arXiv subjects

Rakesh Tibrewala

Publications and source records attributed to Rakesh Tibrewala.

At least 19 recordsLinked to original sources

arXiv2025

Baryogenesis from a Majorana Fermion Coupled to Quarks

In the theory with a Majorana fermion ($X$) coupled to quark-like fermions ($Q$) via a dimension-six four-fermion vector-vector interaction, we have computed in an earlier work the baryon asymmetry generated in the decay and scattering processes of the $X$ with $Q$. In this work we consider such processes in the expanding early Universe, set up the Boltzmann equations governing the $X$ and net baryon number densities, and numerically solve them in example benchmark points, taking the thermally averaged decay and scattering rates and their temperature dependence from the earlier study. We find that starting from a baryon symmetric Universe at early time, the presently observed baryon asymmetry of the Universe (BAU) can be explained in this theory over a wide range of mass scales, $M_χ\in (10^4,10^{16})$ GeV for appropriately chosen couplings. We find that scattering processes play a crucial role in generating the baryon asymmetry in this theory. We present our results in a general manner that should be useful not just in our theory, but also in other related theories that share the essential ingredients. Our results should help guide promising ways to probe such new physics in terrestrial experiments. For instance, in regions of parameter space that yield the observed BAU, we present the rate for neutron-antineutron oscillation and discuss the prospects for observing this in upcoming experiments.

hep-ph↗

arXiv2025

Generalized Landau Yang Theorem

Landau Yang theorem is well known for the past several decades. It prohibits the decay of a massive spin 1 particle to two photons. This emerges simply from the representation theory of the Poincare group and Bose Statistics. It does not require any action or Lagrangian. We generalize this theorem to theories with supersymmetry (SUSY) which disallows even decay to two photinos (Majorana fermions) as well as the decay of a zino to a photon and a photino. We will prove that if the photon has a mass, howsoever small, this theorem can be evaded. We also show that the supersymmetric selection rule above can also be evaded through the Stueckelberg mass term. Further interesting implications are also pointed out.

hep-th↗

arXiv2025

Baryon Asymmetry from the Decay and Scattering of a Majorana Fermion Pair Coupled to Quarks

We compute the baryon asymmetry in decay and scattering processes involving the electromagnetically charge-neutral fermion $χ$ that carries nonzero baryon number and interacts with quark-like fermions $U,D$ via a vector-vector dimension-six effective operator, in the theory we developed in our earlier work. Majorana masses for the $χ$ break baryon number and split the Dirac fermion $χ$ into a pair of Majorana fermions $X_n$ with indefinite baryon number. We identify loop amplitudes for $X_n$ decay and scattering processes that are sensitive to the baryon number violation. The phases in the Majorana mass and couplings, in conjunction with the phase from intermediate onshell states, lead to $C$ and $CP$ violation in these processes. For some representative parameter choices, we numerically compute the decay and scattering baryon asymmetries between the process and its conjugate process, and find that the asymmetry generated is very interesting for explaining the baryon asymmetry of the Universe.

hep-ph↗

arXiv2023

Effective Theory for Baryogenesis with a Majorana Fermion Pair Coupled to Quarks

With a goal toward explaining the observed baryon asymmetry of the Universe, we extend the standard model (SM) by adding a vector-vector dimension-six effective operator coupling a new Dirac fermion $χ$, uncharged under the SM gauge symmetries but charged under baryon number, to a quark-like up-type fermion and two identical down-type fermions. We introduce baryon number violation by adding Majorana masses to $χ$, which splits the Dirac fermion into two Majorana fermions with unequal masses. We speculate on the origin of the effective operator, the Majorana mass, and the new physics sector connection to the SM, by considering some ultraviolet completion examples. In addition to the baryon number violation, we show that $C$ and $CP$ invariances can be violated in the theory, and the interference between tree and loop amplitudes with on-shell intermediate states can lead to a baryon asymmetry in $χ$ decay and scattering processes. We write down the Boltzmann equation for baryon number in the early Universe incorporating the decay and scattering baryon asymmetries. We provide numerical estimates for the baryon asymmetry generated, and for the neutron-antineutron oscillation rate.

hep-ph↗

arXiv2016

Midisuperspace quantization: possibilities for fractional and emergent spacetime dimensions

