arXiv subjects

Avijit K. Ganguly

Publications and source records attributed to Avijit K. Ganguly.

At least 19 recordsLinked to original sources

arXiv2022

Magnetized matter effects on dilaton photon mixing

Dilatons ($ϕ(x)$) are a class of bosonic scalar particles associated with scaling symmetry and its compensation (under the violations of the same). Due to two photon coupling, they can produce optical signatures in a magnetic field. In vacuum or plain matter they couple to one of the transversely polarized state of the photon. But in a magnetized matter, they couple to both the transversely polarized state of photon (due to emergence of a parity violating part of photon self energy contribution from a magnetized matter). A part of this work is directed towards understanding the issue of mixing of scalar with various polarizations states of photon in a medium ( magnetized or unmagnetized ) due to the constraints from different discrete (CPT) symmetries associated with the interaction. Based on these symmetry aided arguments, the structure of the mixing matrix is found to be $3 \times 3$. Thus there exists non-zero finite probabilities of oscillation between different polarization states of photon to dilaton. Our analytical and numerical analysis show no existence of periodic oscillation length either in temporal or spatial direction for most general values of the parameters in the theory. Possible astrophysical consequences of these results, those can be detected through observations are discussed.

hep-ph↗

arXiv2022

Differentiating Dilatons from Axions by their mixing with photons

According to the model ($Λ$CDM), based on deep cosmological observations, the current universe is constituted of 5$\%$ baryonic matter and 25 $\%$ non-baryonic cold dark matter (of speculative origin). These include quanta of scalar filed like dilaton($ϕ$) of scale symmetry origin and quanta of pseudoscalar field of extra standard model symmetry ( Peccei-Quinn) origin, like axion ($ϕ'$). These fields couple to di-photons through dim-5 operators. In magnetized medium, they in principle can interact with the three degrees of freedom (two transverse ($A_{\parallel,\perp}$) and one longitudinal ($A_{L}$)) of photon($γ$) as long as the total spin is conserved. Because of intrinsic spin being zero, both $ϕ$ and $ϕ'$ could in principle have interacted with $A_{L}$, (having $s_{z}=0$). However, out of $ϕ$ and $ϕ'$ only one interacts with $A_{L}$. Furthermore, the ambient external magnetic field and media, breaks the intrinsic Lorentz symmetry of the system invoking Charge conjugation, Parity and Time reversal symmetries, we analyse the mixing dynamics of $ϕγ$ and $ϕ'γ$ systems and the structural {\it difference} of their mixing pattern. The strength of electromagnetic (EM) signals due to $ϕγ$ and $ϕ'γ$ mixing as a result would be {\it different}. We conclude by commenting on the possibility of detecting this {\it difference} -- in polarimetric observables the EMS -- using the existing space-borne detectors.

hep-ph↗

arXiv2020

Exploring scalar-photon interactions in energetic astrophysical events

Scalar fields like dilaton appear in quantum field theory (QFT) due to scale symmetry breaking. Their appeal also extends to modified theories of gravity, like $F(R)$ gravity, Horva Lifshitz gravity etc. In unified theories they make their appearance through compactification of the extra dimension. Apart from resolving the issues of compactification scale and size, the particles of their fields can also turn out to be excellent candidate to solve the dark energy (DE) and dark matter (DM) problem of the universe. In this work we study their mixing dynamics with photons in a magnetized media, by incorporating the effect of parity violating part of the photon polarization tensor, evaluated in a finite density magnetized media. This piece, though in general is odd in the external magnetic field strength $eB$; in this work we however have retained terms to $O$($eB$). We are able to demonstrate in this work that, in magnetized medium a dilatonic scalar field $(ϕ)$ can excite the two transverse degrees of freedom (DOF) of the photons. One due to direct coupling and the other indirectly through the parity violating term originating due to magnetized medium effects. This results in the mixing dynamics being governed by, $3\times 3$ mixing matrices. This mixing results in making the underlying media optically active. In this work we focus on the spectro-polarimetric imprints of these particles, on the spectra of the electromagnetic (EM) fields of Gamma Ray Bursters (GRB). Focusing on a range of parameters (i.e., magnetic field strength, plasma frequency $(ω_{p})$, size of the magnetized volume, coupling strength to photons and their mass) we make an attempt to point out how space-borne detectors should be designed to optimise their detection possibility.

