arXiv subjects

Rabindra N. Mohapatra

Publications and source records attributed to Rabindra N. Mohapatra.

At least 19 recordsLinked to original sources

arXiv2026

UV-Complete Models for a Light Axial Gauge Boson

We present new anomaly free gauge models where the gauge field only has axial vector couplings to both quarks and leptons. We use the left-right symmetric universal seesaw models as the basis for this construction with an extra $U(1)_a$ as the axial gauge group. We present three main versions of the model, denoted as models A, B and C (and their variations), with different properties depending on the way the gauge anomaly is canceled. We show how the models can accommodate small neutrino masses. The models allow for a new Dirac fermion coupled via the $U(1)_a$ gauge portal to the SM fields which can be the dark matter. The models A and its variation have the novel property that there is an upper limit on the $U(1)_a$ gauge coupling $g_a$, due to the fact that the standard model Higgs doublet shares the $U(1)_a$ quantum number. For models B and C, we discuss the phenomenological constraints on the gauge coupling $g_a$ and gauge boson mass $m_{\cal A}$ from current low energy observations where, unlike in models A, $g_a$ depends on $m_{\mathcal{A}}$ through a single vacuum expectation value.

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arXiv2026

Radiative Seesaw Model with Baryon Number Violation and Upper Limit on Neutron-anti-Neutron Transition Time

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 reheating 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 $\frac{1}{Λ^2}N u_Rd_Rd_R$. This model can lead to observable neutron-anti-neutron ($n-\bar{n}$) oscillation. We show that if RHNs are produced non-thermally, we can get a cosmological upper limit on the transition time $τ_{n-\bar{n}}$, which is within the reach of the planned ESS HIBEAM/NNBAR experiment. The proton stability is guaranteed by the scotogenic $Z_2$ invariance, which prevents the appearance of the Dirac mass term for the neutrino

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arXiv2025

Avoiding Blindness in Baryon Number Violating Processes: Free-Beam and Intranuclear Paths to Neutron-Antineutron Transitions

Experimental searches for neutron--antineutron ($n \rightarrow \bar n$) 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 $n \rightarrow \bar n$, 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 $n\rightarrow \bar{n}$ that is akin to those developed for electric dipole moments and other systems for which short-distance new physics must be studied in-medium.

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arXiv2025

Matter-Dark Matter Coincidence and Mirror World

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 towards 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 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. $ΔN_{eff} \geq 0.007$.

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arXiv2025

Accidental Peccei-Quinn Symmetry From Gauged U(1) and a High Quality Axion

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.

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arXiv2024

Hybrid SO(10) Axion Model Without Quality Problem

Invisible axion models that solve the strong CP 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 $SO(10)\times U(1)_a$ 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 $4\times 10^{11}$ 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.

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arXiv2024

Radiative Origin of Fermion Mass Hierarchy in Left-Right Symmetric Theory

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.

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arXiv2024

Effects of Neutron-Antineutron Transitions in Neutron Stars

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.

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arXiv2024

Predictive Dirac Neutrino Spectrum with Strong CP Solution in $\pmb{SU(5)_L \times SU(5)_R}$ Unification

We develop a grand unified theory of matter and forces based on the gauge symmetry $SU(5)_L\times SU(5)_R$ with parity interchanging the two factor groups. Our main motivation for such a construction is to realize a minimal GUT embedding of left-right symmetric models that provide a parity solution to the strong CP problem without the axion. We show how the gauge couplings unify with an intermediate gauge symmetry $SU(3)_{cL}\times SU(2)_{2L}\times U(1)_{L}\times SU(5)_R$, and establish its consistency with proton decay constraints. The model correctly reproduces the observed fermion masses and mixings and leads to naturally light Dirac neutrinos with their Yukawa couplings suppressed by a factor $M_I/M_G$, the ratio of the intermediate scale to the GUT scale. We call this mechanism type II-Dirac seesaw. Furthermore, the model predicts $δ_{CP} = \pm (130.4 \pm 1.2)^\circ $ and $m_{ν_1} = (4.8-8.4)$ meV for the Dirac CP phase and the lightest neutrino mass. We demonstrate how the model solves the strong CP problem via parity symmetry.