Recently, motivated by certain loop quantum gravity inspired corrections, it was shown that for spherically symmetric midisuperspace models infinitely many second derivative theories of gravity exist (as revealed by the presence of three arbitrary functions in the corresponding Lagrangian/Hamiltonian) and not just those allowed by spherically symmetric general relativity. This freedom can be interpreted as the freedom to accommodate certain quantum gravity corrections in these models even in the absence of higher curvature terms (at a semi-classical level, at least). For a particular choice of the arbitrary functions it is shown that the new theories map to spherically symmetric general relativity in arbitrary number of (integer) dimensions thus explicitly demonstrating that when working with midisuperspace models, one loses the information about the dimensionality of the full spacetime. In addition, it is shown that these new theories can accommodate scenarios of fractional spacetime dimensions as well as those of emergent spacetime dimensions -- a possibility suggested by various approaches to quantum gravity.

gr-qc↗

arXiv2015

Fermionic edge states and new physics

We investigate the properties of the Dirac operator on manifolds with boundaries in presence of the Atiyah-Patodi-Singer boundary condition. An exact counting of the number of edge states for boundaries with isometry of a sphere is given. We show that the problem with the above boundary condition can be mapped to one where the manifold is extended beyond the boundary and the boundary condition is replaced by a delta function potential of suitable strength. We also briefly highlight how the problem of the self-adjointness of the operators in the presence of moving boundaries can be simplified by suitable transformations which render the boundary fixed and modify the Hamiltonian and the boundary condition to reflect the effect of moving boundary.

hep-th↗

arXiv2015

New second derivative theories of gravity for spherically symmetric spacetimes

We present new second derivative, generally covariant theories of gravity for spherically symmetric spacetimes (general covariance is in the $t-r$ plane) belonging to the class where the spherically symmetric Einstein-Hilbert theory is modified by the presence of $g_{θθ}$ dependent functions. In $3+1$ dimensional vacuum spacetimes there is three-fold infinity of freedom in constructing such theories as revealed by the presence of three arbitrary $g_{θθ}$ dependent functions in the Hamiltonian (matter Hamiltonian also has the corresponding freedom). This result is not a contradiction to the theorem of Hojman et. al. [1] which is applicable to the full theory whereas the above conclusion is for symmetry reduced sector of the theory (which has a much reduced phase space). In the full theory where there are no special symmetries, the result of Hojman et. al. will continue to hold. In the process we also show that theories where the constraint algebra is deformed by the presence of $g_{θθ}$ dependent functions - as is the case in the presence of inverse triad corrections in loop quantum gravity - can always be brought to the form where they obey the standard (undeformed) constraint algebra by performing a suitable canonical transformation. We prove that theories obtained after performing canonical transformation are inequivalent to the symmetry reduced general relativity and that the resulting theories fall within the purview of the theories mentioned above.

gr-qc↗

arXiv2014

Zero modes and divergence of entanglement entropy

We investigate the cause of the divergence of the entanglement entropy for the free scalar fields in $(1+1)$ and $(D + 1)$ dimensional space-times. In a canonically equivalent set of variables, we show explicitly that the divergence in the entanglement entropy of the continuum field in $(1 + 1)-$ dimensions is due to the accumulation of large number of near-zero frequency modes as opposed to the commonly held view of divergence having UV origin. The feature revealing the divergence in zero modes is related to the observation that the entropy is invariant under a hidden scaling transformation even when the Hamiltonian is not. We discuss the role of dispersion relations and the dimensionality of the space-time on the behavior of entanglement entropy.

hep-th↗

arXiv2014

Spherically symmetric Einstein-Maxwell theory and loop quantum gravity corrections

Effects of inverse triad corrections and (point) holonomy corrections, occuring in loop quantum gravity, are considered on the properties of Reissner-Nordström black holes. The version of inverse triad corrections with unmodified constraint algebra reveals the possibility of occurrence of three horizons (over a finite range of mass) and also shows a mass threshold beyond which the inner horizon disappears. For the version with modified constraint algebra, coordinate transformations are no longer a good symmetry. The covariance property of spacetime is regained by using a \emph{quantum} notion of mapping from phase space to spacetime. The resulting quantum effects in both versions of these corrections can be associated with renormalization of either mass, charge or wave function. In neither of the versions, Newton's constant is renormalized. (Point) Holonomy corrections are shown to preclude the undeformed version of constraint algebra as also a static solution, though time-independent solutions exist. A possible reason for difficulty in constructing a covariant metric for these corrections is highlighted. Furthermore, the deformed algebra with holonomy corrections is shown to imply signature change.