astro-ph.HE↗

arXiv2019

Mixing dynamics of dimension-five interactions (scalar/pseudoscalar-photon) in magnetized medium

In many extentions of standard model, dimension-5 scalar di-photon ($g_{γγϕ}ϕ$ $F^{μν}F_{μν}$ ) or pseudoscalar di-photon ($g_{γγa}a\tilde{F}^{μν} F_{μν} $, ) interaction materializes due to scale symmetry breaking or $U_A(1)$ symmetry breaking. In a magnetized vacuum (i.e., in an external background field $\bar{F}_{μν} $) the transverse degrees of freedom of the photons-- for such systems-- can be described in terms of the form factors constructed out of the background field strength tensor ($\bar{F}_{μν} $) and the same for dynamical photon ($f^{μν}$); they happen to be $\bar{F}_{μν}f^{μν}$ and ${\tilde{\bar{F}}_{μν}f^{μν}}$. These form factors transform differently under CP transformation. While $\bar{F}_{μν}f^{μν}$ (describing polarization orthogonal to B ($|γ_{||} >$)) is CP even, the other one, ${\tilde{\bar{F}}_{μν}f^{μν}}$( describing polarization along B ($|γ_\bot >$), is CP odd. In the interaction Lagrangian, if the scalar is interchanged with the pseudoscalar, the role of the two form factors just gets interchanged. Thus for nearly degenerate strengths of the coupling constants ( $g_{γγϕ}$ and $g_{γγ_a}$ ) and masses ($m_ϕ$ and $m_a$ ) of the respective candidates, proper identification of one from the other may become very difficult in laboratory or astrophysics based experiments. The basic motivation of this investigation is to reduce this uncertainty through incorporation of parity violating ({\it originating through magnetized medium effects }) part of the photon self-energy in the effective Lagrangian. This step, in turn affects the (Pseudo) Scalar Photon mixing dynamics drastically and brings out a significant change in the spectrum of the electromagnetic beam undergoing such interaction.

hep-ph↗

arXiv2016

Lorentz symmetry violating low energy dispersion relations from a dimension-five photon scalar mixing operator

Dimension-five photon $(γ)$ scalar $(ϕ)$ interaction terms usually appear in the bosonic sector of unified theories of electromagnetism and gravity. In these theories the three propagation eigenstates are different from the three field eigenstates. The dispersion relation in an external magnetic field shows that, for a non- zero energy $(ω)$, out of the three propagating eigenstates one has superluminal phase velocity $v_p$. During propagation, another eigenstate undergoes amplification or attenuation, showing signs of an unstable system. The remaining one maintains causality. In this paper, using techniques from optics as well as gravity, we identify the energy $(ω)$ interval outside which $v_p \le c$ for the field eigenstates $|γ_{\parallel} > $ and $ |ϕ> $, and stability of the system is restored. The behavior of group velocity $v_g$ is also explored in the same context. We conclude by pointing out its possible astrophysical implications.

hep-ph↗

arXiv2013

Signatures Of Scalar Photon Interaction In Astrophysical Situations

Dimension-5 photon ($γ$) scalar ($ϕ$) interaction terms usually appear in the Lagrangian of bosonic sector of unified theories of electromagnetism and gravity. This interaction makes the medium dichoric and induces optical activity. We have modelled the propagation of photons with this interaction in the environment of cold a magnetized compact star (White Dwarf (WD) or Neutron Star (NS)), assuming synchro-curvature process as the dominant mechanism of emission. We have tried to outline the polarimetric implications of photon-scalar coupling on the produced spectrum of these emissions in the stellar atmosphere. Further more assuming the 'emission-energy vs emission-altitude' relation, that is believed to hold in such (i.e. cold magnetized WD or NS) environments, we have tried to point out the possible modifications to the radiation spectrum when the same is incorporated along with dim-5 photon scalar mixing operator.