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arXiv2024

Long-lived doubly charged scalars in the left-right symmetric model: catalyzed nuclear fusion and collider implications

We show that the doubly charged scalar from the $SU(2)_R$-triplet Higgs field in the Left-Right Symmetric Model has its mass governed by a hidden symmetry so that its value can be much lower than the $SU(2)_R$ breaking scale. This makes it a long-lived particle while being consistent with all existing theoretical and experimental constraints. Such long-lived doubly charged scalars have the potential to trigger catalyzed fusion processes in light nuclei, which may have important applications for energy production. We show that it could also bear consequences on the excess of large ionization energy loss ($dE/dx$) recently observed in collider experiments.

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arXiv2024

Explanation of the 95 GeV $γγ$ and $b\bar{b}$ excesses in the Minimal Left-Right Symmetric Model

We propose a simple interpretation of the $γγ$ excesses reported by both CMS and ATLAS groups at 95 GeV together with the LEP excess in the $Zb\bar{b}$ channel around the same mass in terms of a neutral scalar field in the minimal left-right symmetric model (LRSM). We point out that the scalar field which implements the seesaw mechanism for neutrino masses has all the right properties to explain these observations, without introducing any extra scalar fields. The key point is that this scalar particle is hardly constrained because it couples only to heavy right-handed particles. As a result, the diphoton decay mode receives contributions from both mixing with the Standard Model (SM) Higgs and the heavy charged bosons in the LRSM, depending on the $SU(2)_R\times U(1)_{B-L}$ symmetry breaking scale $v_R$. The complete allowed parameter space for explaining the 95 GeV excesses in this model can be probed with the high-precision measurements of the SM Higgs mixing with other scalars at the high-luminosity LHC and future Higgs factories.

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arXiv2023

Parity Solution to the Strong CP Problem and a Unified Framework for Inflation, Baryogenesis, and Dark Matter

It has been known for some time that asymptotic parity invariance of weak interactions can provide a solution to the strong CP problem without the need for the axion. Left-right symmetric theories which employ a minimal Higgs sector consisting of a left-handed and a right-handed doublet is an example of such a theory wherein all fermion masses arise through a generalized seesaw mechanism. In this paper we present a way to understand the origin of matter-antimatter asymmetry as well as the dark matter content of the universe in these theories using the Affleck-Dine (AD) leptogenesis mechanism and inflaton decay, respectively. Three gauge singlet fermions are needed for this purpose, two of which help to implement the Dirac seesaw for neutrino masses while the third one becomes the non-thermal warm dark matter candidate. A soft lepton number breaking term involving the AD scalar field is used to generate lepton asymmetry which suffers no wash-out effects and maintains the Dirac nature of neutrinos. This framework thus provides a unified description of many of the unresolved puzzles of the standard model that require new physics.

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arXiv2023

Gauged $L_e-L_μ-L_τ$ symmetry, fourth generation, neutrino mass and dark matter

We present two models where the familiar leptonic symmetry $L_e-L_μ-L_τ$ is a gauge symmetry. We show how anomaly cancellation constrains the allowed theories, with one of them requiring a fourth sequential chiral standard model fermion generation and a second one with three generations, requiring gauging of $(L_e-L_μ-L_τ)-(B_1-B_2-B_3)$ with $B_a$ representing the baryon number of the $a$th generation quarks. Unlike global $L_e-L_μ-L_τ$ models which always leads to inverted mass hierarchy for neutrinos, the gauged version can lead to normal hierarchy. We show how to construct realistic models in both the cases and discuss the dark matter candidate in both. In our model, the breaking of $U(1)_{L_e-L_μ-L_τ}$ is responsible for neutrino mass via type-I mechanism whereas the real part of $U(1)_{L_e-L_μ-L_τ}$ breaking scalar field (called $ϕ$ here) plays the role of freeze-in dark matter candidate. Since $ϕ$ is unstable, for it to qualify as dark matter, its lifetime must be larger than the age of the Universe, implying that the relic of $ϕ$ is generated through freeze-in mechanism and its mass must be less than an MeV. We also discuss the possibility of explaining both muon and electron $(g-2)$ while being consistent with the dark matter relic density and lifetime constraints.

