arXiv subjects

Urjit A. Yajnik

Publications and source records attributed to Urjit A. Yajnik.

At least 37 records · Page 2Linked to original sources

arXiv2020

New ultraviolet operators in supersymmetric SO(10) GUT and consistent cosmology

We consider the minimal supersymmetric grand unified model (MSGUT) based on the group $\mathrm{SO}(10)$, and study conditions leading to possible domain wall (DW) formation. It has been shown earlier that the supersymmetry preserving vacuum expectation values (vev's) get mapped to distinct but degenerate set of vev's under action of $D$ parity, leading to formation of domain walls as topological pseudo-defects. The metastability of such walls can make them relatively long lived and contradict standard cosmology. Thus we are led to consider adding a nonrenormalisable Planck scale suppressed operator, that breaks $\mathrm{SO}(10)$ symmetry but preserves Standard Model symmetry. For a large range of right handed breaking scales $M_R$, this is shown to give rise to the required pressure difference to remove the domain walls without conflicting with consistent big bang nucleosynthesis (BBN) while avoiding gravitino overproduction. However, if the walls persist till the onset of weak (thermal) inflation, then a low $\sim 10 - 100$ TeV $M_R$ becomes problematic.

hep-ph↗

arXiv2020

Evolution of black hole shadow in the presence of ultralight bosons

Kerr black holes coupled to quantized bosonic fields display a special version of the Hawking effect, governed by the superradiance condition. This leads to rapid growth of boson cloud through spontaneous creation, leading to slowing down of the black hole, and detectable as growth of the black hole shadow. This can be developed into a technique for searching or constraining the existence of ultralight bosons. We study this phenomenon for spin-0 bosons in the shadow of a black hole, with a detailed analysis of Sgr$A^*$, and put estimates on the evolution time scales and subsequent change in the black hole shadow features. Our study shows that there is a small window of parameters where the increase of shadow of a supermassive black hole may be visible, but only if the sensitivity of measurements increases from current 25 $μ$as to about 0.1 $μ$as.

gr-qc↗

arXiv2019

$0νββ$ in left-right theories with Higgs doublets and gauge coupling unification

We consider a version of Left-Right Symmetric Model in which the scalar sector consists of a Higgs bidoublet ($Φ$) with $B-L=0$, Higgs doublets ($H_{L,R}$) with $B-L=1$ and a charged scalar ($δ^+$) with $B-L=2$ leading to radiatively generated Majorana masses for neutrinos and thereby, leads to new physics contributions to neutrinoless double beta decay ($0νββ$). We show that such a novel framework can be embedded in a non-SUSY $SO(10)$ GUT leading to successful gauge coupling unification at around $10^{16}$ GeV with the scale of left-right symmetry breaking around $10^{10}$ GeV. The model can also be extended to have left-right symmetry breaking at TeV scale, enabling detection of $W_R, Z_R$ bosons in LHC and future collider searches. In the context of neutrinoless double beta decay, this model can saturate the present bound from GERDA and KamLAND-Zen experiments. Also, we briefly explain how keV-MeV range RH neutrino arising from our model can saturate various astrophysical and cosmological constraints and can be considered as warm Dark Matter (DM) candidate to address various cosmological issues. We also discuss on left-right theories with Higgs doublets without having scalar bidoublet leading to fermion masses and mixings by inclusion of vector like fermions.

hep-ph↗

arXiv2019

The comprehensive theory of light

Modern attempts to understand light go back to Newton who considered light to be particles, the so called corpuscular theory, and the other school of Huygens, Young and others. Huygens and Young viewpoint emphasised the wave property. This difference of opinions persisted for close to two centuries till Maxwell theory solidly established light as a wave phenomenon associated with Electromagnetism. A serious schism so to speak was introduced into the theory of light with the understanding of "Light gas", the so called Black Body radiation. Einstein's photon hypothesis and Bose's successful derivation of the Planck spectrum put the corpuscular theory back in place, while classical description continues to be used as well. One may then wonder if this being a classical limit may have to be corrected when dealing with quantum phenomena. To most people's surprise, the classical description was in fact a subset of the full quantum description, and the classical states of light could be shown to be subsumed within the fully quantum description without having to take the limit of Planck's constant h going to zero. This was the resolution provided by E. C. G. Sudarshan's Diagonal representation as the most general formalism for dealing with light.