hep-ph↗

arXiv2008

Photon & Axion Oscillation In a Magnetized Medium: A Covariant Treatment

Pseudoscalar particles, with almost zero mass and very weak coupling to the visible matter, arise in many extensions of the standard model of particle physics. Their mixing with photons in the presence of an external magnetic field leads to many interesting astrophysical and cosmological consequences. This mixing depends on the medium properties, the momentum of the photon and the background magnetic field. Here we give a general treatment of pseudoscalar-photon oscillations in a background magnetic field, taking the Faraday term into account. We give predictions valid in all regimes, under the assumption that the frequency of the wave is much higher than the plasma frequency of the medium. At sufficiently high frequencies, the Faraday effect is negligible and we reproduce the standard pseudoscalar-photon mixing phenomenon. However at low frequencies, where Faraday effect is important, the mixing formulae are considerably modified. We explicitly compute the contribution due to the longitudinal mode of the photon and show that it is negligible.

hep-ph↗

arXiv2007

Self Interacting Dark Matter in the Solar System

Weakly coupled, almost massless, spin 0 particles have been predicted by many extensions of the standard model of particle physics. Recently, the PVLAS group observed a rotation of polarization of electromagnetic waves in vacuum in the presence of transverse magnetic field. This phenomenon is best explained by the existence of a weakly coupled light pseudoscalar particle. However, the coupling required by this experiment is much larger than the conventional astrophysical limits. Here we consider a hypothetical self-interacting pseudoscalar particle which couples weakly with visible matter. Assuming that these pseudoscalars pervade the galaxy, we show that the solar limits on the pseudoscalar-photon coupling can be evaded.

hep-ph↗

arXiv2005

Pseudoscalar Photon Mixing In A Magnetized Medium

Axions are pseudo-scalar particles, those arise because of breaking of Peccei Queen (PQ) symmetry. Axions have a tree level coupling to two photons. As a consequence there exists a tree level coupling of axion to photon in a magnetic field. However, in an external magnetic field, there exists a new loop induced, axion photon vertex, that gives rise to axion photon coupling. The strength of the tree level axion photon coupling in magnetic field is known to be model dependent. However in a magnetic field, the new loop induced coupling has some interesting features. This note discusses the new axion photon vertex in a magnetized medium and the corrections arising from there. The magnitude of the correction to axion photon coupling, because of magnetized vacuum and matter is estimated in this note. While making this estimate we note that the form of the axion photon vertex is related to the axial polarization tensor. This vertex is shown to satisfy the Ward identity. The coupling is shown to have a momentum dependent piece in it. Astrophysical importance of this extra modification is also pointed out.

hep-ph↗

arXiv2003

Optical Activity From Extra Dimension

Optical activity, like Faraday effect, is a rotation of the plane of polarization of propagating light in a medium and can be attributed to different sources with distinct signatures. In this note we discuss the effect of optical activity {\it{in vacuum}} due to Kaluza-Klein scalar field $ϕ$, in the presence of an external electro-magnetic field. The astrophysical implication of this effect is indicated. We also point out the possibility of observing the same in laboratory conditions.

astro-ph↗

arXiv2003

The Axialvector-Vector Amplitude and Neutrino Effective Charge in a Magnetized Medium

To one loop the effective neutrino photon interaction takes place through the vector-vector type and the axialvector-vector type amplitude. In this work we explicitly write down the form of the axialvector-vector amplitude to all orders in the external magnetic field in a medium. We then infer upon its zero external momentum limit which contributes to the effective charge of the neutrinos inside a magnetized medium. We further show its gauge invariance properties.

hep-ph↗

arXiv2002

Neutrino Photon Interaction in a Magnetized Medium II

In the presence of a thermal medium or an external electro-magnetic field, neutrinos can interact with photon, mediated by the corresponding charged leptons (real or virtual). The effect of a medium or an electromagnetic field is two-fold - to induce an effective $νγ$ vertex and to modify the dispersion relations of all the particles involved to render the processes kinematically viable. It has already been noted that in a medium neutrinos acquire an effective charge, which in the standard model of electroweak interaction comes from the vector type vertex of weak interaction. On the other hand in a magnetized plasma, the axial vector part also start contributing to the effective charge of a neutrino. This contribution corresponding to the axial vector part in the interaction Lagrangian is denoted as the axial polarisation tensor. In an earlier paper we explicitly calculated the form of the axial polarisation tensor to all odd orders in external magnetic field. In this note we complete that investigation by computing the same, to all even orders in external magnetic field. We further show its gauge invarience properties. Finally we infer upon the zero external momentum limit of this axial polarisation tensor.

