Browsing by Author "Layek, Biswanath"

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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, Biswanath
We 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.
Azimuthal flow of decay photons in relativistic nuclear collisions
(APS, 2006-10) Layek, Biswanath
An overwhelming fraction of photons from relativistic heavy-ion collisions has its origin in the decay of π0 and η mesons. We calculate the azimuthal asymmetry of the decay photons for several azimuthally asymmetric pion distributions. We find that the kT dependence of the elliptic flow parameter v2 for the decay photons closely follows the elliptic flow parameter vπ02 evaluated at pT≈kT+δ, where δ≈0.1−0.2 GeV, for typical pion distributions measured in nucleus-nucleus collisions at relativistic energies. Similar results are obtained for photons from the 2−γ decay of η mesons. Assuming that the flow of π0 is similar to those for π+ and π− for which independent measurements would be generally available, this ansatz can help in identifying additional sources for photons. Taken along with quark number scaling suggested by the recombination model, it may help to estimate v2 of the parton distributions in terms of azimuthal asymmetry of the decay photons at large kT.
Baryogenesis via density fluctuations with a second order electroweak phase transition
(World Scientific, 2003) Layek, Biswanath
We consider the presence of cosmic string induced density fluctuations in the universe at temperatures below the electroweak phase transition temperature. Resulting temperature fluctuations can restore the electroweak symmetry locally, depending on the amplitude of fluctuations and the background temperature. The symmetry will be spontaneously broken again in a given fluctuation region as the temperature drops there (for fluctuations with length scales smaller than the horizon), resulting in the production of baryon asymmetry. The time scale of the transition will be governed by the wavelength of fluctuation and, hence, can be much smaller than the Hubble time. This leads to strong enhancement in the production of baryon asymmetry for a second order electroweak phase transition as compared to the case when transition happens due to the cooling of the universe via expansion. For a two-Higgs extension of the Standard Model (with appropriate CP violation), we show that one can get the required baryon to entropy ratio if fluctuations propagate without getting significantly damped. If fluctuations are damped rapidly, then a volume factor suppresses the baryon production. Still, the short scale of the fluctuation leads to enhancement of the baryon to entropy ratio by at least 3–4 orders of magnitude compared to the conventional case of second order transition where the cooling happens due to expansion of the universe.
Baryon inhomogeneities due to cosmic string wakes at the quark-hadron transition
(Springer, 2003-05) Layek, Biswanath
Baryon 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.
Baryon inhomogeneity generation in the quark-gluon plasma phase
(APS, 2006-05) Layek, Biswanath
We discuss the possibility of generation of baryon inhomogeneities in a quark-gluon plasma phase due to moving Z(3) interfaces. By modeling the dependence of effective mass of the quarks on the Polyakov loop order parameter, we study the reflection of quarks from collapsing Z(3) interfaces and estimate resulting baryon inhomogeneities in the context of the early universe. We argue that in the context of certain low energy scale inflationary models, it is possible that large Z(3) walls arise at the end of the reheating stage. Collapse of such walls could lead to baryon inhomogeneities which may be separated by large distances near the QCD scale. Importantly, the generation of these inhomogeneities is insensitive to the order, or even the existence, of the quark-hadron phase transition. We also briefly discuss the possibility of formation of quark nuggets in this model, as well as baryon inhomogeneity generation in relativistic heavy-ion collisions.
Baryon inhomogeneity generation via cosmic strings at QCD scale and its effects on nucleosynthesis
(APS, 2003-04) Layek, Biswanath
We have earlier shown that cosmic strings moving through the plasma at the time of a first order quark-hadron transition in the early universe can generate large scale baryon inhomogeneities. In this paper, we calculate detailed structure of these inhomogeneities at the quark-hadron transition. Our calculations show that the inhomogeneities generated by cosmic string wakes can strongly affect nucleosynthesis calculations. A comparison with observational data suggests that such baryon inhomogeneities should not have existed at the nucleosynthesis epoch. If this disagreement holds with more accurate observations, then it will lead to the conclusions that cosmic string formation scales above 1014–1015 GeV may not be consistent with nucleosynthesis and CMBR observations. Alternatively, some other input in our calculation should be constrained, for example, if the average string velocity remains sufficiently small so that significant density perturbations are never produced at the QCD scale, or if strings move ultrarelativistically so that string wakes are very thin, trapping negligible amount of baryons. Finally, if the quark-hadron transition is not of first order then our calculations do not apply.
