arXiv subjects

U. A. Yajnik

Publications and source records attributed to U. A. Yajnik.

At least 19 recordsLinked to original sources

arXiv2024

Left-Right Symmetry Breaking and Gravitational Waves : A Tale of Two Phase Transitions

We study possible ways gravitational waves (GW) are sourced in a theory with minimal left-right symmetry breaking. Generically first order phase transitions (FOPT) lead to gravitational waves sourced by bubble dynamics, while second order phase transitions (SOPT) do not. However, due the presence of two degenerate fields, we obtain domain walls in the putative SOPT case, giving rise to GW via disintegrating domain walls, testable at experiments such as IPTA, DECIGO, and LISA. On the other hand, for the case of FOPT, we get the usual signal from spontaneously created bubbles, but there also arises a late forming domain wall structure separating the two types of vacua. The disintegration of these walls provides an additional source of GW. Thus the parameter range signalling FOPT case gives rise to two distinct peaks in the spectrum of GW. This is verifiable for the low symmetry breaking scales $10^4 - 10^6$ GeV, but a high scale such as $\sim10^{10}$ GeV remains beyond the reach of currently planned experiments. Finally, we point out that a version of the left-right symmetric model which separates the scale of parity breaking from that of gauge symmetry breaking is also subject to domain wall formation and amenable to GW observations.

astro-ph.CO↗

arXiv2021

What is the Gravitational Field of a Mass in a Spatially Nonlocal Quantum Superposition?

The study of the gravitational field produced by a spatially non-local, superposed quantum state of a massive particle is a thrilling area of modern physics. One question to be answered is whether the gravitational field behaves as the classical superposition of two particles separated by a spatial distance with half the mass located at each position or as a quantum superposition with a far more interesting and subtle behaviour for the gravitational field. Quantum field theory is ideally suited to probe exactly this kind of question. We study the scattering of a massless scalar on such a spatially nonlocal, quantum superposition of a massive particle. We compute the differential scattering cross section corresponding to the interaction coming from the exchange of one graviton. We find that the scattering cross section is not at all represented by the Schrödinger-Newton picture of potential scattering from two localized sources with half the mass at each source. We discuss how our result would be lethal to the Schrödinger-Newton description of gravitation interacting with quantum matter and would be conducive to considering the gravitational field to be quantized. We comment on the experimental feasibility of observing such effects.

gr-qc↗

arXiv2021

Ferromagnetic instability in PAAI in the sky

We study an idealised plasma of fermions, coupled through an abelian gauge force $U(1)_X$, and which is asymmetric in that the masses of the oppositely charged species are greatly unequal. The system is dubbed PAAI, plasma asymétrique, abélien et idéalisé. It is argued that due to the ferromagnetic instability that arises, the ground state gives rise to a complex of domain walls. This complex being held together by stresses much stronger than cosmic gravity, does not evolve with the scale factor and along with the heavier oppositely charged partners simulates the required features of Dark Energy with mass scale for the lighter fermions in the micro-eV to nano-eV range. Further, residual $X$-magnetic fields through mixture with standard magnetic fields, can provide the seed for cosmic-scale magnetic fields. Thus the scenario can explain several cosmological puzzles including Dark Energy.

hep-th↗

arXiv2020

PAAI in the sky : towards a particulate mechanism for Dark Energy and concordant Dark Matter

We propose the origins of Dark Energy in a hidden sector with a pair of very light fermions, oppositely charged under an abelian gauge force $U(1)_X$ but of unequal mass. The system is dubbed PAAI, plasma which is abelian, asymmetric and idealised. For a range of the hidden fine structure constant values and the value of mass of the lightest fermion the PAAI is argued to simulate Dark Energy. Additional fermions from the same sector are shown to account for Dark Matter. Further, residual $X$-magnetic fields can mix with Maxwell electromagnetism to provides the seed for cosmic-scale magnetic fields. Thus the scenario can explain several cosmological puzzles from within the same hidden sector.

hep-ph↗

arXiv2019

Cosmic Ferromagnetism of Magninos

We study the physical conditions for the occurrence of ferromagnetic instability in a neutral plasma of fermions. We consider a system of two species $M$ and $Y$ which are oppositely charged under a local $U(1)_{X}$, with $M$ much lighter than $Y$. The leading correction to free quasiparticle behaviour for the lighter species arises from the exchange interaction, while the heavier species remain spectators. This plasma, which is abelian, asymmetric and idealised, is shown to be naturally susceptible to the formation of a completely spin-imbalanced ferromagnetic state for the lighter species (dubbed a magnino) in large parts of parameter space. It is shown that the domain structure formed by this ferromagnetic state can mimic Dark Energy, determining the masses of the two fermion species involved, depending on their abundance relative to the standard photons. Incomplete cancellation of the X-magnetic fields among the domains can give rise to residual long range $X$-magnetic fields. Under the assumption that this $U(1)_{X}$ mixes with Maxwell electromagnetism, this provides a mechanism for the seed for cosmic-scale magnetic fields. An extended model with several flavours $M_a$ and $Y_a$ of the species can incorporate Dark Matter. Thus the scenario shows the potential for explaining the large scale magnetic fields, and what are arguably the two most important outstanding puzzles of cosmology: Dark Matter and Dark Energy.

