arXiv subjects

Ajit M. Srivastava

Publications and source records attributed to Ajit M. Srivastava.

At least 19 recordsLinked to original sources

arXiv2026

Secondary Hadron--Nucleus Collisions of Short-Lived Hadrons in Ultra-Relativistic Fixed-Target Heavy-Ion Interactions

Ultra-relativistic heavy nuclei traversing a solid target undergo successive nuclear encounters separated by atomic lattice spacings. At sufficiently high beam energies, Lorentz contraction reduces the proper time between collisions to $\mathcal{O}(10^4)$~fm$/c$ in the center-of-mass frame of the first interaction. We then consider the fragmentation region of this first collision, and show that short-lived hadrons produced in this region, with additional Lorentz boost, can reach the next nucleus before decaying. We show that this geometry enables secondary hadron--nucleus collisions involving species that cannot be realized as conventional secondary beams or in subsequent hadron--nucleus interactions in cosmic-ray cascades. For a $2.76$ TeV-per-nucleon Pb beam incident on a solid Pb lattice, we determine which forward-produced hadrons can survive to a second interaction, estimate their collision probabilities, and analyze potential observable consequences. In particular, we identify some representative hadrons whose proper lifetimes are of order $10^3$ fm/c, e.g. specific mesons ($η^\prime$) and heavy-flavor resonances ($J/ψ, D^*(2010)$), as projectile species that become accessible through this collision space-time geometry. At substantially higher beam energies (for example, with 10 TeV per-nucleon Pb beam), the survival probabilities are significantly enhanced. This can make even very short lived hadrons with life times of few tens fm ( $Ξ(1530)$, $ω(782)$, $ϕ(1020)$) available for this secondary hadron-nucleus collision, providing an additional motivation for future ultra-relativistic fixed-target heavy-ion experiments.

nucl-th↗

arXiv2026

Pulse Modulation as a Signature of the Asteroid-Neutron Star Collision Model for High-Energy Transients

Asteroid-neutron star collision models have been proposed as possible sources of high-energy transients, such as gamma-ray bursts (GRBs) and fast radio bursts (FRBs). The sequence of events following the impact of the asteroid and finally dissolving into the neutron star can have several other observable consequences. We propose that due to the development of the off-diagonal moment of inertia (MI) components, the merger's aftermath can lead to the wobbling of the pulsar (assuming the neutron star happens to be a pulsar). Using sample values of various parameters, viz., size, shape, the locations of the deposits, and the pre-existing pulsar deformation parameter ($η$), we calculate the detailed pulse profile modulation of the pulsar. We observe a distinct pattern of pulse profile modulation on a characteristic timescale enhanced by a factor of $1/η$ compared to the pulse timing. Importantly, even small changes in the MI components, of order $ε$, can produce large pulse profile modulations of order $ε/η$ (depending on the relative location of asteroid material deposition). Thus, if an asteroid-neutron star collision is responsible for a high-energy transient, the associated pulse profile modulation may serve as a falsifiable observational signature of such an event.

astro-ph.HE↗

arXiv2025

Doppler shifted Hawking radiation from acoustic black holes in ultra-relativistic heavy-ion collisions

In a hydrodynamic flow, with flow becoming supersonic at some point, the subsonic-supersonic boundary behaves as the horizon of a black hole. Possibility of detecting Hawking radiation from such acoustic black holes has been investigated in a variety of laboratory systems, ranging from cold atom systems, to condensed matter systems with hydrodynamic flow of electrons, to relativistic heavy-ion collisions (at relatively lower collision energies). Ultra-relativistic heavy-ion collisions, with boost-invariant longitudinal flow of the quark-gluon plasma (QGP) in a wide rapidity window has eluded this remarkable possibility because in this case the black hole horizon is dynamical, moving away from center with sound velocity, leading to infinite red shift of Hawking radiation. We show here that such a conclusion is premature. The QGP flow at very large rapidities, necessarily deviates from Bjroken boost invariant flow. Due to this, an observer close to that region sees black hole horizon with a finite redshift. It leads to non-trivial prediction of Hawking radiation affecting particle momentum distributions for a window of rapidities, leaving near central rapidity regions unaffected.

