the weak pion decay constant $f_{\pi }.$ The quarks are confined in the sense that the quark propagator does not have a pole at timelike momenta. We determine the ground-state mass $m_0$ in the vector channel from the Euclidean time Fourier transform of the correlator, which falls off as $e^{-m_{0}T}$ at large T. $m_0$ lies around $590\hspace{0.5em}\mathrm{MeV},$ and is almost independent of the model form of the gluon propagator. This method allows us to stay in Euclidean space and to avoid analytic continuation of the quark or gluon propagators into the timelike region.

The sensitivity of polarization observables in the reaction $N\overrightarrow{N}\mathrm{NN}\phi$ slightly above the threshold is studied with respect to the details of the one-boson exchange model and a possible admixture of hidden strangeness in the nucleon. It is shown that the finite-energy predictions differ strongly from the threshold predictions. A measurement of the beam-target asymmetry and the φ decay anisotropy can help to disentangle the role of various reaction mechanisms.

The characteristics of the midrapidity and target sources (apparent temperatures, velocities, and neutron multiplicities) extracted from the neutron energy spectra, have been measured for various quasiprojectile (QP) excitation energies, reconstructed from charged particles of well defined peripheral events in the ${}^{35}{\mathrm{C}\mathrm{l}+}^{\mathrm{nat}}\mathrm{Ta}$ reaction at 43 MeV/nucleon. The reconstructed excitation energy of the QP is always smaller than the excitation energy calculated from its velocity, assuming pure dissipative binary collision. The latter observation combined with the neutron multiplicity at midrapidity and the apparent temperature suggests important preequilibrium and/or dynamical effects in the entrance channel. The midrapidity source moves at a velocity lower than the nucleon-nucleon center of mass velocity showing the importance of the attractive mean-field potential from the target even at 43 MeV/nucleon. The above picture is confirmed by comparison to Boltzman-Nordheim-Vlasov (BNV) simulations.

High spin states have been populated in ${}^{102}\mathrm{Rh}$ using the reaction ${}^{70}\mathrm{Zn}{(}^{36}\mathrm{S},p3n\gamma )$ at 130 MeV. The γ rays have been detected with the EUROGAM2 array. The level structure of ${}^{102}\mathrm{Rh}$ has been investigated. Several bands have been identified and established over a wide range of spin. They are interpreted using the Nilsson-Strutinsky cranking formalism and explained in terms of band terminations. Their configurations are built from the valence particles and valence holes relative to a ${}^{90}\mathrm{Zr}$ core: $g_{9/2}$ protons (and $N=3\mathrm{}$ proton holes) and $d_{5/2},$ $g_{7/2},$ and $h_{11/2}$ neutrons. After ${}^{102}\mathrm{Pd},$ ${}^{102}\mathrm{Rh}$ is the second heavy nucleus and the first odd-odd nucleus in which configurations in the valence space are followed from low spin up to their termination.

High-spin states in ${}^{182}\mathrm{Re}$ were studied using the ${}^{176}\mathrm{Yb}{(}^{11}\mathrm{B},5n)$ reaction at 70 MeV. The previously known four-quasiparticle isomer at 2256 keV is assigned spin and parity of ${16}^{-}.$ The rotational band built upon this multiquasiparticle state was identified and examination of the ${(g}_K-g_R{)/Q}_0$ values revealed that it arises from the $\pi {(9/2\mathrm{}}^{-}\left[514\right])\otimes {\nu }^3{(7/2\mathrm{}}^{-}[514\left]{,7/2\mathrm{}}^{-}\right[503\left]{,9/2\mathrm{}}^{+}\right[624\left]\right)$ configuration. The decay of the ${16}^{-}$ isomer is found to be less hindered when compared to those observed for similar four-quasiparticle long-lived states in neighboring odd-odd ${}^{176,178,180}\mathrm{Ta}$ isotopes. A possible explanation of this feature, involving a “local” mixing with a nearby ${16}^{-}$ collective state, is discussed.

Fission cross sections have been measured for the systems of ${}^{11}\mathrm{B},$ ${}^{16}\mathrm{O},$ ${}^{19}{\mathrm{F}+}^{232}\mathrm{Th}$ in the sub-barrier to extreme-sub-barrier energy (ESBE) region. Two position sensitive avalanche detectors were employed to measure the fission events due to fusion-fission (FF) and transfer fission (TF) processes using the fission fragment folding angle technique. The cross sections for fusion-fission reactions could be well accounted by the coupled channel calculations over a large energy range except in the ESBE region. The transfer induced fission cross sections are observed to be large at extreme sub-barrier energies. The ratio of transfer fission cross section to that of total fission is observed to increase rapidly as the energy is decreased in the sub-barrier region for all the systems. Also in the ESBE region a number of low momentum transfer fission (LMTF) events having folding angle close to $180°$ are observed with a rather weak dependence on the bombarding energy. These events were investigated by several control experiments. Considering the uncertainties involved in the control experiments, it appears difficult to rule out if they originate completely from background neutron induced reactions or from any new process such as Coulomb fission as conjectured in some earlier studies. The results on different components of the fission cross sections have been discussed.

We present an interpretation of oscillations in the energy autocorrelation functions for ${}^{12}\mathrm{C}{+}^{24}\mathrm{Mg}$ scattering assuming that the scattering process can be treated in terms of formation and decay of a dinucleus with strongly overlapping resonances. It is shown that these oscillations can be interpreted in terms of a slow spin decoherence and time-space localization of a coherently rotating hyperdeformed intermediate system. This coherent rotation allows us to reconstruct the time evolution of the long-lived intermediate dinucleus back into the past as far as the moment of its formation. Our analysis indicates that for spin values $\simeq 20–25ħ$ and at ≃50 MeV excitation energy hyperdeformation (3:1) of ${}^{36}\mathrm{Ar}$ terminates and is replaced by (2:1) deformation. New experiments are proposed to test our interpretation.

The half-lives $T_{1/2}$ for proton decay from the nuclei ${}_{53}^{109}\mathrm{I},$ ${}_{63}^{131}\mathrm{Eu},$ and ${}_{67}^{141}\mathrm{Ho}$ are evaluated exactly, assuming that the emitted proton moves in a deformed single particle Nilsson level. The calculated values of $T_{1/2},$ consistent with experiments, suggest a deformation of $\beta \approx 0.14$ for ${}_{53}^{109}\mathrm{I}$ and $\beta \approx 0.3–0.4$ for ${}_{63}^{131}\mathrm{Eu}$ and ${}_{67}^{141}\mathrm{Ho}.$ The corresponding most likely candidate states as ground states are ${K=1/2\mathrm{}}^{+},$ ${K=5/2\mathrm{}}^{+},$ and ${K=7/2\mathrm{}}^{-}$ for the above three nuclei, respectively. The recently found isomeric state in ${}_{67}^{141}\mathrm{Ho}$ is interpreted as a ${K=1/2\mathrm{}}^{+}$ state.

A quantum mechanical analysis of the bremsstrahlung in α decay of ${}^{210}\mathrm{Po}$ is performed in close reference to a semiclassical theory. We clarify the contribution from the tunneling, mixed, outside barrier regions, and from the wall of the inner potential well to the final spectral distribution, and discuss their interplay. We also comment on the validity of semiclassical calculations, and the possibility to eliminate the ambiguity in the nuclear potential between the alpha particle and daughter nucleus using the bremsstrahlung spectrum.