hep-ph↗

arXiv2002

Effective Neutrino Photon Interaction in a Magnetized Medium

Neutrino-photon processes, forbidden in vacuum, can take place in presence of a thermal medium or an external electro-magnetic field, mediated by the corresponding charged leptons (real or virtual). The effect of a medium or an electromagnetic field is two-fold - to induce an effective $ν-γ$ vertex and to modify the dispersion relations of all the particles involved to render the processes kinematically viable. It has already been noted that in presence of a thermal medium such an electromagnetic interaction translates into the neutrino acquiring a small effective charge. In this work, we extend this concept to the case of a thermal medium in presence of an external magnetic field and calculate the effective charge of a neutrino in the limit of a weak magnetic field. We find that the effective charge is direction dependent which is a direct effect of magnetic field breaking the isotropy of the space.

hep-ph↗

arXiv2000

Absorption of Electro-magnetic Waves in a Magnetized Medium

In continuation to our earlier work, in which the structure of the vacuum polarisation tensor in a medium was analysed in presence of a background electro-magnetic field, we discuss the absorptive part of the vacuum polarization tensor. Using the real time formalism of finite temperature field theory we calculate the absorptive part of 1-loop vacuum polarisation tensor in the weak field limit ($eB < m^2$). Estimates of the absorption probability are also made for different physical conditions of the background medium.

hep-ph↗

arXiv1999

Faraday effect : a field theoretical point of view

We analyze the structure of the vacuum polarization tensor in the presence of a background electromagnetic field in a medium. We use various discrete symmetries and crossing symmetry to constrain the form factors obtained for the most general case. From these symmetry arguments, we show why the vacuum polarization tensor has to be even in the background field when there is no background medium. Taking then the background field to be purely magnetic, we evaluate the vacuum polarization to linear order in it. The result shows the phenomenon of Faraday rotation, i.e., the rotation of the plane of polarization of a plane polarized light passing through this background. We find that the usual expression for Faraday rotation, which is derived for a non-degenerate plasma in the non-relativistic approximation, undergoes substantial modification if the background is degenerate and/or relativistic. We give explicit expressions for Faraday rotation in completely degenerate and ultra-relativistic media.

hep-ph↗

arXiv1998

Renormalisation Group Improved Thermal Coupling Constant In An External Field

Starting from renormalised Effective Lagrangian, in the presence of an external Chromo-Electric field at finite temperature, the expression for thermal coupling constant ($α= (g^2)/(4 π)$) as a function of temperature and external field is derived, using finite temperature two parameter renormalisation group equation of Matsumoto, Nakano and Umezawa. For some values of the parameters, the coupling constant is seen to be approaching a value $\sim unity$.

hep-th↗

arXiv1998

Pre-Equilibrium Evolution of QCD Plasma :An Appraisal

We discuss, in this review, the existing models for the production of QGP in Relativistic Heavy Ion Collisions. We shall try to give a brief description of the models available for the pre-equilibrium production of QGP, ranging from soft process dominated QCD regime to the pQCD dominated regime. We also present the estimates for the initial energy density, the temperature and the thermalisation time -- available from each of these studies. Finally, we shall address the questions regarding thermalisation, color equilibration and chemical equilibration of the produced QGP in different experimental facilities.

nucl-th↗

arXiv1998

Comment on Fermionic and Bosonic Pair-creation in an External Electric Field at Finite Temperature

We show that contrary to the claim made by Hallin and Liljenberg in Phys. Rev. D52 1150,(1995), (hep-th/9412188) the thermal correction to the thermal decay or pair production rate for a system placed in a heat bath in the presence of an external electric field, is always nonzero in the finite as well as infinite time limit. Using the formalism outlined there, we reestimate the decay rate for different values of temperature, mass and time.We also try to identify the parameter ranges where the quantity of interest agrees with that computed previously, at high temperature (in the infinite time limit), from the imaginary part of the effective action using imaginary time and real time formalism of thermal field theory. We also point out that in the strictly infinite time limit, the correct decay rate as obtained from the work of Hallin et. al. tends to diverge.

hep-th↗