arXiv subjects

Urjit A. Yajnik

Publications and source records attributed to Urjit A. Yajnik.

At least 19 recordsLinked to original sources

arXiv2025

Relativistic quantum mechanics and quantum field theory

Relativistic quantum mechanics can be considered to have begun with a search for wave equations corresponding to each intrinsic spin. However, relativistic quantum physics differs fundamentally from the non-relativistic wave mechanics. It requires a formalism allowing \ creation and destruction of particles. This gets proper treatment only in a framework called quantum field theory. This article is a semi-historic account of the intriguing new features which emerge as a part of quantum field theory. Such a discussion is impossible without a basic presentation of the formalism itself. Hence some mathematics is included in finer print. The article is directed mostly to those familiar with essential classical mechanics and basic quantum mechanics, though I strive to provide a flavour of the subject to the keenly interested non-physics reader.

physics.hist-ph↗

arXiv2024

Gravitational production of massive scalars in the context of inflation

We set up a formalism for calculating the energy density generated in a quantized massive scalar field in the course of the drastic change in spacetime geometry at the end of the inflationary era. The calculation relies on the notion of adiabatic vacuum. The Bogolubov coefficients are computed by employing the sudden approximation. After obtaining a general formula, we calculate explicitly the energy density generated in a particle species with $m/H \ll1$, where $m$ is the particle mass and $H$ is the Hubble constant during the inflationary epoch. We find the contribution of the long-wavelength modes to be $\propto H^5/m$. If such particles are very weakly interacting, they can come to dominate the total energy density in the Universe. Other cosmological implications are also discussed.

astro-ph.CO↗

arXiv2024

Reheating constraints on modified quadratic chaotic inflation

The Reheating era of inflationary universe can be parameterized by various parameters like reheating temperature $T_{\text{re}}$, reheating duration $N_{\text{re}}$ and average equation of state parameter $\overline{ω}_{\text{re}}$, which can be constrained by observationally feasible values of scalar power spectral amplitude $A_{\text{s}}$ and spectral index $n_{\text{s}}$. In this work, by considering the quadratic chaotic inflationary potential with logarithmic-correction in mass, we examine the reheating era in order to place some limits on model's parameter space. By investigating the reheating epoch using Planck 2018+BK18+BAO data, we show that even a small correction can make the quadratic chaotic model consistent with latest cosmological observations. We also find that the study of reheating era helps to put much tighter constraints on model and effectively improves accuracy of model.

astro-ph.CO↗

arXiv2024

Collider Signatures of $W_R$ boson in the Alternative Left-Right Model

Alternative Left-Right Models offer an attractive option to left-right models. Emerging from $E_6$ grand unification, these models are consistent with light scalars which do not induce flavour-changing neutral currents due to the presence of exotic quarks. Here we investigate the signature at the LHC collider of the charged $W_R$ boson, which can be lighter than in left-right models. We include constraints from collider data and show that $W_R$ can be produced in pairs, or in conjunction with a light charged Higgs boson. The final decay products involve leptons or jets. We explore all production and decay possibilities and indicate which ones are most promising to be observed at the colliders. Our analysis shows that signals of $W_R$ bosons can be observed at the LHC at 27 TeV, some for lower luminosity, and under most favourable conditions, even at 13 TeV.

hep-ph↗

arXiv2024

Gravitational wave signature of generic disappearance of $Z_2$-symmetry breaking domain walls

Breaking of discrete parity at high scale gives rise to $Z_2$-domain walls (DW). The metastability of such walls can make them relatively long lived and contradict standard cosmology. We consider two classes of theories with similar underlying feature, the left right symmetric theories and two Higgs doublet models. Both of them possess some breaking of $Z_2$ discrete symmetries. As a first step, domains form at a high energy scale during parity breaking. In the second step, these domains further decompose into subdomains due to $Z_2$ symmetry breaking in two Higgs doublet models closer to the electroweak scale. We show that after this two step formation of domains and subdomains, a QCD instanton induced energy difference can remove the domain walls as well as the subdomain walls at around the same time successfully. The removal occurs purely as the result of a chance event taking place with probability very close to 0.25, and does not require one to introduce any non-renormalisable $Z_2$-symmetry breaking term to the Lagrangian. We then investigate the gravitational waves arising from the collapse of such domain walls and show that the peak frequency of these waves lies in the $10^{-7}$--$10^{-6}~\mbox{Hz}$ band, corresponding to annihilation temperatures of $1$--$10$ GeV. This frequency band is sensitive to pulsar timing array based experiments such as SKA and NANOGrav. The recent NANOGrav results rule out our DW collapse model for higher values of parity breaking scale above $10^7$ GeV. Our DW collapse model with parity breaking scales below $10^7$ GeV remains consistent with the current NANOGrav results and has a good chance of being seriously tested in future pulsar timing based experiments.