hep-ph↗

arXiv2024

Detecting superfluid transition in the pulsar core

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 CFL 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 such that the random vortex network decays, obeying specific scaling laws leading to universal features for the detection of superfluid transitions occurring in a pulsar core.

astro-ph.HE↗

arXiv2024

Status of Astronomy Education in India: A Baseline Survey

We present the results of a nation-wide baseline survey, conducted by us, for the status of Astronomy education among secondary school students in India. The survey was administered in 10 different languages to over 2000 students from diverse backgrounds, and it explored multiple facets of their perspectives on astronomy. The topics included students' views on the incorporation of astronomy in curricula, their grasp of fundamental astronomical concepts, access to educational resources, cultural connections to astronomy, and their levels of interest and aspirations in the subject. We find notable deficiencies in students' knowledge of basic astronomical principles, with only a minority demonstrating proficiency in key areas such as celestial sizes, distances, and lunar phases. Furthermore, access to resources such as telescopes and planetariums remain limited across the country. Despite these challenges, a significant majority of students expressed a keen interest in astronomy. We further analyze the data along socioeconomic and gender lines. Particularly striking were the socioeconomic disparities, with students from resource-poor backgrounds often having lower levels of access and proficiency. Some differences were observed between genders, although not very pronounced. The insights gleaned from this study hold valuable implications for the development of a more robust astronomy curriculum and the design of effective teacher training programs in the future.

physics.ed-ph↗

arXiv2024

Pulsar as a Weber detector of gravitational waves and a probe to its internal phase transitions

It is believed that cores of neutron stars provide a natural laboratory where exotic high baryon density QCD phases may exist.The theoretically well established {\it neutron superfluid phase} is also believed to be found only inside neutron stars. Focus on neutron stars has intensified in recent years with the direct detection of gravitational waves (GWs) from binary neutron star (BNS) merger, which has allowed the possibility of directly probing the properties of the interior of a neutron star. A remarkable phenomenon manifested by rapidly rotating neutron stars is in their {\it avatar} as {\it Pulsars}. The accuracy of pulsar timing allowed the first indirect detection of GWs from a BNS system and opened up a few exciting possibilities. Any pulsar deformation, even if incredibly tiny, can leave imprints on the pulses by introducing tiny perturbations of the moment of inertia (MI) tensor components. While the diagonal MI components of the perturbed MI tensor affect the pulse timings, the off-diagonal components lead to the pulsar's wobbling and affecting the pulse profile. This opens up an opportunity to explore various phase transitions inside a pulsar core by induced density fluctuations through the observable effects on the pulse timing and 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 a neutron star. With the possibility of detecting any minute changes in its configuration through pulse observations, the neutron star has the potential to perform as a Weber detector of GWs. This brief review focuses on these specific aspects of a pulsar, specifically on the type of physics that can be probed by utilizing the effect of changes in the MI tensor on pulse properties.

astro-ph.HE↗

arXiv2024

Percolating Cosmic String loops from evaporating primordial black holes

The Pulsar timing data from NANOGrav Collaboration has regenerated interest in the possibility of observing stochastic gravitational wave background arising from cosmic strings. In the standard theory, the cosmic string network forms during spontaneous symmetry breaking (SSB) phase transition in the whole universe via the so called Kibble mechanism. This scenario would not be possible, e.g., in models of low energy inflation, where the reheat temperature is much lower than the energy scale of cosmic strings. We point out a very different possibility, where a network of even high energy scale cosmic strings can form when the temperature of the Universe is much lower. We consider local heating of plasma in the early universe by evaporating primordial black holes (PBHs). It is known that for suitable masses of PBHs, their Hawking radiation may re-heat the surrounding plasma to high temperatures, restoring certain symmetries {\it locally} which are broken at the ambient temperature at that stage. Expansion of the hot plasma cools it so that the {\it locally restored symmetry} is spontaneously broken again. If this SSB supports formation of cosmic strings, then string loops will form in this region around the PBH. Further, resulting temperature gradients lead to pressure gradients such that plasma develops radial flow with the string loops getting stretched as they get dragged by the flow. For a finite density of PBHs of suitable masses, one will get local hot spots, each one contributing to expanding cosmic string loops. For suitable PBH density, the loops from different regions may intersect. Intercommutation of strings can then lead to percolation, leading to the possibility of formation of infinite string network, even when the entire universe never goes through the respective SSB phase transition.