gr-qc↗

arXiv2014

Inhomogeneities, loop quantum gravity corrections, constraint algebra and general covariance

Loop quantum gravity corrections, in the presence of inhomogeneities, can lead to a deformed constraint algebra. Such a deformation implies that the effective theory is no longer generally covariant. As a consequence, the geometrical concepts used in the classical theory lose their meaning. In the present paper we propose a method, based on canonical transformation on the phase space of the spherically symmetric effective theory, to systematically recover the classical constraint algebra in the presence of the inverse triad corrections as well as in the presence of the holonomy corrections. We show, by way of explicit example, that this also leads to the recovery of general covariance of the theory in the presence of inverse triad corrections, implying that one can once again use the geometrical concepts to analyze the solutions in the presence of these quantum gravity corrections.

gr-qc↗

arXiv2013

Numerical evaluation of the three-point scalar-tensor cross-correlations and the tensor bi-spectrum

Utilizing the Maldacena formalism and extending the earlier efforts to compute the scalar bi-spectrum, we construct a numerical procedure to evaluate the three-point scalar-tensor cross-correlations as well as the tensor bi-spectrum in single field inflationary models involving the canonical scalar field. We illustrate the accuracy of the adopted procedure by comparing the numerical results with the analytical results that can be obtained in the simpler cases of power law and slow roll inflation. We also carry out such a comparison in the case of the Starobinsky model described by a linear potential with a sudden change in the slope, which provides a non-trivial and interesting (but, nevertheless, analytically tractable) scenario involving a brief period of deviation from slow roll. We then utilize the code we have developed to evaluate the three-point correlation functions of interest (and the corresponding non-Gaussianity parameters that we introduce) for an arbitrary triangular configuration of the wavenumbers in three different classes of inflationary models which lead to features in the scalar power spectrum, as have been recently considered by the Planck team. We also discuss the contributions to the three-point functions during preheating in inflationary models with a quadratic minimum. We conclude with a summary of the main results we have obtained.

astro-ph.CO↗

arXiv2012

Modified constraint algebra in loop quantum gravity and spacetime interpretation

Classically the constraint algebra of general relativity, which generates gauge transformations, is equivalent to spacetime covariance. In LQG, inverse triad corrections lead to an effective Hamiltonian constraint which can lead to a modified constraint algebra. We show, using example of spherically symmetric spacetimes, that a modified constraint algebra does not correspond to spacetime coordinate transformation. In such a scenario the notion of black hole horizon, which is based on spacetime notions, also needs to be reconsidered. A possible modification to the classical trapping horizon condition leading to consistent results is suggested. In the case where the constraint algebra is not modified a spacetime picture is valid and one finds mass threshold for black holes and small corrections to Hawking temperature.

gr-qc↗

arXiv2011

LHC Signatures of a Vector-like b'

Many beyond the standard model extensions predict the existence of heavy vector-like fermions. We study the LHC signatures of one such heavy vector-like fermion, called b', with electromagnetic charge -1/3 like the SM b-quark, but which could generically have different SU(2)_L and U(1)_Y quantum numbers. Our emphasis will be on the phenomenology due to b <-> b' mass-mixing, present after electroweak symmetry breaking. We focus on aspects which distinguish a vector-like b' from a chiral b' and include tree-level decays of the b' into t W, b Z and b h final states. While our analysis is largely model-independent, we take as a motivating example warped-space models in which a vector-like b' appears as the custodial partner of the top-quark.

hep-ph↗

arXiv2011

Novel black hole bound states and entropy

We solve for the spectrum of the Laplacian as a Hamiltonian on $\mathbb{R}^{2}-\mathbb{D}$ and in $\mathbb{R}^{3}-\mathbb{B}$. A self-adjointness analysis with $\partial\mathbb{D}$ and $\partial\mathbb{B}$ as the boundary for the two cases shows that a general class of boundary conditions for which the Hamiltonian operator is essentially self-adjoint are of the mixed (Robin) type. With this class of boundary conditions we obtain "bound state" solutions for the Schroedinger equation. Interestingly, these solutions are all localized near the boundary. We further show that the number of bound states is finite and is in fact proportional to the perimeter or area of the removed \emph{disc} or \emph{ball}. We then argue that similar considerations should hold for static black hole backgrounds with the horizon treated as the boundary.

