arXiv subjects

Debashis Pachhar

Publications and source records attributed to Debashis Pachhar.

6 recordsLinked to original sources

arXiv2026

Thermal Leptogenesis in the BNT Model of Neutrino Mass

We investigate neutrino mass and thermal leptogenesis in the Babu-Nandi-Tavartkiladze (BNT) model featuring a scalar quadruplet ($\Phi$) and a pair of vector-like fermion triplets ($\Sigma$). In this framework, neutrino masses are generated via an effective dimension-7 operator $LLHH(H^{\dagger}H)/\Lambda^3$ at the tree level and via the dimension-5 operator $LLHH/\Lambda$ at the one-loop level. It naturally accommodates sub-eV neutrino masses even if the new physics scale $\Lambda$ is $\mathcal{O}(\rm TeV)$, thus making the model a compelling target for experimental searches. We explore the viability of thermal leptogenesis in this model, which is distinct from the canonical seesaw-based leptogenesis due to the presence of vector-like fermions. We find that leptogenesis is viable for $M_\Sigma \gtrsim 10^{7}$ GeV for a hierarchical spectrum of fermion triplets. However, in the quasi-degenerate regime, resonant enhancement of the $CP$ asymmetry lowers this scale down to $\mathcal{O}({\rm TeV})$, reconciling successful leptogenesis with the originally motivated TeV-scale phenomenology and testability of the model at colliders.

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arXiv2026

Neutrino mass variables in 3 active and 2 sterile neutrino scenario

The three-flavor framework of neutrino oscillations successfully explains most experimental results, but persistent anomalies at short- and long-baseline experiments hint at the existence of additional light sterile states. In particular, eV-scale sterile neutrinos are motivated by LSND and MiniBooNE results, while sub-eV sterile states with mass-squared differences at the $10^{-2}$ and $10^{-5}$~eV$^2$ scales have been proposed to address the T2K--NO$\nu$A tension and the absence of the expected upturn in the solar neutrino energy spectrum, respectively. Such sterile states are singlets under the Standard Model gauge group and mix only through their admixture with active neutrinos. In this work, we investigate the phenomenology of the $3+2$ scenario, incorporating one eV-scale sterile neutrino together with a sub-eV state, and analyze their impact on absolute-mass related observables: the sum of neutrino masses $\Sigma$ constrained by cosmology, the effective electron neutrino mass $m_\beta$ from beta decay, and the effective Majorana mass $m_{\beta\beta}$ probed in neutrinoless double beta decay. We demonstrate that the presence of two sterile states can significantly modify the allowed parameter space compared to the three-flavor and $3+1$ frameworks, with some mass-ordering schemes already disfavored by current cosmological and laboratory limits. Finally, we assess the implications of upcoming sensitivities from KATRIN, Project~8, and LEGEND-1000, highlighting the complementary role of sub-eV sterile neutrinos in probing physics beyond the minimal three-flavor paradigm.

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arXiv2025

Scalar-induced Neutrinoless Double Beta Decay in $SU(5)$

We discuss the role of heavy scalar fields in mediating neutrinoless double beta decay $(0\nu\beta\beta)$ within the $SU(5)$ Grand Unified Theory framework, extended suitably to include neutrino mass. In such a minimal realistic $SU(5)$ setup for fermion masses, the scalar contributions to $0\nu\beta\beta$ are extremely suppressed as a consequence of the proton decay bound. We circumvent this problem by imposing a discrete ${\cal Z}_3$ symmetry. However, the scalar contributions to $0\nu\beta\beta$ remain suppressed in this $SU(5) \times {\cal Z}_3$ model due to the neutrino mass constraint. We find that the $0\nu\beta\beta$ contribution can be enhanced by extending the scalar sector with an additional $\mathbf{15}$-dimensional scalar representation with suitable ${\cal Z}_3$ charge. Such an extension not only yields realistic fermion mass spectra but also leads to experimentally testable predictions in upcoming ton-scale $0\nu\beta\beta$ searches, which can be used as a sensitive probe of the new scalars across a broad range, from LHC-accessible scales up to $\sim 10^{10}\,\text{GeV}$.