hep-ph↗

arXiv2023

Multi-charged moments and symmetry-resolved Rényi entropy of free compact boson for multiple disjoint intervals

We study multi-charged moments and symmetry-resolved Rényi entropy of free compact boson for multiple disjoint intervals. The Rényi entropy evaluation involves computing the partition function of the theory on Riemann surfaces with genus g>1. This makes Rényi entropy sensitive to the local conformal algebra of the theory. The free compact boson possesses a global U(1) symmetry with respect to which we resolve Rényi entropy. The multi-charged moments are obtained by studying the correlation function of flux-generating vertex operators on the associated Riemann surface. Symmetry-resolved Rényi entropy is then obtained from the Fourier transforms of the charged moments. Rényi entropy is shown to have the familiar equipartition into local charge sectors up to the leading order. The multi-charged moments are also essential in studying the symmetry resolution of mutual information. The multi-charged moments of the self-dual compact boson and massless Dirac fermion are also shown to match for the cases when the associated reduced density matrix moments are known to be the same. Finally, we numerically check our results against the tight-binding model.

hep-th↗

arXiv2023

Symmetry Resolved Entanglement Entropy in Hyperbolic de Sitter Space

In this paper, we study the relation between entanglement and global internal symmetries on de Sitter space. We consider two symmetric causally disconnected regions in the hyperbolic chart on de Sitter space. Since entanglement measures characterises correlations, the study of entanglement between the two causally disconnected regions gives information about the long range correlations in de Sitter space. When a theory possesses an additive global internal symmetry, the entanglement measures for a state with fixed global charge may be decomposed into local charge sectors in either subsystem and thus providing a finer resolution of entanglement. Here we will consider two theories: free complex scalar field, and free Dirac field on de Sitter space. Both theories possess global internal $U(1)$ symmetry. We study the symmetry resolved entanglement entropy for both theories in the Bunch-Davies vacuum state. We find that the symmetry resolved entanglement entropy has equipartition into local charge sectors upto the terms that scale as $V_{H_3}^0$ in the limit of large $V_{H_3}$, where $V_{H_3}$ is the volume of either region. This equipartition however is only broken by the terms of order $O(1/V_{H_3})$. Consequently, we have equipartition of symmetry resolved entanglement entropy in the limit of infinite volume.

hep-th↗

arXiv2023

Cogenesis of visible and dark sector asymmetry in a minimal seesaw framework

Recently there is a renewed interest in exploring the Dark sector of the universe in a more constrained way. Particularly in [1], the FIMP ( Feebly Interacting Massive Particle) scenario was shown to be realized with a minimal extension of the SM with three sterile neutrinos in the spirit of νMSM. In this paper, we show that without invoking any additional symmetries of the model, we can realize the idea of ADM (Asymmetric Dark Matter) signaling a common origin of the matter-anti-matter asymmetries in visible as well as Dark sectors. The model allows for a range of dark matter masses, 1 - 100keV, with correct active neutrino masses through the Type-I seesaw mechanism. Thus, the model explains the neutrino masses, Dark Matter abundance and replicates matter-anti-matter asymmetry of the visible sector in the Dark sector, all in a version of the νMSM. The asymmetry in the dark sector is manifested in the predominance of one parity of the heavy neutrinos in the comoving frame, which should be determinable in the ongoing experiments.

hep-ph↗

arXiv2023

Charge imbalance resolved Rényi negativity for free compact boson: Two disjoint interval case

In this paper, we study the symmetry decomposition of Rényi negativity into charge imbalance sectors for the 1+1 dimensional free compact boson field with a global U(1) symmetry in the ground state for the case of two disjoint intervals. We obtain multi-charged and charged Rényi negativity moments by computing the four-point correlator of flux-generating vertex operators on the Riemann surface. We then obtain charge imbalance resolved Rényi negativity by taking the Fourier transform of the charged moments. Finally, we match our results against the tight-binding model as a numerical check.

