arXiv subjects
Publications and source records attributed to Sibaji Raha.
The mechanism for the propagation of strangelets with low baryon number through the atmosphere of the Earth has been explored. It has been shown that under suitable initial conditions, such strangelets may indeed reach depths near mountain altitudes with mass numbers and charges close to the observed values in cosmic ray experiments.
One of the abiding mysteries in the so-called standard cosmological model is the nature of the dark matter. It is universally accepted that there is an abundance of matter in the universe which is non-luminous, due to their very weak interaction, if at all, with the other forms of matter, excepting of course the gravitational attraction. Speculations as to the nature of dark matter are numerous, often bordering on exotics, and searches for such exotic matter is a very active field of astroparticle physics at the dawn of the new century. Nevertheless, in recent years, there has been experimental evidence for at least one form of dark matter - the massive compact halo objects detected through gravitational microlensing effects proposed by Paczynski some years ago. To date, no clear consensus as to what these objects, referred to in the literature as well as in the following by the acronym MACHO, are made of; for a brief discussion of some of the suggestions, see below. In this work, we show that they find a natural explanation as leftover relics from the putative first order cosmic quark - hadron phase transition that is predicted by the standard model of particle interactions to have occurred during the microsecond epoch of the early universe.
The Chandrasekhar limit for quark stars is evaluated from simple energy balance relations, as proposed by Landau for white dwarfs or neutron stars. It has been found that the limit for quark stars depends on, in addition to the fundamental constants, the Bag constant.
One of the confident predictions of QCD is that at sufficiently high temperature and/or density, hadronic matter should undergo a thermodynamic phase transition to a colour deconfined state of matter - popularly called the Quark-Gluon Plasma (QGP). In low energy effective theories of Quantum Chromodynamics (QCD), one usually talks of the chiral transition for which a well defined order parameter exists. We investigate the dissociation of pions and kaons in a medium of hot quark matter decsribed by the Nambu - Jona Lasinio (NJL) model. The decay widths of pion and kaon are found to be large but finite at temperature much higher than the critical temperature for the chiral (or deconfinement) transition, the kaon decay width being much larger. Thus pions and even kaons (with a lower density compared to pions) may coexist with quarks and gluons at such high temperatures. On the basis of such premises, we investigate the process of hadronization in quark-gluon plasma with special emphasis on whether such processes shed any light on acceptable low energy effective theories of QCD.
The abundance and size distribution of quark nuggets (QN), formed a few microseconds after the big bang due to first order QCD phase transition in the early universe, has been estimated. It appears that stable QNs could be a viable candidate for cosmological dark matter. The evolution of baryon inhomogeneity due to evaporated (unstable) QNs are also examined.
We calculate the size distribution of quark nuggets, which could be formed due to first order QCD phase transition in the early universe. We find that there are a large number of stable Quark Nuggets which could be a viable candidate for cosmological dark matter.
Two recent experiments have demonstrated that the effective $ρ$-mass in nuclear medium, as extracted from the $^3He(γ, π^+ π^-)$ reaction, is substantially reduced. This has been advocated as an indication of partial restoration of chiral symmetry in nuclear matter. We show that even in the absence of chiral symmetry, effective mean field nuclear matter models can explain these findings quantitatively.
A new dynamical model for the propagation of strangelets through the terrestrial atmosphere is proposed.
We present a microscopic approach to dynamical pionisation of the hot quark-gluon matter formed in ultrarelativistic heavy ion collisions. The time evolution of the system is described assuming that quarks undergo Brownian motion in a thermal bath provided by the gluons. The rate of hadronization as well as the time dependence of the temperature of the system are seen to be quite sensitive to the QCD $Λ$ parameter. Even in a non-equilibrium scenario, we find that there appears a clear hint of a first order phase transition.
The Jacobi polynomial has been advocated by many authors as a useful tool to evolve non-singlet structure functions to higher $Q^2$. In this work, it is found that the convergence of the polynomial sum is not absolute, as there is always a small fluctuation present. Moreover, the convergence breaks down completely for large $N$.
Pion and kaon dissociation in a medium of hot quark matter is studied in the Nambu Jona-Lasinio model. The decay width of pion and kaon are found to be large but finite at temperatures much higher than the so called critical temperature of chiral or deconfinement transition, kaon decay width being larger. Consequently, pions and even kaons (with a lower density compared to pions) should coexist with quarks and gluons at such high temperatures. The implication of the above result in the study of Quark-Gluon plasma is discussed.
Pion dissociation in a medium of hot quark matter is studied. The decay width of pion is found to be large but finite at temperatures much higher than the so called critical temperature of chiral or deconfinement transition. Consequently, pions should coexist with quarks and gluons at such high temperatures. The result is in agreement with the lattice calculations. The implication of the above result in the study of Quark-Gluon plasma is discussed.
The relativistic Fokker Planck equation has been used to study the evolution of the quark distribution in the quark gluon phase expected to be formed in ultra-relativistic heavy ion collisions. The effect of thermal masses for quarks and gluons is incorporated to take account of the in-medium properties. We find that the kinetic equilibrium is achieved before the system reaches the critical temperature of quark hadron phase transition. We find that chemical equilibrium is not achieved during this time. We have evaluated the electromagnetic probes of quark gluon plasma from the non-equilibrated quark gluon phase and compared them with those in completely equilibrated scenario. The hard QCD production rates for the electromagnetic ejectiles as well as the heavy quark production rates are also calculated.
The cosmic first order phase transition from quarks to hadrons, occurring a few microseconds after the Big Bang, would lead to the formation of quark nuggets which would be stable on a cosmological time scale, if the associated baryon number is larger than a critical value. We examine the possibility that these surviving quark nuggets may not only be viable candidates for cold dark matter but even close the universe.
We apply the momentum integrated Boltzmann transport equation to study the time evolution of various quark flavours in the central region of ultra-relativistic heavy ion collisions. The effects of thermal masses for quarks and gluons are incorporated to take into account the in-medium properties of these ingredients of the putative quark gluon plasma. We find that even under very optimistic conditions, complete chemical equilibration in the quark gluon plasma appears unlikely.
The effects of dissipation on the space time evolution of matter formed in ultra-relativistic heavy ion collision is discussed. The thermal photon spectra for RHIC and LHC energies with viscous flow is considered. The effects of viscosity in the thermal single photon spectra is seen to be important in QGP phase as compared to the hadronic phase. Recently available data from WA80 group at SPS energies for $S+Au$ collision are compared with theoretical calculations. The experimental data do not appear to be compatible with the formation of matter in the pure hadronic phase.
The variation of kaon mass in dense, charge-neutral baryonic matter at beta-equilibrium has been investigated. The baryon interaction has been included by means of nonlinear Walecka model, with and without hyperons and the interaction of kaons with the baryons has been incorporated through the Nelson-Kaplan model. A self-consistant, one-loop level calculation has been carried out. We find that at the mean field level, the presence of the hyperons makes the density-dependence of the kaon mass softer. Thus, the kaon condensation threshold is pushed up in the baryon density. The loop diagrams tend to lower the kaon condensation point for lower values of $a_3 m_s$. We also find that the S-wave kaon-nucleon interaction plays the dominant role in determining the on-set of kaon condensation and the contribution of the P-wave interaction is insignificant.
Effects of finite temperature correction to the dilepton emission rate in ultrarelativistic heavy ion collisions have been looked into. It has been seen that although the $ρ$-peak in the dilepton spectra from the hadronic sector is suppressed at very high temperatures, almost no effect can be observed in the space time integrated count rate.