astro-ph.CO↗

arXiv2019

Vacuum Decay Induced by False Skyrmions

We consider the Skyrme model modified by the addition of mass terms which explicitly break chiral symmetry and pick out a specific point on the model's target space as the unique true vacuum. However, they also allow the possibility of false vacua, local minima of the potential energy. These false vacuum configurations admit metastable skyrmions, which we call false skyrmions. False skyrmions can decay due to quantum tunnelling, consequently causing the decay of the false vacuum. We compute the rate of decay of the false vacuum due to the existence of false skyrmions.

hep-th↗

arXiv2013

The Battle of the Bulge: Decay of the Thin, False Cosmic String

We consider the decay of cosmic strings that are trapped in the false vacuum in a theory of scalar electrodynamics in 3+1 dimensions. We restrict our analysis to the case of thin-walled cosmic strings which occur when large magnetic flux trapped inside the string. Thus the string looks like a tube of fixed radius, at which it is classically stable. The core of the string contains magnetic flux in the true vacuum, while outside the string, separated by a thin wall, is the false vacuum. The string decays by tunnelling to a configuration which is represented by a bulge, where the region of true vacuum within, is ostensibly enlarged. The bulge can be described as the meeting, of a kink soliton anti-soliton pair, along the length of the string. It can be described as a bulge appearing in the initial string, starting from the string of small, classically stable radius, expanding to a fat string of large, classically unstable (to expansion) radius and then returning back to the string of small radius along its length. This configuration is the bounce point of a corresponding O(2) symmetric instanton, which we can determine numerically. Once the bulge appears it explodes in real time. The kink soliton anti-soliton pair recede from each other along the length of the string with a velocity that quickly approaches the speed of light, leaving behind a fat tube. At the same time the radius of the fat tube that is being formed, expands (transversely) as it is no longer classically stable, converting false vacuum to the true vacuum with ever diluting magnetic field within. The rate of this expansion is determined by the energy difference between the true vacuum and the false vacuum. Our analysis could be applied to a network, of cosmic strings formed in the very early universe or vortex lines in a superheated superconductor.

hep-th↗

arXiv2013

Tunneling decay of false vortices

We consider the decay of vortices trapped in the false vacuum of a theory of scalar electrodynamics in 2+1 dimensions. The potential is inspired by models with intermediate symmetry breaking to a metastable vacuum that completely breaks a U(1) symmetry, while in the true vacuum the symmetry is unbroken. The false vacuum is unstable through the formation of true vacuum bubbles; however, the rate of decay can be extremely long. On the other hand, the false vacuum can contain metastable vortex solutions. These vortices contain the true vacuum inside in addition to a unit of magnetic flux and the appropriate topologically nontrivial false vacuum outside. We numerically establish the existence of vortex solutions which are classically stable; however, they can decay via tunneling. In general terms, they tunnel to a configuration which is a large, thin-walled vortex configuration that is now classically unstable to the expansion of its radius. We compute an estimate for the tunneling amplitude in the semi-classical approximation. We believe our analysis would be relevant to superconducting thin films or superfluids.

hep-th↗

arXiv2010

Fate of the false monopoles: induced vacuum decay

We study a gauge theory model where there is an intermediate symmetry breaking to a meta- stable vacuum that breaks a simple gauge group to a U (1) factor. Such models admit the existence of meta-stable magnetic monopoles, which we dub false monopoles. We prove the existence of these monopoles in the thin wall approximation. We determine the instantons for the collective coordinate that corresponds to the radius of the monopole wall and we calculate the semi-classical tunneling rate for the decay of these monopoles. The monopole decay consequently triggers the decay of the false vacuum. As the monopole mass is increased, we find an enhanced rate of decay of the false vacuum relative to the celebrated homogeneous tunneling rate due to Coleman [1].

hep-th↗

arXiv2008

Cosmology for Particle Physicists

In these notes we present a selection of topics, each section approximately amounting to one lecture. We begin with a brief recapitulation of General Relativity, and the Standard Model of Cosmology. This is followed by lectures on important signatures of the remote past. These include : (i) inflation, (ii) density perturbations leading to galaxy formation, (iii) study of hot and cold relics decoupled from the remaining constituents, some of which can be candidates for Dark Matter, (iv) baryon asymmetry of the Universe.