Bursts of gravitational waves due to crustquake from pulsars
(Oxford, 2020-09) Layek, Biswanath
Pulsars undergoing crustquake release strain energy, which can be absorbed in a small region inside the inner crust of the star and excite the free superfluid neutrons therein. The scattering of these neutrons with the surrounding pinned vortices may unpin a large number of vortices and effectively reduce the pinning force on vortex lines. Such unpinning by neutron scattering can produce glitches for Crab-like pulsars and Vela pulsar of size in the range of ∼10−8–10−7 and ∼10−9–10−8, respectively. Although we discuss here the crustquake-initiated excitation, the proposal is very generic and equally applicable for any other sources, which can excite the free superfluid neutrons, or can be responsible for superfluid – normal phase transition of neutron superfluid in the inner crust of a pulsar.
Cosmic string induced sheetlike baryon inhomogeneities at the quark-hadron transition
(APS, 2001-03) Layek, Biswanath
Cosmic strings moving through matter produce wakes where the density is higher than the background density. We investigate the effects of such wakes occurring at the time of a first order quark-hadron transition in the early universe and show that they can lead to a separation of the quark-gluon plasma phase in the wake region, while the region outside the wake converts to the hadronic phase. Moving interfaces then trap large baryon densities in sheetlike regions which can extend across the entire horizon. A typical separation between such sheets, at formation, is of the order of 1 km. Regions of baryon inhomogeneity of this nature, i.e., having a planar geometry and separated by such large distance scales, appear to be well suited for recent models of inhomogeneous nucleosynthesis to reconcile the large baryon to photon ratio implied by the recent measurements of the cosmic microwave background power spectrum.
Detecting superfluid transition in the pulsar core
(OUP, 2024-07) Layek, Biswanath
It 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.
Effects of phase transition induced density fluctuations on pulsar dynamics
(Elsevier, 2015-07) Layek, Biswanath; Das, Arpan
We show that density fluctuations during phase transitions in pulsar cores may have non-trivial effects on pulsar timings, and may also possibly account for glitches and anti-glitches. These density fluctuations invariably lead to non-zero off-diagonal components of the moment of inertia, leading to transient wobbling of star. Thus, accurate measurements of pulsar timing and intensity modulations (from wobbling) may be used to identify the specific pattern of density fluctuations, hence the particular phase transition, occurring inside the pulsar core. Changes in quadrupole moment from rapidly evolving density fluctuations during the transition, with very short time scales, may provide a new source for gravitational waves.
Elliptic flow of decay photons
(IAEA, 2006) Layek, Biswanath
The 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 checked
Excited hadrons as a signal for quark–gluon plasma formation
(World Scientific, 2006) Layek, Biswanath
At the quark–hadron transition, when quarks get confined to hadrons, certain orbitally excited states, namely those which have excitation energies above the respective L = 0 states of the same order as the transition temperature Tc, may form easily because of thermal velocities of quarks at the transition temperature. We propose that the ratio of multiplicities of such excited states to the respective L = 0 states can serve as an almost model independent signal for the quark–gluon plasma (QGP) formation in relativistic heavy-ion collisions. For example, the ratio R* of multiplicities of and when plotted with respect to the center-of-mass energy of the collision (or vs. centrality/number of participants), should show a jump at the value of beyond which the QGP formation occurs. This should happen irrespective of the shape of the overall plot of R* vs. . Recent data from RHIC on Λ*/Λ vs. Npart for large values of Npart may be indicative of such a behavior, though there are large error bars. We give a list of several other such candidate hadronic states.
Glitches due to quasineutron-vortex scattering in the superfluid inner crust of a pulsar
(APS, 2023-01) Layek, Biswanath
We revisit the mechanism of vortex unpinning caused by the neutron-vortex scattering [B. Layek and P. R. Yadav, Mon. Not. R. Astron. Soc. 499, 455 (2020)] in the inner crust of a pulsar. The strain energy released by the crustquake is assumed to be absorbed in some part of the inner crust and causes pair-breaking quasineutron excitations from the existing free neutron superfluid in the bulk of the inner crust. The scattering of these quasineutrons with the vortex core normal neutrons unpins a large number of vortices from the thermally affected regions and results in pulsar glitches. We consider the geometry of a cylindrical shell of the affected pinning region to study the implications of the vortex unpinning in the context of pulsar glitches. We find that a pulsar can release about ∼1011–1013 vortices by this mechanism. These numbers are equivalent to the glitch size of orders ∼10−11–10−9 for Vela-like pulsars with the characteristic age τ≃104 years. We also suggest a possibility of a vortex avalanche triggered by the movement of the unpinned vortices. A rough estimate of the glitch size caused by an avalanche shows an encouraging result.