physics.hist-ph↗

arXiv2019

Bose's derivation, the crucial link to quantum mechanics

In this brief note I address the question not frequently asked, namely, why did it take two decades between Einstein's first proposal of photons and derivation of the full Planck formula from first principles of Statistical Mechanics, albeit with a fundamental new approach to counting of states. Secondly, why did it fall to an independent inquirer, S. N. Bose, in far away Dacca to arrive at the correct derivation of this formula, arguably a most crucial one of the first half of the twentieth century? Reasonable hypotheses are proposed for answers to both. I also argue that the timing of Bose's communication to Einstein played a crucial role in Einstein's approval of de Broglie's thesis and hence the emergence of the definitive version of quantum mechanics in the late 1920's.

physics.hist-ph↗

arXiv2018

Reconciling low multipole anomalies and reheating in single field inflationary models

Reheating phase of inflationary Universe can be modeled by parameters $T_{\text{reh}}$, $\bar{w}_{\text{reh}}$ and $N_{\text{reh}}$, which can be constrained by the scalar spectral amplitude $A_{s}$ and the scalar spectral index $n_{s}$. On the other hand the low multipole anomalies in the CMB can be modeled by suitable features in the inflaton potential. We show that the parameters of these features in the inflaton potential provide additional constraints on the reheating parameters. For several single field models we find that the reheating parameters are substantially more constrained by the requirement of compatibility with the proposed explanation for low multipole anomalies.

astro-ph.CO↗

arXiv2018

Topological pseudodefects of a supersymmetric $SO(10)$ model and cosmology

Obtaining realistic supersymmetry preserving vacua in the minimal renormalizable supersymmetric $Spin(10)$ GUT model introduces considerations of the non-trivial topology of the vacuum manifold. The $D$-parity of low energy unification schemes gets lifted to a one-parameter subgroup $U(1)_D$ of $Spin(10)$. Yet, the choice of the fields signaling spontaneous symmetry breaking leads to disconnected subsets in the vacuum manifold related by the $D$-parity. The resulting domain walls, existing due to topological reasons but not stable, are identified as topological pseudodefects. We obtain a class of one-parameter paths connecting $D$-parity flipped vacua and compute the energy barrier height along the same. We consider the various patterns of symmetry breaking which can result in either intermediate scale gauge groups or a supersymmetric extension of the Standard Model. If the onset of inflation is subsequent to GUT breaking, as could happen also if inflation is naturally explained by the same GUT, the existence of such pseudodefects can leave signatures in the CMB. Specifically, this could have an impact on the scale invariance of the CMB fluctuations and LSS data at the largest scale.

hep-ph↗

arXiv2017

Non-perturbative to perturbative QCD via the FFBRST

Recently a new type of quadratic gauge was introduced in QCD in which the degrees of freedom are suggestive of a phase of abelian dominance. In its simplest form it is also free of Gribov ambiguity. However this gauge is not suitable for usual perturbation theory. The finite field dependent BRST (FFBRST) transformation is a method established to interrelate generating functionals for different effective versions of gauge fixed field theories. In this paper we propose a FFBRST transformation suitable for transforming the theory in the new quadratic gauge into the standard Lorenz gauge Faddeev-Popov version of the effective lagrangian. The task is made interesting by the fact the BRST invariance obeyed by the two effective lagrangians are not the same, however suitable extension of the previous procedures accomplishes the required result. We are thus able to identify a field redefinition to go from a non-perturbative phase of QCD to perturbative QCD.

hep-th↗

arXiv2017

Tunneling decay of false vortices with gravitation

We study the effect of vortices on the tunneling decay of a symmetry-breaking false vacuum in three spacetime dimensions with gravity. The scenario considered is one in which the initial state, rather than being the homogeneous false vacuum, contains false vortices. The question addressed is whether, and, if so, under which circumstances, the presence of vortices has a significant catalyzing effect on vacuum decay. After studying the existence and properties of vortices, we study their decay rate through quantum tunneling using a variety of techniques. In particular, for so-called thin-wall vortices we devise a one-parameter family of configurations allowing a quantum-mechanical calculation of tunneling. Also for thin-wall vortices, we employ the Israel junction conditions between the interior and exterior spacetimes. Matching these two spacetimes reveals a decay channel which results in an unstable, expanding vortex. We find that the tunneling exponent for vortices, which is the dominant factor in the decay rate, is half that for Coleman-de Luccia bubbles. This implies that vortices are short-lived, making them cosmologically significant even for low vortex densities. In the limit of the vanishing gravitational constant we smoothly recover our earlier results for the decay of the false vortex in a model without gravity.

