arXiv subjects

Ajit M. Srivastava

Publications and source records attributed to Ajit M. Srivastava.

At least 37 records · Page 2Linked to original sources

arXiv2015

Power spectrum of flow fluctuations in relativistic heavy-ion collisions

We carry out hydrodynamical simulation of the evolution of fluid in relativistic heavy-ion collisions with random initial fluctuations. The time evolution of power spectrum of momentum anisotropies shows very strong correspondence with the physics of cosmic microwave anisotropies as was earlier predicted by some of us. In particular our results demonstrate suppression of superhorizon fluctuations and the correspondence between the location of the first peak in the power spectrum of momentum anisotropies and the length scale of fluctuations and expected freezeout time scale (more precisely, the sound horizon size at freezeout).

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arXiv2015

Possibility of DCC formation in pp collisions at LHC energy via reaction-diffusion equation

There are indications of formation of a thermalized medium in high multiplicity pp collisions at LHC energy. It is possible that such a medium may reach high enough energy density/temperature so that a transient stage of quark-gluon plasma, where chiral symmetry is restored, may be achieved. Due to rapid 3-dimensional expansion, the system will quickly cool undergoing spontaneous chiral symmetry breaking transition. We study the dynamics of chiral field, after the symmetry breaking transition, for such an event using reaction-diffusion equation approach which we have recently applied for studying QCD transitions in relativistic heavy-ion collisions. We show that the interior of such a rapidly expanding system is likely to lead to the formation of a single large domain of disoriented chiral condensate (DCC) which has been a subject of intensive search in earlier experiments. We argue that large multiplicity pp collisions naturally give rise to required boundary conditions for the existence of slowly propagating front solutions of reaction-diffusion equation with resulting dynamics of chiral field leading to the formation of a large DCC domain.

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arXiv2015

Reaction-diffusion equation for quark-hadron transition in heavy-ion collisions

Reaction-diffusion equations with suitable boundary conditions have special propagating solutions which very closely resemble the moving interfaces in a first order transition. We show that the dynamics of chiral order parameter for chiral symmetry breaking transition in heavy-ion collisions, with dissipative dynamics, is governed by one such equation, specifically, the Newell-Whitehead equation. Further, required boundary conditions are automatically satisfied due to the geometry of the collision. The chiral transition is, therefore, completed by a propagating interface, exactly as for a first order transition, even though the transition actually is a crossover for relativistic heavy-ion collisions. Same thing also happens when we consider the initial confinement-deconfinement transition with Polyakov loop order parameter. The resulting equation, again with dissipative dynamics, can then be identified with the reaction-diffusion equation known as the Fitzhugh-Nagumo equation which is used in population genetics. We discuss the implications of these results for heavy-ion collisions. We also discuss possible extensions for the case of early universe.

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arXiv2015

Effects of Phase Transition induced density fluctuations on pulsar dynamics

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.

astro-ph.HE↗

arXiv2014

Probing Dynamics of Phase Transitions occurring inside a Pulsar

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 star's moment of inertia. Our results suggest that these changes may be observable, and may possibly account for glitches and (recently observed) anti-glitches. Accurate measurements of pulsar timing and intensity modulations (arising from wobbling of star due to development of the off-diagonal components of moment of inertia) 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 arising from these rapidly evolving density fluctuations.

astro-ph.HE↗

arXiv2014

Quarkonia Disintegration due to time dependence of the $q \bar{q}$ potential in Relativistic Heavy Ion Collisions

Rapid thermalization in ultra-relativistic heavy-ion collisions leads to fast changing potential between a heavy quark and antiquark from zero temperature potential to the finite temperature one. Time dependent perturbation theory can then be used to calculate the survival probability of the initial quarkonium state. In view of very short time scales of thermalization at RHIC and LHC energies, we calculate the survival probability of $J/ψ$ and $Υ$ using sudden approximation. Our results show that quarkonium decay may be significant even when temperature of QGP remains low enough so that the conventional quarkonium melting due to Debye screening is ineffective.

hep-ph↗

arXiv2014

A Novel Mechanism for $J/ψ$ Disintegration in Relativistic Heavy Ion Collisions

In this paper we discuss the possibility of $J/ψ$ disintegration due the $Z(3)$ domain walls that are expected to form in QGP medium. These domain walls give rise to localised color electric field which disintegrates $J/ψ$, on interaction, by changing the color composition and simultaneously exciting it to higher states of $c\bar{c}$ system.

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arXiv2014

Spontaneous CP violating quark scattering from asymmetric $Z\left(3\right)$ interfaces in QGP

In this paper, we extend our earlier study of spontaneous CP violating scattering of quarks and anti-quarks from QCD $Z\left(3\right)$ domain walls for the situation when these walls have asymmetric profiles of the Polyakov loop order parameter $l(x)$. Dynamical quarks lead to explicit breaking of $Z(3)$ symmetry, which lifts the degeneracy of the $Z(3)$ vacua arising from spontaneous breaking of the $Z(3)$ symmetry in the quark-gluon plasma (QGP) phase. Resulting domain walls have asymmetric profile of $l(x)$ (under reflection $x \rightarrow -x$ for a domain wall centered at the origin). We calculate the background gauge field profile $A_0$ associated with this domain wall profile. Interestingly, even with the asymmetric $l(x)$ profile, quark-antiquark scattering from the corresponding gauge field configuration does not reflect this asymmetry. We show that the expected asymmetry in scattering arises when we include the effect of asymmetric profile of $l(x)$ on the effective mass of quarks and antiquarks and calculate resultant scattering. We discuss the effects of such asymmetric Z(3) walls in generating quark and antiquark density fluctuations in cosmology, and in relativistic heavy-ion collisions e.g. event-by-event baryon fluctuations.

