Department of Physics
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Item Probing phase transitions in a pulsar core through the observable effects on pulse profile modulation(Springer, 2024-07) Layek, BiswanathThere are compelling arguments in favour of various baryon-rich exotic QCD phases in the core of a pulsar. We suggested a technique to probe such phases by studying the effects of phase transition-induced density fluctuations on pulse profile modulation. Such density fluctuations cause the initial moment of inertia tensor (MI) of an oblate shape pulsar to get random additional contributions for each component. These contributions are assumed to be Gaussian of width , which characterizes the strength of density fluctuations. Using sample values of and the pulsar’s deformation parameter , we solve Euler’s equations for the rotational dynamics of the pulsar to observe the effects of wobbling through the modifications of pulse profiles. Our results show a specific pattern in the perturbed pulses. The wobbling of the pulsar dies away once the density fluctuations fade away. This feature distinguishes the transient pulse modulations from the pre-existing wobbling. The decay time of these modulations, being directly related to the relaxation time of density fluctuations, it provides valuable information about the nature of phase transition.Item Unpinning of superfluid vortices through quasineutron-vortex scattering and pulsar glitches(Springer, 2024-07) Layek, BiswanathThe model of pinning and unpinning of superfluid vortices is considered the most popular explanation behind pulsar glitches. However, the reason behind the almost instantaneous unpinning of a large number of vortices still needs a proper mechanism. Some of us [5] proposed that the neutron-vortex scattering in the inner crust of a pulsar may be responsible for such vortex unpinning. The strain energy released by the crustquake is assumed to be absorbed in some part of the inner crust. It causes pair-breaking quasi-neutron excitations from the existing free neutron superfluid in the bulk of the inner crust. The scattering of these quasi-neutrons with the vortex core neutrons should unpin a large number of vortices from the thermally affected regions and result in pulsar glitches. Here, we consider a cylindrical geometry of the affected pinning region to study the implications of the vortex unpinning in the context of pulsar glitches. We find that a Vela-like pulsars can release about vortices by this mechanism and results in glitches of size . We also explored the possibility of a vortex avalanche triggered by the movement of the unpinned vortices. An estimate of the glitch size caused by an avalanche shows a favourable result. The time scales associated with various events are compatible with glitch observations.Item Pulsar as a weber detector of gravitational waves and a probe to its internal phase transitions(World Scientific, 2024) Layek, BiswanathIt is believed that cores of neutron stars provide a natural laboratory where exotic high baryon density phases of quantum chromo dynamics (QCD) may exist. In fact, the theoretically well-established neutron superfluid phase is also believed to be found only inside neutron stars. Focus on neutrons stars has tremendously intensified in recent years with the direct detection of gravitational waves (GWs) by LIGO/Virgo from binary neutron star (BNS) merger events which has allowed the possibility of directly probing the properties of the interior of a neutron star. A truly remarkable phenomenon manifested by rapidly rotating neutron stars is in their avatar as Pulsars. The accuracy of pulsar timing can reach the level of one part in 1015, comparable to that of atomic clocks. Indeed, it was such a great accuracy which had allowed the first indirect detection of GWs from a BNS system. Such an incredible accuracy of pulse timings points to a very interesting possibility. Any deformation of the pulsar, even if it is extremely tiny, has the potential of leaving its imprints on the pulses through introduction of tiny perturbations in the entire moment of inertia (MI) tensor. While, the diagonal components of perturbed MI tensor affect the pulse timings, the off-diagonal components lead to wobbling of pulsar, directly affecting the pulse profile. This opens up a new window of opportunity for exploring various phase transitions occurring inside a pulsar core, through induced density fluctuations, which may be observable as perturbations in the pulse timing as well as its profile. Such perturbations also naturally induce a rapidly changing quadrupole moment of the star, thereby providing a new source of GW emission. Another remarkable possibility arises when we consider the effect of an external GW on neutron star. With the possibility of detecting any minute changes in its configuration through pulse observations, the neutron star has the potential of performing as a Weber detector of GW. This brief review will focus on these specific aspects of a pulsar. Specifically, the focus will be on the type of physics which can be probed by utilizing the effect of changes in the MI tensor of the pulsar on pulse properties.Item Detecting superfluid transition in the pulsar core(OUP, 2024-07) Layek, BiswanathIt is believed that the core of a neutron star can be host to various novel phases of matter, from nucleon superfluid phase to exotic high baryon density quantum chromodynamics (QCD) phases. Different observational signals for such phase transitions have been discussed in the literature. Here, we point out a unique phenomenon associated with phase transition to a superfluid phase, which may be the nucleon superfluid phase or a phase like the colour-flavour locked phase, allowing for superfluid vortices. In any superfluid phase transition, a random network of vortices forms via the so-called Kibble–Zurek mechanism, which eventually mostly decays away, finally leaving primarily vortices arising from the initial angular momentum of the core. This transient, random vortex network can have a non-zero net angular momentum for the superfluid component, which will generally be oriented in an arbitrary direction. This is in contrast to the final vortices, which arise from initial rotation and hence have the initial angular momentum of the neutron star. The angular momentum of the random vortex network is balanced