Rabindra N. Mohapatra

The minimal scotogenic model where small neutrino masses arise via radiative seesaw, is known to provide a unified framework for neutrino mass and origin of matter via leptogenesis. However if the inflation reheat temperature of the universe is below the sphaleron decoupling temperature, then leptogenesis fails and one way to understand the origin of matter would be to add an effective interaction involving the right-handed neutrino (RHN) N of the form 1 Λ 2 N u R d R d R . This model can lead to observable neutron-antineutron ( n − n ¯ ) oscillation if N is a Majorana fermion. We show that if RHNs are produced nonthermally, we can get a cosmological upper limit on the transition time τ n − n ¯ , which is within the reach of the planned European spallation source high-intensity baryon extraction and measurement/neutron-antineutron experiment. Proton stability is guaranteed by the Z 2 invariance, which prevents the appearance of the Dirac mass term for the neutrino.

Experimental searches for neutron-antineutron ( n → n ¯ ) transitions can be considered via two approaches: conversion in free-neutron beams and intranuclear transformation leading to matter instability in large-mass detectors. Plans for next-generation searches make it timely to highlight the complementarity, necessity, and limitations of each method. Converting the bound neutron limit into one for free neutrons traditionally utilizes nucleus-specific estimates of the in-medium suppression of n → n ¯ , obtained within mean-field theory under a single-operator assumption. This paper highlights how this suppression can be scenario-dependent, which can lead to deviations from the standard approach that can span several orders of magnitude. A further goal of the paper is to point out the need for a broader phenomenology program for n → n ¯ that is akin to those developed for electric dipole moments and other systems for which short-distance new physics must be studied in medium. We find that moderate deviations from the standard suppression factor arise generically via multioperator interference effects with the largest enhancements corresponding to regions of parameter space with partial phase alignment between operator contributions.

A bstract: We construct explicit models that solve the axion quality problem originating from quantum gravitational effects. The general strategy we employ is to supplement the Standard Model and its grand unified extensions by an anomaly-free axial U(1) _a symmetry that is gauged. We show that for several choices of the gauge quantum numbers of the fermions, this setup leads to an accidental U(1) symmetry with a QCD anomaly which can be identified as the Peccei-Quinn (PQ) symmetry that solves the strong CP problem. The U(1) _a gauge symmetry controls the amount of explicit PQ symmetry violation induced by quantum gravity, resulting in a high quality axion. We present two classes of models employing this strategy. In the first class (models I and II), the axial U(1) _a gauge symmetry acts on vector-like quarks leading to an accidental KSVZ-type axion. The second class (model III) is based on SO(10) grand unified theory extended by a gauged U(1) _a symmetry that leads to a hybrid KSVZ-DFSZ type axion. The couplings of the axion to the electron and the nucleon are found to be distinct in this class of hybrid models from those in the KSVZ and DFSZ models, when the axion is identified as the dark matter of the universe, which can be used to test these models. Interestingly, all models presented here have domain wall number of one, which is free of cosmological problems that typically arise in axion models.

Why matter and dark matter contents of the Universe are of the same order of magnitude, is one of the puzzles of modern cosmology. At the face of it, this would seem to point toward a basic similarity between matter and dark matter, suggesting perhaps the widely discussed mirror world picture as an ideal setting for a discussion of this issue. Here we outline a new and simple mirror world scenario to explain this puzzle. Our model uses the Affleck-Dine mechanism to generate baryon asymmetry and dark matter relic density leading to an asymmetric dark matter picture. We find that, for a certain parameter range of the model, the mirror electron is the unique possibility for dark matter whereas, in the complementary parameter range, the mirror baryons constitute the dark matter. In either case, the mirror photon must have mass in the MeV range for consistency with observations. For the case of mirror electron dark matter, the model predicts a lower bound on the amount of dark radiation, i.e., Δ N eff ≥ 0.007 .

Invisible axion models that solve the strong C P problem via the Peccei-Quinn (PQ) mechanism typically have a quality problem that arises from quantum gravity effects which violate all global symmetries. These models therefore require extreme fine-tuning of parameters for consistency. We present a new solution to the quality problem in a unified SO ( 10 ) × U ( 1 ) a gauge model, where U ( 1 ) a is an anomaly free axial gauge symmetry. PQ symmetry emerges as an accidental symmetry in this setup, which admits a PQ breaking scale as large as 7 × 10 11 GeV , allowing for the axion to be the cosmological dark matter. We call this a hybrid axion model due to its unique feature that it interpolates between the popular KSVZ and DFSZ axion models. Its predictions for the experimentally measurable axion couplings to the nucleon and electron are distinct from those of the usual models, a feature that can be used to test it. Furthermore, the model has no domain wall problem and it provides a realistic and predictive framework for fermion masses and mixings.

We analyze effects of neutron-antineutron transitions in neutron stars, specifically on (i) cooling, (ii) rotation rate, and (iii) for binary pulsars, the increase in the orbital period. We show that these effects are negligibly small.

A bstract: Despite the remarkable success of the Standard Model, the hierarchy and patterns of fermion masses and mixings remain a profound mystery. To address this, we propose a model employing the rank mechanism, where the originally massless quarks and leptons sequentially get masses. The third generation masses originate from the seesaw mechanism at the tree-level, while those of the second and first generations emerge from one-loop and two-loop radiative corrections, respectively, with a progressive increase in the rank of the mass matrix. This approach does not require new discrete or global symmetries. Unlike other theories of this type that require the introduction of additional scalars, we employ the double seesaw mechanism within a left-right symmetric framework, which allows us to realize this scenario solely through gauge interactions.

Despite the remarkable success of the Standard Model, the hierarchy and patterns of fermion masses and mixings remain a profound mystery. To address this, we propose a model employing the rank mechanism, where the originally massless quarks and leptons sequentially get masses. The third-generation masses originate from the seesaw mechanism at the tree level, while those of the second and first generations emerge from one-loop and two-loop radiative corrections, respectively, with a progressive increase in the rank of the mass matrix. This approach does not require new discrete or global symmetries. Unlike other theories of this type that require the introduction of additional scalars, we employ the double seesaw mechanism within a left-right symmetric framework, which allows us to realize this scenario solely through gauge interactions.