hep-th↗

arXiv2017

Why PeV scale left-right symmetry is a good thing

Left-right symmetric gauge theory presents a minimal paradigm to accommodate massive neutrinos with all known conserved symmetries duly gauged. The work presented here is based on the argument that the see-saw mechanism does not force the new right handed symmetry scale to be very high, and as such some of the species from the spectrum of the new gauge and Higgs bosons can have masses within a few orders of magnitude of the TeV scale. The scale of the left-right parity breaking in turn can be sequestered from the Planck scale by supersymmetry. We have studied several formulations of such Just Beyond Standard Model (JBSM) theories for their consistency with cosmology. Specifically the need to eliminate phenomenologically undesirable domain walls gives many useful clues. The possibility that the exact left-right symmetry breaks in conjunction with supersymmetry has been explored in the context of gauge mediation, placing restrictions on the available parameter space. Finally we have also studied a left-right symmetric model in the context of metastable supersymmetric vacua and obtained constraints on the mass scale of Right handed symmetry. In all the cases studied, The mass scale of right handed neutrino $M_R$ remains bounded from above, and in some of the cases the scale $10^9$ GeV favourable for supersymmetric thermal leptogenesis is disallowed. On the other hand PeV scale remains a viable option, and the results warrant a more detailed study of such models for their observability in collider and astroparticle experiments.

hep-ph↗

arXiv2017

The conception of photons

In the year 1900 Max Planck was led by experimental observations to propose a strange formula for the intensity as a function of frequency for light emitted by a cavity. It relied on peculiar properties to be obeyed by the emitters and absorbers in the cavity. I highlight the mathematically suggestive nature of the formula, accessible even to a high school student, that could have provided a clue to the physical reasoning of Planck. In 1905, Einstein made the bold photon hypothesis and was able to predict the formula for the photoelectric effect which was not fully explored then. The first part of this article concerns the period 1905 to 1923 with a possible explanation for the difficulties in acceptance of the new concept. In the second part of the article I present how S. N. Bose's 1924 paper provided a systematic derivation of Planck formula by consistently adopting the conception of photons within Boltzmann's framework. In conclusion we discuss the implications of Quantum Mechanics that eventually emerged, showing that the seeds of some of its uncanny conceptual content were already foreshadowed in Einstein's proposal.

physics.hist-ph↗

arXiv2016

Stability longevity and all that : false vacua and topological defects

I present two interesting studies related to the role of solitons in theories with spontaneous symmetry breaking. Quantised fermions coupled to solitons are known to induce fractional fermion number. I present an example where an unstable topological solution binding a zero mode of a majorana fermion gets stabilised due to the induced fractional number. Thus neither is the cosmic string stable nor is the fermion number conserved, yet the bound state then becomes stable due to Quantum Mechanics. The other phenomenon concerns a metastable vacuum getting destabilised due to the presence of a cosmic string. This happens because the scalar field signalling spontaneous symmetry breaking acquires a vacuum expectation value approaching its true vacuum value in the core of the string. Thus the string acts like a seed nucleating the true vacuum. The instanton mediating between the false vacuum and the exit point into the true vacuum is shown to exist and graphically displayed through a numeric calculation. T. Padmanabhan has been a long time friend and was collaborator for early investigations in this direction. I review these works done subsequently with other collaborators as a felicitation to him on his sixtieth birthday.