hep-ph↗

arXiv2004

B-L Cosmic strings and Baryogenesis

Cosmic strings arising from breaking of the $U(1)_{B-L}$ gauge symmetry that occurs in a wide variety of unified models can carry zero modes of heavy Majorana neutrinos. Decaying and/or repeatedly self-interacting closed loops of these ``$B-L$'' cosmic strings can be a non-thermal source of heavy right-handed Majorana neutrinos whose decay can contribute to the observed baryon asymmetry of the Universe (BAU) via the leptogenesis route. The $B-L$ cosmic strings are expected in GUT models such as SO(10), where they can be formed at an intermediate stage of symmetry breaking well below the GUT scale $\sim 10^{16}$ GeV; such light strings are not excluded by the CMB anisotropy data and may well exist. We estimate the contribution of $B-L$ cosmic string loops to the baryon-to-photon ratio of the Universe in the light of current knowledge on neutrino masses and mixings implied by atmospheric and solar neutrino measurements. We find that $B-L$ cosmic string loops can contribute significantly to the BAU for $U(1)_{B-L}$ symmetry breaking scale $η_{B-L}\gsim 1.7\times 10^{11}\gev$. At the same time, in order for the contribution of decaying $B-L$ cosmic string loops not to exceed the observed baryon-to-photon ratio inferred from the recent WMAP results, the lightest heavy right-handed Majorana neutrino mass $M_1$ must satisfy the constraint $M_1 \leq 2.4 \times 10^{12}(η_{B-L}/10^{13}\gev)^{1/2}\gev$. This may have interesting implications for the associated Yukawa couplings in the heavy neutrino sector and consequently for the light neutrino masses generated through see-saw mechanism.

hep-ph↗

arXiv2003

Leptogenesis with Left-Right domain walls

The presence of domain walls separating regions of unbroken $SU(2)_L$ and $SU(2)_R$ is shown to provide necessary conditions for leptogenesis which converts later to the observed Baryon aymmetry. The strength of lepton number violation is related to the majorana neutrino mass and hence related to current bounds on light neutrino masses. Thus the observed neutrino masses and the Baryon asymmetry can be used to constrain the scale of Left-Right symmetry breaking.

hep-ph↗

arXiv2002

Quantum Mechanical Spectra of Charged Black Holes

It has been argued by several authors, using different formalisms, that the quantum mechanical spectrum of black hole horizon area is discrete and uniformly spaced. Recently it was shown that two such approaches, namely the one involving quantisation on a reduced phase space, and the algebraic approach of Bekenstein and Gour are equivalent for spherically symmetric, neutral black holes (hep-th/0202076). The observables of one can be mapped to those of the other. Here we extend that analysis to include charged black holes. Once again, we find that the ground state of the black hole is a Planck size remnant.

hep-th↗

arXiv2002

Transient domain walls and lepton asymmetry in the Left-Right symmetric model

It is shown that the dynamics of domain walls in Left-Right symmetric models, separating respective regions of unbroken SU(2)_L and SU(2)_R in the early universe, can give rise to baryogenesis via leptogenesis. Neutrinos have a spatially varying complex mass matrix due to CP-violating scalar condensates in the domain wall. The motion of the wall through the plasma generates a flux of lepton number across the wall which is converted to a lepton asymmetry by helicity-flipping scatterings. Subsequent processing of the lepton excess by sphalerons results in the observed baryon asymmetry, for a range of parameters in Left-Right symmetric models.

hep-ph↗

arXiv2002

Black hole area quantization

It has been argued by several authors that the quantum mechanical spectrum of black hole horizon area must be discrete. This has been confirmed in different formalisms, using different approaches. Here we concentrate on two approaches, the one involving quantization on a reduced phase space of collective coordinates of a Black Hole and the algebraic approach of Bekenstein. We show that for non-rotating, neutral black holes in any spacetime dimension, the approaches are equivalent. We introduce a primary set of operators sufficient for expressing the dynamical variables of both, thus mapping the observables in the two formalisms onto each other. The mapping predicts a Planck size remnant for the black hole.

hep-th↗

arXiv2001

Discussion on a possible neutrino detector located in India

We have identified some important and worthwhile physics opportunitites with a possible neutrino detector located in India. Particular emphasis is placed on the geographical advantage with a stress on the complimentary aspects with respect to other neutrino detectors already in operation.

hep-ph↗

arXiv2001

Neutrino and astroparticle physics : Working group report

The contributions made to the Working Group activities on neutrino and astroparticle physics are summarised in this article. The topics discussed were leptogenesis in Left-Right symmetric model, inflationary models in Raman-Sundrum scenarios, ultra high energy cosmic rays and neutrino oscillations in 4 flavour and decaying neutrino models.

hep-ph↗

arXiv2001

Baryogenesis

Developments in understanding of Baryogenesis are reviewed. We start with early motivations and the proposals in the context of GUTs. Next, the importance of the sphaleron solution and its implications are discussed. Studies of the Standard Model reveal that the latter has a Higgs structure incompatible with existence of observed \B asymmetry. We then discuss a generic scenario for electroweak baryogenesis relying on bubble wall dynamics. We also summarise the status of the MSSM, and alternative scenarios utilising topological defects as the source of non-equilibrium behaviour and leptogenesis.

hep-ph↗