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arXiv2014

Reviving quark nuggets as a candidate for dark matter

We discuss a novel mechanism for segregation of baryons and anti-baryons in the quark-gluon plasma phase which can lead to formation of quark and antiquark nuggets in the early universe, irrespective of the order of the quark-hadron phase transition. This happens due to CP violating scattering of quarks and antiquarks from moving $Z(3)$ domain walls. CP violation here is spontaneous in nature and arises from the nontrivial profile of the background gauge field ($A_{0}$ ) between different $Z(3)$ vacua. We study the effect of this spontaneous CP violation on the baryon transport across the collapsing large $Z(3)$ domain walls (which can arise in the context of certain low energy scale inflationary models). Our results show that this CP violation can lead to large concentrations of baryons and anti-baryons in the early universe. The quark and antiquark nuggets, formed by this alternate mechanism, can provide a viable dark matter candidate within standard model without violating any observational constraints.

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arXiv2012

Domain growth and fluctuations during quenched transition to QGP in relativistic heavy-ion collisions

We model the initial confinement-deconfinement transition in relativistic heavy-ion collisions as a rapid quench in view of expected rapid thermalization to a QGP state. The transition is studied using the Polyakov loop model, with the initial field configuration (in the confining phase) covering a small neighborhood of the confining vacuum $l \simeq 0$, as appropriate for $T < T_c$. Quench is implemented by evolving this initial configuration with the effective potential at a temperature $T > T_c$. We study the formation of Z(3) domain structure and its evolution during the transition as $l$ rolls down in different directions from the top of the central hill in the effective potential of $l$. When explicit Z(3) symmetry breaking effects (arising from dynamical quark effects) are small, then we find well defined Z(3) domains which coarsen in time. Remarkably, the magnitude plot of $l$ shows vacuum bubble like configurations arising during the quench. This first order transition like behavior occurs even though there is no metastable vacuum separated by a barrier from the true vacuum for the parameter values used. When the initial field configuration everywhere rolls down roughly along the same direction (as will happen with large explicit symmetry breaking) then we do not find such bubble-like configurations. However, in this case we find huge oscillations of $l$ with large length scales. We show that such large oscillations can lead to large fluctuations in the evolution of flow anisotropies compared to the equilibrium transition case.

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arXiv2012

Spontaneous CP violation in quark scattering from QCD Z(3) interfaces

In this paper, we explore the possibility of spontaneous CP violation in the scattering of quarks and anti-quarks from QCD Z(3) domain walls. The CP violation here arises from the nontrivial profile of the background gauge field $(A_{0})$ between different Z(3) vacua. We calculate the spatial variation of $A_{0}$ across the Z(3) interface from the profile of the Polyakov loop $L(\vec{x})$ for the Z(3) interface and calculate the reflection of quarks and antiquarks using the Dirac equation. This spontaneous CP violation has interesting consequences for the relativistic heavy-ion collision experiments, such as baryon enhancement at high $P_{T}$. It also acts as a source of additional J/$ψ$ suppression. We also discuss its implications for the early universe.

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arXiv2011

Analyzing flow anisotropies with excursion sets in relativistic heavy-ion collisions

We show that flow anisotropies in relativistic heavy-ion collisions can be analyzed using a certain technique of shape analysis of excursion sets recently proposed by us for CMBR fluctuations to investigate anisotropic expansion history of the universe. The technique analyzes shapes (sizes) of patches above (below) certain threshold value for transverse energy/particle number (the excursion sets) as a function of the azimuthal angle and rapidity. Modeling flow by imparting extra anisotropic momentum to the momentum distribution of particles from HIJING, we compare the resulting distributions for excursion sets at two different azimuthal angles. Angles with maximum difference in the two distributions identify the event plane, and the magnitude of difference in the two distributions relates to the magnitude of momentum anisotropy, i.e. elliptic flow.

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arXiv2011

Effects of quarks on the formation and evolution of Z(3) walls and strings in relativistic heavy-ion collisions

We investigate the effects of explicit breaking of Z(3) symmetry due to the presence of dynamical quarks on the formation and evolution of Z(3) walls and associated QGP strings within Polyakov loop model. We carry out numerical simulations of the first order quark-hadron phase transition via bubble nucleation (which may be appropriate, for example, at finite baryon chemical potential) in the context of relativistic heavy-ion collision experiments. Using appropriate shifting of the order parameter in the Polyakov loop effective potential, we calculate the bubble profiles using bounce technique, for the true vacuum as well as for the metastable Z(3) vacua, and estimate the associated nucleation probabilities. These different bubbles are then nucleated and evolved and resulting formation and dynamics of Z(3) walls and QGP strings is studied. We discuss various implications of the existence of these Z(3) interfaces and the QGP strings, especially in view of the effects of the explicit breaking of the Z(3) symmetry on the formation and dynamical evolution of these objects.