hep-th↗

arXiv2014

Production and decay rates of excited leptons in a left-right symmetric scenario

We merge two leading Beyond Standard Model scenarios, namely compositeness and left-right symmetry, and probe the resulting collider signatures in the leptonic case. The constraints on composite models for fermions leave open the possibility of vector like excitations of Standard Model (SM) fermions. Here we consider the possibility of low scale left-right gauge symmetry $SU(2)_{R} \times SU(2)_{L} \times U(1)_ {B-L}$, with the simplifying assumption that the right like excited sector of fermions is significantly heavier than the excitations of the left chiral fermions. It is found that the right handed currents still contribute to observable processes, and alter the existing bounds on the scale of compositeness. The cross section times branching ratio of the photon decay channel is strongly depressed, bringing down the exclusion limit of the mass of excited electrons from about 2 TeV to below 1 TeV. On the other hand, cross section times branching ratio of the Z decay channel is significantly enhanced and remains greater than that of the photon channel. We thus propose analyzing the Z decay channel in existing collider data in order to search for signature of left-right symmetry as well as excited leptons with masses above 1 TeV.

hep-ph↗

arXiv2014

Infra-red Abelian dominance without Abelian-projection

Maximal Abelian gauge has been a particular choice to study dynamical generation of off-diagonal gluon masses in QCD. This gauge is a special case of Abelian projection. Massive off-diagonal gluons are considered a clue to Abelian dominance. Here we propose a gauge condition which is quadratic in fields and which does not fall in the class of Abelian projection. However it does generate off-diagonal gluon masses dynamically thus hinting at Abelian dominance in this gauge too.

hep-th↗

arXiv2014

Spontaneous parity breaking with broken supersymmetry : cosmological constraint

Unified models incorporating the right handed neutrino in a symmetric way generically possess parity symmetry. If this is broken spontaneously it results in the formation of domain walls in the early Universe, whose persistence is unwanted. A generic mechanism for destabilisation of such walls is a small pressure difference signalled by difference in the free energy across the walls. It is interesting to explore the possibility of such effects in conjunction with the effects that break supersymmetry in a phenomenologically acceptable way. Realising this possibility in the context of several scenarios of supersymmetry breaking results in an upper bound on the scale of spontaneous parity breaking, often much lower than the GUT scale. In the left-right symmetric models studied, the upper bound is no higher than $10^{11}$GeV but a scale as low as $10^5$GeV is acceptable.

hep-th↗

arXiv2012

Spontaneous parity breaking and supersymmetry breaking in metastable vacua with consistent cosmology

We study the compatibility of spontaneous breaking of parity and successful cosmology in a left-right symmetric model where supersymmetry breaking is achieved in metastable vacua. We show that domain walls formed due to this breaking can be removed due to Planck scale suppressed terms, provided the parity breaking scale $M_R$ is constrained to remain smaller than $10^{10}-10^{11}$ GeV. Ensuring metastability is achieved naturally even if the entire mechanism operates at low scales, within a few orders of magnitude of the TeV scale. Taking $M_R$ as high as permitted, close to the acceptable reheat temperature after inflation, would require the magnetic phase of the Supersymmetric Quantum Chromodynamics (SQCD) to have set in before the end of inflation.

hep-ph↗

arXiv2011

Supersymmetric Left-Right models with Gauge Coupling Unification and Fermion Mass Universality

We explore the unification of gauge couplings and fermion masses in two different types of supersymmetric left-right models, one with Higgs triplets and the other with both Higgs triplets as well as bitriplets. The minimal versions of these models do not give rise to the desired unification and some extra fields have to be added. After such a modification, it is possible in one model to get gauged $B-L$ symmetry to be unbroken down to TeV scale. We also identify the parameter space at the electroweak scale which gives rise to fermion mass unification at a high scale $M_{G}$. Type I seesaw emerges as the natural explanation of the small neutrino masses in both the models.

hep-ph↗

arXiv2011

Dark Energy from ferromagnetic condensation of cosmic magninos

It is proposed that an ultra-light fermionic species, dubbed cosmic magnino has condensed into a ferromagnetic state in the Universe. The extended structure of domain walls associated with this ferromagnetism accounts for the observed Dark Energy. In modification of the situation with an electron gas, it is proposed that the Stoner criterion is satisfied due to magnetic dipolar repulsion. The cosmological requirements then yeild a lower bound on the magnetic moment of the cosmic magnino. The proposed magnetism is supposed to be associated with a new non-standard electromagnetism. If the magnino is also electrically charged under this electromagnetism, the corresponding oppositely charged heavier species would account partially or entirely for the Dark Matter in the Universe.

astro-ph.CO↗