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arXiv2023

Conformal B-L and Pseudo-Goldstone Dark Matter

We show that a conformal extension of the standard model with local B-L symmetry and two complex scalars breaking B-L can provide a unified description of neutrino mass, origin of matter and dark matter. There are two hierarchical B-L breaking vacuum expectation value (VEV) scales in the model, the higher denoted by $v_B$ and the lower by $v_A$. The higher breaking scale is dynamically implemented via the Coleman-Weinberg mechanism and plays a key role in the model since it induces electroweak symmetry breaking as well as the lower B-L breaking scale. It is also responsible for neutrino masses via the seesaw mechanism and origin of matter. The imaginary part of the complex scalar with lower B-L breaking VEV plays the role of a pseudo-Goldstone dark matter (DM). The DM particle is unstable with its lifetime naturally longer than $10^{28}$ seconds. We show that its relic density arises from the freeze-in mechanism for a wide parameter domain. Due to the pseudo-Goldstone boson nature of the DM particle, the direct detection cross section is highly suppressed. The model also predicts the dark matter to be heavier than 100 TeV and it decays to two high energy neutrinos which can be observable at the IceCube, providing a test of this model.

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arXiv2022

Constraints on Neutron-Mirror-Neutron Oscillation from Neutron Star Cooling

We address a method of limiting neutron-mirror neutron mixing ($ε_{nn'}$) by analyzing its effect on neutron star (NS) heating. This method employs observational bounds on the surface temperature of NSs to constrain $ε_{nn'}$. It has been suggested that the bound obtained this way is so stringent that it would exclude any discovery of $n-n'$ oscillation in the currently planned terrestrial experiments at various laboratories. This conclusion motivated us to critically analyze this suggestion in more detail. In this note, we point out a very interesting new effect present in nearly exact mirror models, which can significantly affect this bound. The new element is that in nearly exact mirror models there is the mirror analog of $β$ decay, i.e. $n' \to p' + e' + \barν'_e$, which creates a cloud of mirror particles $n'$, $p'$, $e'$, $D'$ and He$'$ inside the NS. The resulting $e'$ can "rob" the energy generated by the $n \to n'$ transition from the NS, via $e-e'$ scattering enabled by the presence of a (minute) millicharge in mirror particles. Such a tiny millicharge on mirror particles is highly likely in these models. This results in energy being emitted as unobserved mirror photons via fast mirror bremsstrahlung, whose effect is to relax the stringent bounds on $ε_{nn'}$.

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arXiv2022

Neutron-Mirror-Neutron Oscillation and Neutron Star Cooling

It was pointed out in a recent paper that the observed cooling rate of old, cold neutron stars (NS) can provide an upper limit on the transition rate of neutron to mirror neutron ($n-n'$). This limit is so stringent that it would preclude any discovery of $n \to n'$ oscillation in the current round of terrestrial searches for the process. Motivated by this crucially important conclusion, we critically analyze this suggestion and note an interesting new effect present in nearly exact mirror models for $n \to n'$ oscillation, which significantly affect this bound. The new element is the $β$ decay $n' \to p'+ e' +\barν'_{e}$, which creates a cloud of mirror particles $n'$, $p'$, $e'$ and $D'$ inside the NS core. The $e'$ can "rob" the energy generated by the $n \to n'$ transition via $e-e'$ scattering enabled by the presence of a (minute) milli-charge in mirror particles. This energy is emitted as unobserved mirror photons via fast mirror bremsstrahlung leading to a relaxation of this upper limit.

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arXiv2022

Affleck-Dine Leptogenesis with One Loop Neutrino Mass and strong CP

We present a unified model that solves four major problems of the standard model i.e. neutrino masses, origin of matter, strong CP problem and dark matter using the framework of Affleck-Dine (AD) mechanism. The AD-field is responsible for inflation, origin of matter and neutrino masses which arise at the one loop level. Neutrino masses are therefore intimately connected to the baryon to photon ratio of the universe. The dark matter in the model is the axion field used to solve the strong CP problem. The model has a near massless Majorana fermion which contributes to $ΔN_{\rm eff}\sim 0.1$ in the early universe, that can be tested in the upcoming CMB-S4 experiment

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arXiv2022

Neutrino Mass from Affleck-Dine Leptogenesis and WIMP Dark Matter

Affleck-Dine (AD) mechanism for leptogenesis involves the cosmological evolution of a complex scalar field (AD field) that carries non-zero lepton number. We show how explicit lepton number breaking terms, which involve the AD field needed to implement this scenario combined with fermionic WIMP dark matter, can generate neutrino mass at the one loop level, thus providing a unified framework for solving four major puzzles of the standard model i.e. inflation, baryogenesis, dark matter and neutrino mass. We discuss some phenomenological implications of this model.

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