High-density QCD phase transitions inside neutron stars: Glitches and gravitational waves
(Springer, 2017-10) Layek, Biswanath; Das, Arpan
We discuss physics of exotic high baryon density QCD phases which are believed to exist in the core of a neutron star. This can provide a laboratory for exploring exotic physics such as axion emission, KK graviton production etc. Much of the physics of these high-density phases is model-dependent and not very well understood, especially the densities expected to occur inside neutron stars. We follow a different approach and use primarily universal aspects of the physics of different high-density phases and associated phase transitions. We study effects of density fluctuations during transitions with and without topological defect production and study the effect on pulsar timings due to changing moment of inertia of the star. We also discuss gravitational wave production due to rapidly changing quadrupole moment of the star due to these fluctuations.
Isocurvature fluctuations through axion trapping by cosmic string wakes
(APS, 2005-03) Layek, Biswanath
We consider wakelike density fluctuations produced by cosmic strings at the quark-hadron transition in the early universe. We show that low momentum axions which are produced through the radiation from the axionic string at an earlier stage, may get trapped inside these wakes due to delayed hadronization in these overdense regions. As the interfaces, bordering the wakes, collapse, the axions pick-up momentum from the walls and finally leave the wake regions. These axions thus can produce large scale isocurvature fluctuations. We have calculated the detailed profile of these axionic density fluctuations and discuss its astrophysical consequences.
Large-scale unpinning and pulsar glitches due to the forced oscillation of vortices
(2024-11) Layek, Biswanath
The 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.
Modulation of pulse profile as a signal for phase transitions in a pulsar core
(OUP, 2022-04) Layek, Biswanath
We calculate detailed modification of pulses from a pulsar arising from the effects of phase transition induced density fluctuations on the pulsar moment of inertia. We represent general statistical density fluctuations using a simple model where the initial moment of inertia tensor of the pulsar (taken to be diagonal here) is assumed to get random additional contributions for each of its component which are taken to be Gaussian distributed with certain width characterized by the strength of density fluctuations ϵ. Using sample values of ϵ, (and the pulsar deformation parameter η) we numerically calculate detailed pulse modifications by solving Euler’s equations for the rotational dynamics of the pulsar. We also give analytical estimates which can be used for arbitrary values of ϵ and η. We show that there are very specific patterns in the perturbed pulses which are observable in terms of modulations of pulses over large time periods. In view of the fact that density fluctuations fade away eventually leading to a uniform phase in the interior of pulsar, the off-diagonal components of MI tensor also vanish eventually. Thus, the modification of pulses due to induced wobbling (from the off-diagonal MI components) will also die away eventually. This allows one to distinguish these transient pulse modulations from the effects of any wobbling originally present. Further, the decay of these modulations in time directly relates to relaxation of density fluctuations in the pulsar giving valuable information about the nature of phase transition occurring inside the pulsar.
Probing dynamics of phase transitions occurring inside a pulsar
(Springer, 2015-12) Layek, Biswanath; Das, Arpan
During the evolution of a pulsar, various phase transitions may occur in its dense interior, such as superfluid transition, as well as transition to various exotic phases of quantum chromodynamics (QCD). We propose a technique which allows to probe these phases and associated transitions by detecting changes in rotation of the star arising from density changes and fluctuations during the transition affecting stars moment of inertia (MI). Our results suggest that these changes may be observable, and may possibly account for glitches and anti-glitches. Accurate measurements of pulsar timing and intensity modulations (from wobbling) may be used to pin down the particular phase transition occurring inside the pulsar core. We also discuss the possibility of observing gravitational waves from the changes in the quadrupole moment (QM) arising from these rapidly evolving density fluctuations.
Probing phase transitions in a pulsar core through the observable effects on pulse profile modulation
(Springer, 2024-07) Layek, Biswanath
There 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.
Pulsar as a weber detector of gravitational waves and a probe to its internal phase transitions
(World Scientific, 2024) Layek, Biswanath
It 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 101⁢5, 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.