hep-th↗

arXiv2023

The seven laws of Quantum Mechanics : banishing the bogeys

The laws of quantum mechanics are couched in subtle mathematical language. The laws are not usually stated in a compact pedagogical form. Here I present a possible way to correct this. Essential facts can be distilled into seven statements that are easy to remember and easily referred back. Also, the current teaching of quantum mechanics is laden with words of negative connotations, originating as they did during the early decades of the subject when the subject was intellectually still puzzling. A wide variety of experiments in the intervening decades, not least those that were awarded the Nobel Prize of 2022 amply affirm the validity and substantial ``reality'' of Quantum Mechanics as a theory. I take a few of the inadequacies of classical framework to illustrate that some of the complaints against Quantum Mechanics are patently misplaced. Finally I discuss the bogeys such as ``wave article duality'', ``uncertainty'', ``indistinguishability'' ``statistics'' and ``entanglement'' and advocate adopting better terminology to save new learners from the old biases.

physics.hist-ph↗

arXiv2022

Dark Matter in the Alternative Left Right Model

The Alternative Left-Right Model is an attractive variation of the usual Left-Right Symmetric Model because it avoids flavour-changing neutral currents, thus allowing the additional Higgs bosons in the model to be light. We show here that the model predicts several dark matter candidates naturally, through introduction of an $R$-parity similar to the one in supersymmetry, under which some of the new particles are odd, while all the SM particles are even. Dark matter candidates can be fermionic or bosonic. We present a comprehensive investigation of all possibilities. We analyze and restrict the parameter space where relic density, direct and indirect detection bounds are satisfied, and investigate the possibility of observing fermionic and bosonic dark matter signals at the LHC. Both the bosonic and fermionic candidates provide promising signals, the first in LHC at 300 fb$^{-1}$, the second at higher luminosity, 3000 fb$^{-1}$. Signals from bosonic candidates are indicative of the presence of exotic $d^\prime$ quarks, while fermionic candidates imply the existence of charged Higgs bosons, all with masses in the TeV region.

hep-ph↗

arXiv2022

Primordial black holes from D-parity breaking in SO(10) grand unified theory

The growing evidence of gravitational waves from binary black hole mergers has renewed the interest in study of primordial black holes (PBH). Here we study a mechanism for the formation of PBH from collapse of pseudo-topological domain walls which form out of equilibrium during inflation and then collapse post inflation. We apply the study to domain wall formation due to $D$-parity embedded in a supersymmetric grand unified theory (GUT) based on $SO(10)$ and compare the abundance of resulting PBH with the existing constraints. Thus the macroscopic relics can then be used to constrain or rule out a GUT, or demand a refinement of the theory of PBH formation in this class of GUTs.

astro-ph.CO↗

arXiv2022

Neutrino mass and charged lepton flavor violation in an extended left-right symmetric model

We consider an $U(1)_{L_μ-L_τ}$ extended left-right symmetric gauge theory where the neutrino masses are generated through inverse seesaw mechanism. In this model the muon $(g-2)$ anomaly is accounted for by the mediation of $Z_{μτ}$, the gauge boson of $U(1)_{L_μ- L_τ}$ symmetry. The symmetries of the model require the light neutrino mass matrix to have a particular two-zero texture, which leads to non-trivial constraints in the minimum neutrino mass. In addition, the model predicts observable charged lepton flavor violation in $μ-τ$ sector.

hep-ph↗

arXiv2022

Determining neutrino mass hierarchy in an extended Left-Right model

We derive the lower bound on absolute scale of lightest neutrino mass for normal hierarchy and inverted hierarchy pattern of light neutrinos by studying the new physics contributions to charged lepton flavour violating decays and neutrinoless double beta decay in the framework of a TeV scale left-right symmetric model. Neutrino mass is generated in the model via type-II seesaw dominance with the help of a heavy sterile neutrino. This scenario allows large light-heavy neutrino mixing and the mixing facilitates new channels for neutrinoless double beta decay and lepton flavour violating decays. We express all the model parameters in terms of oscillation parameters in order to constrain light neutrino mass scale and extract information on neutrino mass hierarchy.

