P. K. Panda

Quark-hadron phase transition in a neutron star under strong magnetic fields

We study the effect of a strong magnetic field on the properties of neutron stars with a quark–hadron phase transition. It is shown that the magnetic field prevents the appearance of a quark phase, enhances the leptonic fraction, decreases the baryonic density extension of the mixed phase and stiffens the total equation of state, including both the stellar matter and the magnetic field contributions. Two parametrizations of a density-dependent static magnetic field, increasing, respectively, fast and slowly with the density and reaching 2–4 × 1018 G in the centre of the star, are considered. The compact stars with strong magnetic fields have maximum mass configurations with larger masses and radii and smaller quark fractions. The parametrization of the magnetic field with density has a strong influence on the star properties.

Particle physics bounds from the Hulse-Taylor binary.

The orbital period of the binary pulsar PSR 1913+16 has been observed to decrease at the rate of 2.40{times}10{sup {minus}12} s/s which agrees with the prediction of the quadropole formula for gravitational radiation to within one percent. The decrease in orbital period may also occur by radiation of other massless particles such as scalars and pseudoscalar Nambu-Goldstone bosons. Assuming that this energy loss is less than one percent of the gravitational radiation, we can establish bounds on couplings of these particles to nucleons. For a scalar nucleon coupling of the form {ital g}{sub {ital s}}{phi}{bar {psi}}{psi} we find that {ital g}{sub {ital s}}{lt}3{times}10{sup {minus}19}. From the radiation loss of massless Goldstone bosons we establish the upper bound {theta}{ital g}{sub {ital p}}{lt}10{sup {minus}16} on the QCD vacuum angle {theta} and the pseudoscalar nucleon coupling constant {ital g}{sub {ital p}}. {copyright} {ital 1996 The American Physical Society.}

FIELD THEORETICAL STUDY OF 4He — A VARIATIONAL APPROACH

We developed a nonperturbative technique in field theory to obtain the ground state properties of 4He nucleus. Here, the σ-meson effects are simulated through isosinglet scalar cloud of pairs of off-shell pions with a coherent state. The analysis de facto replaces the scalar isoscalar potential in nuclear physics by multipion condensates. Also, we discuss the two hard photons production from the off-shell π+π− pair as present through pion cloud within the framework of the model.

Short range correlations in relativistic nuclear matter models

In this short note, we consider the unitary operator method as proposed by Villars [3], which automatically guarantees that the correlated state is normalized. The general idea of introducing short range correlations in systems with short range interactions exists for a long time [4, 5] but has not been pursued for the relativistic case. Non-relativistic calculations based on realistic NN potentials predict equilibrium points which do not reproduce simultaneously the binding energy and saturation density. Either the saturation density is reproduced but the binding energy is too small, or the binding energy is reproduced at too high a density [6]. In order to solve this problem, the existence of a repulsive potential or density-dependent repulsive mechanism [7] is usually assumed. Due to Lorentz covariance and self-consistency, relativistic mean field theories [8] include automatically contributions which are equivalent to n-body repulsive potentials in non-relativistic approaches. The relativistic quenching of the scalar field provides a mechanism for saturation, though, by itself it may lead to too small an effective mass and too large incompressibility of nuclear matter, a situation which is encountered in the Walecka model [8]. In the non-relativistic case, we find a wound in the relative wave function if the vector interaction is stronger than the scalar interaction. However, this may not be the case if the interaction is strong enough for short distances. Then, the effective potential can actually turn attractive at short distances and the wave function may well have a node, as proposed by V. Neudatchin [9] and advocated by S. Moszkowski [10], on the basis of the the quark structure of the nucleon. The situation with short range correlations may be more subtle than might be thought from a simple nonrelativistic model. We show that a short range node in the relative wave function may be encountered in relativistic models, although it remains to be seen to what extent the relativistic description simulates the quark structure. In non-relativistic models the saturation arises from the interplay between a long range attraction and a short range repulsion, so strong that it is indispensable to take short range correlations into account. In relativistic mean field models, the parameters are phenomenologically fitted to the saturation properties of nuclear matter. Although in this approach short range correlation effects may be accounted for, to some extent, by the model parameters, it is our aim to study explicitly the consequences of actual short range correlations. In a previous publication [12] we have discussed the effect of the correlations in the ground state properties of nuclear matter in the framework of the Hartree-Fock approximation using an effective Hamiltonian derived from the σ − ω Walecka model. We have shown, for interactions mediated only by sigma and omega mesons, that the equation of state (EOS) becomes considerably softer when correlations are taken into account, provided the correlation function is treated variationally, always paying careful attention to the constraint imposed by the “healing distance” requirement. In the present note we will work within the same approach and will include also the exchange of pions and ρ-mesons. Preliminary results of the present work have been presented in [11] We start by considering the effective Hamiltonian [12]

Compact stars and the symmetry energy

The effect of the symmetry energy on some properties of compact stars which contain strange degrees of freedom is discussed. Both the onset of hyperons or kaon condensation will be considered. The hyperon-meson couplings are chosen according to experimental values of the hyperon nuclear matter potentials and possible uncertainties are considered. It is shown that a softer symmetry energy affects the onset of strangeness, namely neutral (negatively charged) strange particles set on at larger (smaller) densities, and gives rise to a smaller strangeness fraction as a function of density. A softer symmetry energy will possibily give rise to maximum mass configurations with larger masses. Hyperon-meson couplings have a strong effect on the mass of the star. It is shown that, for stars with masses above 1 M⊙, the radius of the star varies linearly with the symmetry energy slope L.

Effect of the symmetry energy on compact stars

The effect of the symmetry energy on the properties of compact stars is discussed. It is shown that, for stars with masses above 1 M⊙, the radius of the star varies linearly with the symmetry energy slope L. The hyperon-meson couplings are chosen according to experimental values of the hyperon nuclear matter potentials, and possible uncertainties are considered. It is shown that a softer symmetry energy gives rise to stars with less hyperons and smaller radius. Hyperon-meson couplings may also have a strong effect on the mass of the star.

PION CORRELATIONS IN NUCLEAR MATTER

The saturation properties of the nuclear matter taking pion correlations into account are studied. We construct a Bogoliubov transformation for the pion pair operators and calculate the energy associated with the pion pairs. The pion dispersion relation is investigated. The correlation energy due to one-pion exchange in nuclear matter and neutron matter at random phase approximation using the generator coordinate method is also studied. The techniques of the charged pion correlations are discussed in the neutron matter calculations. We observe that there is no sign of the pion condensation in this model.

Warm stellar matter within the quark-meson-coupling model

In the present article, we investigate stellar matter obtained within the quark-meson-coupling (QMC) model for fixed temperature and with the entropy of the order of 1 or 2 Boltzmann units per baryon for neutrino-free matter and matter with trapped neutrinos. A new prescription for the calculation of the baryon effective masses in terms of the free energy is used. Comparing the results of the present work with those obtained from the nonlinear Walecka model, smaller strangeness and neutrino fractions are predicted within QMC. As a consequence, QMC has a smaller window of metastability for conversion into a low-mass blackhole during cooling.

Tensor interaction and short range correlations in relativistic nuclear models

Short range correlations are introduced using a Jastrow factor in a relativistic approach to the equation of state of the infinite nuclear matter in the framework of the Hartree-Fock approximation. The pion exchange, including the tensor contribution, is taken into account. It is shown that both the tensor contribution of pion exchange and short range correlations soften the equation of state. Neutron matter with correlations presents no minimum at low densities.

Pentaquarks in the medium in the quark-meson coupling model

We calculate the properties of the pentaquarks