gr-qc↗

arXiv2022

Probing shapes of microbes using liquid crystal textures

We propose a novel technique to probe shape of a single microbe embedded in a nematic liquid crystal (NLC) sample by observing geometry of dark brushes with optical microscope using a cross-polarizer set up. Assuming certain anchoring conditions for the NLC director at the surface of the microbe, we determine the resulting shapes of brushes using numerical simulations. Our results suggest that for asymmetrical microbes (such as cylindrical shaped bacteria/viruses), resulting brushes may carry the imprints of this asymmetry (e.g. the aspect ratio of cylindrical shape) at relatively large distances to be able to be seen using simple optical microscopy even for microbe sizes in few tens to few hundred nanometer range.

cond-mat.soft↗

arXiv2022

Hawking radiation from acoustic black holes in hydrodynamic flow of electrons

Acoustic black holes are formed when a fluid flowing with subsonic velocities, accelerates and becomes supersonic. When the flow is directed from the subsonic to supersonic region, the surface on which the normal component of fluid velocity equals the local speed of sound acts as an acoustic horizon. This is because no acoustic perturbation from the supersonic region can cross it to reach the subsonic part of the fluid. One can show that if the fluid velocity is locally irrotational, the field equations for acoustic perturbations of the velocity potential are identical to that of a massless scalar field propagating in a black hole background. One, therefore, expects Hawking radiation in the form of a thermal spectrum of phonons. There have been numerous investigations of this possibility, theoretically, as well as experimentally, in systems ranging from cold atom systems to quark-gluon plasma formed in relativistic heavy-ion collisions. Here we investigate this possibility in the hydrodynamic flow of electrons. Resulting Hawking radiation in this case should be observable in terms of current fluctuations. Further, current fluctuations on both sides of the acoustic horizon should show correlations expected for pairs of Hawking particles.

gr-qc↗

arXiv2022

Modulation of pulse profile as a signal for phase transitions in a pulsar core

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.

astro-ph.HE↗

arXiv2021

Hawking radiation from acoustic black holes in relativistic heavy ion collisions

We propose a new analogue model of gravity - the evolving quark gluon plasma (QGP) produced in relativistic heavy ion collisions. This quark gluon plasma is the "most inviscid" fluid known. Such low kinematic viscosity is believed to reflect strongly correlated nature for QGP in these experiments. Hence, it may provide a good example of a quantum fluid naturally suited to studies of acoustic Hawking radiation. Due to rapid longitudinal expansion, presence of a sonic horizon is also naturally guaranteed here, though, in general, this horizon is not static. Using Ultra relativistic quantum molecular dynamics (UrQMD) simulations, we show that, under certain conditions, the longitudinal velocity of the plasma, near the sonic horizon, can become time independent for a short span during the evolution of the system. During this period, we can have a conformally static acoustic metric with a (conformal) Killing horizon coinciding with the apparent horizon. An asymptotic observer will then see a thermal flux of phonons, constituting the Hawking radiation, coming from the horizon. For the relatively low energy collision considered here, where the resulting QCD system is governed by non-relativistic hydrodynamics, we estimate the Hawking temperature to be about 4-5 MeV (with the temperature of the QCD fluid being about 135 MeV). We discuss the experimental signatures of this Hawking radiation in terms of a thermal component in the rapidity dependence of the transverse momentum distribution of detected particles. We also discuss extension to ultra-relativistic case which should lead to a higher Hawking temperature, along with the effects of dynamical horizon leading to blue/red shift of the temperature.