by an equal and opposite angular momentum in the normal fluid due to the conservation of angular momentum, thereby imparting an arbitrarily oriented angular momentum component to the outer shell of the neutron star. This will affect the pulse timing and pulse profile of a pulsar. These changes in the pulses will decay away in a characteristic manner that this as the random vortex network decays, obeying specific scaling laws leading to universal features for the detection of superfluid transitions occurring in a pulsar core.Item Large-scale unpinning and pulsar glitches due to the forced oscillation of vortices(2024-11) Layek, BiswanathThe basic framework of the superfluid vortex model for pulsar glitches, though, is well accepted; there is a lack of consensus on the possible trigger mechanism responsible for the simultaneous release of a large number (∼1017) of superfluid vortices from the inner crust. Here, we propose a simple trigger mechanism to explain such catastrophic events of vortex unpinning. We treat a superfluid vortex line as a classical massive straight string with well-defined string tension stretching along the rotation axis of pulsars. The crustquake-induced lattice vibration of the inner crust can act as a driving force for the transverse oscillation of the string. Such forced oscillation near resonance causes the bending of the vortex lines, disturbing their equilibrium configuration and resulting in the unpinning of vortices. We consider unpinning from the inner crust's so-called {\it strong (nuclear)} pinning region, where the vortices are likely pinned to the nuclear sites. We also comment on vortex unpinning from the interstitial pinning region of the inner crust. We sense that unifying crustquake with the superfluid vortex model can naturally explain the cause of large-scale vortex unpinning and generation of large-size pulsar glitches.Item Baryon inhomogeneities due to cosmic string wakes at the quark-hadron transition(Springer, 2003-05) Layek, BiswanathBaryon inhomogeneities generated during the quark-hadron transition may alter the abundances of light elements if they persist up to the time of nucleosynthesis. These inhomogeneities survive up to the nucleosynthesis epoch if they are separated by a distance of at least a few metres. In this work we present a model where large sheets of these inhomogeneities separated by a distance of a few km are formed by cosmic string wakes during the quark-hadron transition. The effect of these sheets on nucleosynthesis will also put constraints on the various cosmic string parameters.Item The 8th workshop on high energy physics phenomenology (WHEPP-8) was held at the Indian Institute of Technology, Mumbai, India during January 5–16, 2004. One of the four working groups, group III was dedicated to QCD and heavy ion physics (HIC). The present manuscript gives a summary of the activities of group III during the workshop (see also [1] for completeness). The activities of group III were focused to understand the collective behaviours of the system formed after the collisions of two nuclei at ultra-relativistic energies from the interactions of the elementary degrees of freedom, i.e. quarks and gluons, governed by non-abelian gauge theory, i.e. QCD. This was initiated by two plenary talks on experimental overview of heavy ion collisions and lattice QCD and several working group talks and discussions.(Springer, 2003-11) Layek, BiswanathWe show that cosmic strings moving through the plasma at the time of a first-order quark-hadron transition in the early universe generate baryon inhomogeneities, which can survive till the nucleosynthesis epoch. We find out how these inhomogeneities actually affect the calculated values of the light element abundances. Recently a wealth of observational data from various experiments have helped to reduce the uncertainties in the values of these abundances. Using these we show that it is possible to derive constraints in the presence of cosmic strings during the quark-hadron transition.Item Working group report: Neutrino and astroparticle physics(Springer, 2004-12) Layek, BiswanathThe 8th workshop on high energy physics phenomenology (WHEPP-8) was held at the Indian Institute of Technology, Mumbai, India during January 5–16, 2004. One of the four working groups, group III was dedicated to QCD and heavy ion physics (HIC). The present manuscript gives a summary of the activities of group III during the workshop (see also [1] for completeness). The activities of group III were focused to understand the collective behaviours of the system formed after the collisions of two nuclei at ultra-relativistic energies from the interactions of the elementary degrees of freedom, i.e. quarks and gluons, governed by non-abelian gauge theory, i.e. QCD. This was initiated by two plenary talks on experimental overview of heavy ion collisions and lattice QCD and several working group talks and discussions.Item Elliptic flow of decay photons(IAEA, 2006) Layek, BiswanathThe elliptic flow of decay photons for several azimuthally asymmetric π0 and η distributions has been studied. The relation between υ2 of decay photons from hadrons and the υ2 of partons where the partons are recombined to form the hadrons has been checkedItem Skyrmion formation in 1+1 dimensions with chemical potential(World Scientific, 2006) Layek, BiswanathFormation of topological objects during phase transitions has been discussed extensively in literature. In all these discussions, defects and antidefects form with equal probabilities. In contrast, many physical situations, such as formation of baryons in relativistic heavy-ion collisions at present energies, flux tube formation in superconductors in the presence of external magnetic field, and formation of superfluid vortices in a rotating vessel, require a mechanism which can bias (say) defects over antidefects. Such a bias can crucially affect defect–antidefect correlations, apart from its effects on defect density. In this paper we initiate an investigation for the basic mechanism of biased formation of defects. For Skyrmions in 1+1 dimensions, we show that incorporation of a chemical potential term in the effective potential leads to a domain structure where order parameter is spatially varying. We show that this leads to biased formation of Skyrmions.
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