gr-qc↗

arXiv2011

Black-hole horizons in modified space-time structures arising from canonical quantum gravity

Several properties of canonical quantum gravity modify space-time structures, sometimes to the degree that no effective line elements exist to describe the geometry. An analysis of solutions, for instance in the context of black holes, then requires new insights. In this article, standard definitions of horizons in spherical symmetry are first reformulated canonically, and then evaluated for solutions of equations and constraints modified by inverse-triad corrections of loop quantum gravity. When possible, a space-time analysis is performed which reveals a mass threshold for black holes and small changes to Hawking radiation. For more general conclusions, canonical perturbation theory is developed to second order to include back-reaction from matter. The results shed light on the questions of whether renormalization of Newton's constant or other modifications of horizon conditions should be taken into account in computations of black-hole entropy in loop quantum gravity.

gr-qc↗

arXiv2009

Non-marginal LTB-like models with inverse triad corrections from loop quantum gravity

Marginal LTB models with corrections from loop quantum gravity have recently been studied with an emphasis on potential singularity resolution. This paper corroborates and extends the analysis in two regards: (i) the whole class of LTB models, including non-marginal ones, is considered, and (ii) an alternative procedure to derive anomaly-free models is presented which first implements anomaly-freedom in spherical symmetry and then the LTB conditions rather than the other way around. While the two methods give slightly different equations of motion, not altogether surprisingly given the ubiquitous sprawl of quantization ambiguities, final conclusions remain unchanged: Compared to quantizations of homogeneous models, bounces seem to appear less easily in inhomogeneous situations, and even the existence of homogeneous solutions as special cases in inhomogeneous models may be precluded by quantum effects. However, compared to marginal models, bouncing solutions seem more likely with non-marginal models.

gr-qc↗

arXiv2008

Lemaitre-Tolman-Bondi collapse from the perspective of loop quantum gravity

Lemaitre-Tolman-Bondi models as specific spherically symmetric solutions of general relativity simplify in their reduced form some of the mathematical ingredients of black hole or cosmological applications. The conditions imposed in addition to spherical symmetry turn out to take a simple form at the kinematical level of loop quantum gravity, which allows a discussion of their implications at the quantum level. Moreover, the spherically symmetric setting of inhomogeneity illustrates several non-trivial properties of lattice refinements of discrete quantum gravity. Nevertheless, the situation at the dynamical level is quite non-trivial and thus provides insights to the anomaly problem. At an effective level, consistent versions of the dynamics are presented which implement the conditions together with the dynamical constraints of gravity in an anomaly-free manner. These are then used for analytical as well as numerical investigations of the fate of classical singularities, including non-spacelike ones, as they generically develop in these models. None of the corrections used here resolve those singularities by regular effective geometries. However, there are numerical indications that the collapse ends in a tamer shell-crossing singularity prior to the formation of central singularities for mass functions giving a regular conserved mass density. Moreover, we find quantum gravitational obstructions to the existence of exactly homogeneous solutions within this class of models. This indicates that homogeneous models must be seen in a wider context of inhomogeneous solutions and their reduction in order to provide reliable dynamical conclusions.

gr-qc↗

arXiv2008

Classical and Quantum Gravitational Collapse in d-dim AdS Spacetime II. Quantum States and Hawking Radiation

In a previous paper we studied the collapse of a spherically symmetric dust distribution (marginally bound LTB) in d-dimensional AdS spacetime and obtained the condition for the formation of trapped surfaces. Here we extend the analysis by giving the canonical theory for the same and subsequently quantize the system by solving the Wheeler-DeWitt equation. We show that for the case of small dust perturbations around a black hole the wave functionals so obtained describe an AdS-Schwarzschild black hole in equilibrium with a thermal bath at Hawking temperature and show the non-trivial dependence of this temperature on the number of spacetime dimensions and the cosmological constant.

gr-qc↗