hep-ph↗

arXiv2022

Vacuum Structure of Alternative Left-Right Model

We investigate an Alternative Left-Right Model (ALRM) with $SU(2)_L$ as well as $SU(2)_R$ gauge groups, but unlike the traditional left-right symmetric models (LRSM) is not symmetric under the exchange of the fermion content. Interestingly, it can be embedded in $E_6$, while its low energy Higgs structure resembles the LRSM, involving Higgs doublets $χ_{L,R}$ and one Higgs bidoublet $Φ$. We analyze the scalar potential and the vacuum structure of the theory analytically to ensure the stability of scalar potential via bounded from below (BFB) and copositivity criteria, accompanied by a numerical study. We establish the necessary criteria for electric charge preserving vacua, yielding constraints on various coupling parameters of the theory. Finally we obtain constraints on the parameters of the model from collider data on the masses of the Higgs scalars.

hep-ph↗

arXiv2021

Effect of large light-heavy neutrino mixing and natural type-II seesaw dominance to lepton flavor violation and neutrinoless double beta decay

We derive the lower bound on the absolute scale of lightest neutrino mass for normal hierarchy and inverted hierarchy pattern of light neutrinos by studying the new physics contributions to charged lepton flavour violations in the framework of a TeV scale left-right symmetric model. In the model, the fermion sector comprises the usual quarks and leptons plus a fermion singlet per generation and the scalar sector consists of isospin doublets, triplets and a bidoublet. The framework allows large light-heavy neutrino mixing where the light neutrino mass formula is governed by a natural type-II seesaw mechanism, unlike the generic type-II seesaw dominance which assumes suppressed light-heavy neutrino mixing. We demonstrate how sizeable loop-induced contribution to light neutrino mass is kept under control such that the light neutrino mass formula is dominantly explained by the type-II seesaw mechanism. We examine the heavy neutrino contributions with large light-heavy neutrino mixing to charged lepton flavour violating processes like $μ\to e γ$, $μ\to 3 e$ and $μ\to e$ conversion inside a nucleus. We present a complementary study between neutrinoless double beta decay and charged lepton flavour violation taking into account single beta decay bound, double beta decay bound and cosmology bounds on the sum of light neutrino masses.

hep-ph↗

arXiv2020

Exploring $0νββ$ and Leptogenesis in the Alternative Left-Right Model

We investigate the possibility of neutrinoless double beta decay ($0νββ$) and leptogenesis within the Alternative Left-Right Model (ALRM). Unlike the usual left-right symmetric model, ALRM features a Majorana right-handed neutrino which does not carry any charge. Further, in this picture the down-type quark and the charged leptons receive mass through the additional left-handed scalar field, rather than the usual doublet. Together, these features conspire to generate significant contributions to the $0νββ$ through vector-scalar ($WH$) mediation. For moderate masses of the relevant charged Higgs boson ($M_{H_1^\pm}\sim 200$ GeV), the half-life of $T_{\frac{1}{2}}^{WH}$ is $\sim 3\times 10^{26}~{\rm yrs}$ for both the case of $^{76}$Ge and $^{136}$Xe, well within the sensitivity expected by future experiments. Invoking the resonant leptogenesis, CP violation arising from the right-handed neutrino decay could be the required order to generate the correct baryogenesis, $ε\sim 10^{-6}$, for small Dirac phases and without any fine tuning.

hep-ph↗

arXiv2020

Reheating constraints to modulus mass for single field inflationary models

We consider string and supergravity motivated scenarios in which moduli fields dominate the energy density of the Universe in a post-inflationary epoch. For the case of a single light modulus it has been shown that considering the evolution of a specific scale from the time of its Hubble crossing during inflation to the present time, a relation can be obtained among the lightest modulus mass, the reheating parameters ($T_{\text{reh}}$, $\bar{w}_{\text{reh}}$ and $N_{\text{reh}}$) and the inflationary observables. By paying closer attention to the role of the $\bar{w}_{\text{reh}}$, we obtain more stringent constraints on the value of the modulus mass and the reheating parameters using the CMB data. Next, the analysis is extended to include features in the inflaton potential as a source of CMB low multipole anomalies, which further constrains the mass of the modulus to be substantially higher than without such a constraint. By both considerations and for several inflation models considered, we find a constraint on the mass of the lightest modulus particle, $m_χ$, generically $\gtrsim10^{15}$GeV, with possible low values $\sim10^{12}$GeV. While a simplification of the reheating phase is assumed, the bounds are reliably suggestive, and the study may be taken as a demonstration that substantial knowledge about reheating phase buried deep in the early epochs of the Universe is accessible through the use of CMB observables today.

astro-ph.CO↗