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arXiv2024

Neutrinoless Double Beta Decay from Scalar Leptoquarks: Interplay with Neutrino Mass and Flavor Physics

We perform a comprehensive analysis of neutrinoless double beta decay and its interplay with low-energy flavor observables in a radiative neutrino mass model with scalar leptoquarks $S_1(\bar{3},1,1/3)$ and $\widetilde{R}_2(3,2,1/6)$. We carve out the parameter region consistent with constraints from neutrino mass and mixing, collider searches, as well as measurements of several flavor observables, such as muon and electron anomalous magnetic moments, charged lepton flavor violation and rare (semi)leptonic kaon and $B$-meson decays, including the recent anomalies in $R_{D^{(*)}}$ and $B\to K\nu\bar{\nu}$ observables. We perform a global analysis to all existing constraints and show the (anti)correlations between all relevant Yukawa couplings satisfying these restrictions. We find that the most stringent constraint on the parameter space comes from $\mu \to e$ conversion in nuclei and $K^{+} \rightarrow\, \pi^{+}\nu \bar{\nu}$ decay. We also point out a tension between the muon and electron $(g-2)$ anomalies in this context. Taking benchmark values from the combined allowed regions, we study the implications for neutrinoless double beta decay including both the canonical light neutrino and the leptoquark contributions. We find that for normal ordering of neutrino masses, the leptoquark contribution removes the cancellation region that occurs for the canonical case. The effective mass in presence of leptoquarks can lie in the desert region between the standard normal and inverted ordering cases, and this can be probed in future ton-scale experiments like LEGEND-1000 and nEXO.

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arXiv2024

Constraining the mass-spectra in the presence of a light sterile neutrino from absolute mass-related observables

The framework of three-flavor neutrino oscillation is a well-established phenomenon, but results from the short-baseline experiments, such as the Liquid Scintillator Neutrino Detector (LSND) and MiniBooster Neutrino Experiment (MiniBooNE), hint at the potential existence of an additional light neutrino state characterized by a mass-squared difference of approximately $1\,\rm eV^2$. The new neutrino state is devoid of all Standard Model (SM) interactions, commonly referred to as a 'sterile' state. In addition, a sterile neutrino with a mass-squared difference of $10^{-2}$ $\rm eV^2$ has been proposed to improve the tension between the results obtained from the Tokai to Kamioka (T2K) and the NuMI Off-axis $\nu_e$ Appearance (NO$\nu$A) experiments. Further, the non-observation of the predicted upturn in the solar neutrino spectra below 8 MeV can be explained by postulating an extra light sterile neutrino state with a mass-squared difference around $10^{-5} \rm eV^2$. The hypothesis of an additional light sterile neutrino state introduces four distinct mass spectra depending on the sign of the mass-squared difference. In this paper, we discuss the implications of the above scenarios on the observables that depend on the absolute mass of the neutrinos, namely the sum of the light neutrino masses $(\Sigma)$ from cosmology, the effective mass of the electron neutrino from beta decay $(m_{\beta})$, and the effective Majorana mass $( m_{\beta\beta})$ from neutrinoless double beta decay. We show that some scenarios can be disfavored by the current constraints of the above variables. The implications for projected sensitivity of Karlsruhe Tritium Neutrino Experiment (KATRIN) and future experiments like Project-8, next Enriched Xenon Observatory (nEXO) have been discussed.

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arXiv2023

Constraints on monopole-dipole potential from the tests of gravity

Ultralight Axion Like Particle (ALP) can mediate a long range monopole-dipole macroscopic force between Earth and Sun if Earth is treated as a polarized source. There are about $10^{42}$ number of polarized electrons in Earth due to the presence of the geomagnetic field. The monopole-dipole interactions between electrons in Earth and nucleons in Sun can influence the perihelion precession of Earth, gravitational light bending and Shapiro time delay. The contribution of monopole-dipole potential is limited to be no larger than the measurement uncertainty. We obtain the first bound on monopole-dipole strength from single astrophysical observations. The perihelion precession of Earth puts the stronger bound on monopole-dipole coupling strength as $g_Sg_P\lesssim 1.75\times 10^{-16}$ for the ALP of mass $m_a\lesssim 1.35\times 10^{-18}~\rm{eV}$. We also obtain constraints on monopole-dipole coupling strength as $g_Sg_P\lesssim 5.61\times 10^{-38}$ from two different astrophysical observations such as the perihelion precession of the planet and the red giant branch. The bound is three orders of magnitude stronger than the E\"ot-Wash experiment and one order of magnitude stronger than the $(\rm{Lab})^N_S\times (\rm{Astro})^e_P$ limit.

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