gr-qc↗

arXiv2020

Initial fluctuations and power spectrum of flow anisotropies in relativistic heavy-ion collisions

Flow has emerged as a crucial probe for the properties of the thermalized medium produced in relativistic heavy-ion collisions. The evolution of initial state fluctuations leaves imprints on the power spectrum of flow coefficients. Therefore flow coefficients are a crucial probe of initial state fluctuations arising from the parton distributions of the colliding nuclei. This has a very strong correspondence with the physics of power spectrum of cosmic microwave background radiation (CMBR) anisotropies which directly probes initial inflationary fluctuations. Much work has been done to probe these interesting interconnections, in particular, in developing techniques for the measurements of higher flow coefficients. We present a short review of these developments. The effect of initial magnetic field on these features will also be reviewed. All this acquires special importance in view of upcoming electron-ion collider which will directly probe initial parton distribution of the colliding nucleus.

nucl-th↗

arXiv2019

Re-visiting gravitational wave events via pulsars

By now many gravitational wave (GW) signals have been detected by LIGO and Virgo, with the waves reaching earth directly from their respective sources. These waves will also travel to different pulsars and will cause (tiny) transient deformations in the pulsar shape. Some of us have recently shown that the resultant transient change in the pulsar moment of inertia may leave an observable imprint on the pulsar signals as detected on earth, especially at resonance. The pulsars may thus act as remotely stationed Weber gravitational wave detectors. This allows us to revisit the past GW events via pulsars. We give here a list of specific pulsars whose future signals will carry the imprints of past GW events, to be specific we constrain it within 50 years. Some interesting cases are, supernova SN1987A with earliest perturbed signals from pulsars J0709-5923 and B0559-57 expected to reach earth in 2023 and 2024 respectively, Crab supernova, with perturbed signal arrival date from pulsar J1856-3754 in 2057, and GW170814 event with its imprints on the signals on the pulsar J0437-4715 reaching earth between 2035-2043. Even the earliest recorded supernova SN185 event may become observable again via pulsar J0900-3144 with the perturbed pulsar signal reaching us sometime between 2033-2066. Importantly, even though the strength of the signal will depend on the interior properties of the pulsar, the expected dates of signal arrival are completely model independent, depending only on the locations of the source and the relevant pulsar.

astro-ph.HE↗

arXiv2018

Formation of topological vortices during superfluid transition in a rotating vessel

Formation of topological defects during symmetry breaking phase transitions via the {\it Kibble mechanism} is extensively used in systems ranging from condensed matter physics to the early stages of the universe. Kibble mechanism uses topological arguments and predicts equal probabilities for the formation of defects and anti-defects. Certain situations, however, require a net bias in the production of defects (or antidefects) during the transition, for example, superfluid transition in a rotating vessel, or flux tubes formation in a superconducting transition in the presence of external magnetic field. In this paper we present a modified Kibble mechanism for a specific system, $^4$He superfluid transition in a rotating vessel, which can produce the required bias of vortices over antivortices. Our results make distinctive predictions which can be tested in superfluid $^4$He experiments. These results also have important implications for superfluid phase transitions in rotating neutron stars and also for any superfluid phases of QCD arising in the non-central low energy heavy-ion collision experiment due to an overall rotation.

cond-mat.other↗

arXiv2018

Pulsars as Weber gravitational wave detectors

A gravitational wave passing through a pulsar will lead to a variation in the moment of inertia of the pulsar affecting its rotation. This will affect the extremely accurately measured spin rate of the pulsar as well as its pulse profile (due to induced wobbling depending on the source direction). The effect will be most pronounced at resonance and should be detectable by accurate observations of the pulsar signal. The pulsar, in this sense, acts as a remotely stationed Weber detector of gravitational waves whose signal can be monitored on earth. With possible gravitational wave sources spread around in the universe, pulsars in their neighborhoods can provide us a family of \textit{remote} detectors all of which can be monitored on earth. Even if GW are detected directly by earth based conventional detectors, such pulsar detectors can provide additional information for accurate determination of the source location. This can be of crucial importance for sources which do not emit any other form of radiation such as black hole mergers. For the gravitational wave events already detected by LIGO (and Virgo), our proposal suggests that one should look for specific pulsars which would have been disturbed by these events, and will transmit this disturbance via their pulse signals in any foreseeable future. If these future pulsar events can be predicted with accuracy then a focused effort can be made to detect any possible changes in the signals of those specific pulsars.