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arXiv2011

Probing the anisotropic expansion history of the universe with cosmic microwave background

We propose a simple technique to detect any anisotropic expansion stage in the history of the universe starting from the inflationary stage to the surface of last scattering from the CMBR data. We use the property that any anisotropic expansion in the universe would deform the shapes of the primordial density perturbations and this deformation can be detected in a shape analysis of superhorizon fluctuations in CMBR. Using this analysis we obtain the constraint on any previous anisotropic expansion of the universe to be less than about 35%.

astro-ph.CO↗

arXiv2011

Enhancement of flow anisotropies due to magnetic field in relativistic heavy-ion collisions

It is known that the presence of background magnetic field in cosmic plasma distorts the acoustic peaks in CMBR. This primarily results from different types of waves in the plasma with velocities depending on the angle between the magnetic field and the wave vector. We consider the consequences of these effects in relativistic heavy-ion collisions where very strong magnetic fields arise during early stages of the plasma evolution. We show that flow coefficients can be significantly affected by these effects when the magnetic field remains strong during early stages due to strong induced fields in the conducting plasma. In particular, the presence of magnetic field can lead to enhancement in the elliptic flow coefficient $v_2$.

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arXiv2010

Simulation of Z(3) walls and string production via bubble nucleation in a quark-hadron transition

We study the dynamics of confinement-deconfinement (C-D) phase transition in the context of relativistic heavy-ion collisions within the framework of effective models for the Polyakov loop order parameter. We study the formation of $Z(3)$ walls and associated strings in the initial transition from the confining (hadronic) phase to the deconfining (QGP) phase via the so called Kibble mechanism. Essential physics of the Kibble mechanism is contained in a sort of domain structure arising after any phase transition which represents random variation of the order parameter at distances beyond the typical correlation length. We implement this domain structure by using the Polyakov loop effective model with a first order phase transition and confine ourselves with temperature/time ranges so that the first order C-D transition proceeds via bubble nucleation, leading to a well defined domain structure. The formation of $Z(3)$ walls and associated strings results from the coalescence of QGP bubbles expanding in the confining background. We investigate the evolution of the $Z(3)$ wall and string network. We also calculate the energy density fluctuations associated with $Z(3)$ wall network and strings which decay away after the temperature drops below the quark-hadron transition temperature during the expansion of QGP. We discuss evolution of these quantities with changing temperature via Bjorken's hydrodynamical model and discuss possible experimental signatures resulting from the presence of $Z(3)$ wall network and associate strings.

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arXiv2009

Using CMBR analysis tools for flow anisotropies in relativistic heavy-ion collisions

Recently we have shown that there are crucial similarities in the physics of cosmic microwave background radiation (CMBR) anisotropies and the flow anisotropies in relativistic heavy-ion collision experiments (RHICE). We also argued that, following CMBR anisotropy analysis, a plot of root-mean square values of the flow coefficients, calculated in a lab fixed frame for RHICE, can yield important information about the nature of initial state anisotropies and their evolution. Here we demonstrate the strength of this technique by showing that elliptic flow for non-central collisions can be directly determined from such a plot without any need for the determination of event-plane.

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arXiv2008

Super-horizon fluctuations and acoustic oscillations in relativistic heavy-ion collisions

We focus on the initial state spatial anisotropies, originating at the thermalization stage, for central collisions in relativistic heavy-ion collisions. We propose that a plot of the root mean square values of the flow coefficients $\sqrt{\bar {v_n^2}} \equiv v_n^{rms}$, calculated in a lab fixed coordinate system, for a large range of $n$, from 1 to about 30, can give non-trivial information about the initial stages of the system and its evolution. We also argue that for all wavelengths $λ$ of the anisotropy (at the surface of the plasma region) much larger than the acoustic horizon size $H_s^{fr}$ at the freezeout stage, the resulting values of $v_n^{rms}$ should be suppressed by a factor of order $2H_s^{fr}/λ$. With initial flow being zero, we discuss the possibility that the resulting flow could show imprints of coherent oscillations in the plot of $v_n^{rms}$ for sub-horizon modes. For gold-gold collision at 200 GeV, these features are expected to occur for n $\ge$ 5, with $n < 4$ modes showing supression. This has strong similarities with the physics of the anisotropies of the cosmic microwave background radiation (CMBR) resulting from inflationary density fluctuations in the universe. It seems possible that the statistical fluctuations due to finite multiplicity may not be able to mask such features in the flow data, or, at least a non-trivial overall shape of the plot of $v_n^{rms}$ may be inferred. In that case, the successes of analysis of CMBR anisotropy power spectrum to get cosmological parameters can be applied for relativistic heavy-ion collisions to learn about various relevant parameters at the early stages of the evolving system.

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