astro-ph.HE↗

arXiv2018

Setting Initial Conditions for Inflation with Reaction-Diffusion Equation

We discuss the issue of setting appropriate initial conditions for inflation. Specifically, we consider natural inflation model and discuss the fine tuning required for setting almost homogeneous initial conditions over a region of order several times the Hubble size which is orders of magnitude larger than any relevant correlation length for field fluctuations. We then propose to use the special propagating front solutions of reaction-diffusion equations for localized field domains of smaller sizes. Due to very small velocities of these propagating fronts we find that the inflaton field in such a ${\it field domain}$ changes very slowly, contrary to naive expectation of rapid roll down to the true vacuum. Continued expansion leads to the energy density in the Hubble region being dominated by the vacuum energy, thereby beginning the inflationary phase. Our results show that inflation can occur even with a single localized field domain of size smaller than the Hubble size. We discuss possible extensions of our results for different inflationary models, as well as various limitations of our analysis (e.g. neglecting self gravity of the localized field domain).

hep-th↗

arXiv2017

Towards laboratory detection of topological vortices in superfluid phases of QCD

Topological defects arise in a variety of systems, e.g. vortices in superfluid helium to cosmic strings in the early universe. There is an indirect evidence of neutron superfluid vortices from glitches in pulsars. One also expects that topological defects may arise in various high baryon density phases of quantum chromodynamics (QCD), e.g. superfluid topological vortices in the color flavor locked (CFL) phase. Though vastly different in energy/length scales, there are universal features, e.g. in the formation of all these defects. Utilizing this universality, we investigate the possibility of detecting these topological superfluid vortices in laboratory experiments, namely heavy-ion collisions. Using hydrodynamic simulations, we show that vortices can qualitatively affect the power spectrum of flow fluctuations. This can give unambiguous signal for superfluid transition resulting in vortices, allowing for check of defect formation theories in a relativistic quantum field theory system, and the detection of superfluid phases of QCD. Detection of nucleonic superfluid vortices in low energy heavy-ion collisions will give opportunity for laboratory controlled study of their properties, providing crucial inputs for the physics of pulsars.

hep-ph↗

arXiv2017

Effects of magnetic field on the plasma evolution in relativistic heavy-ion collisions

Very strong magnetic fields can arise in non-central heavy-ion collisions at ultrarelativistic energies, which may not decay quickly in a conducting plasma. We carry out relativistic magnetohydrodynamics (RMHD) simulations to study the effects of this magnetic field on the evolution of the plasma and on resulting flow fluctuations in the ideal RMHD limit. Our results show that magnetic field leads to enhancement in elliptic flow for small impact parameters while it suppresses it for large impact parameters (which may provide a signal for initial stage magnetic field). Interestingly, we find that magnetic field in localized regions can temporarily increase in time as evolving plasma energy density fluctuations lead to reorganization of magnetic flux. This can have important effects on chiral magnetic effect. Magnetic field has non-trivial effects on the power spectrum of flow fluctuations. For very strong magnetic field case one sees a pattern of even-odd difference in the power spectrum of flow coefficients arising from reflection symmetry about the magnetic field direction if initial state fluctuations are not dominant. We discuss the situation of nontrivial magnetic field configurations arising from collision of deformed nuclei and show that it can lead to anomalous elliptic flow. Special (crossed body-body) configurations of deformed nuclei collision can lead to presence of quadrupolar magnetic field which can have very important effects on the rapidity dependence of transverse expansion (similar to {\it beam focusing} from quadrupole fields in accelerators).

hep-ph↗