arXiv subjects

Rathin Adhikari

Publications and source records attributed to Rathin Adhikari.

At least 19 recordsLinked to original sources

arXiv2026

Light neutrinos, Dark matter and leptogenesis near electroweak scale and $Z_4$ symmetry

Considering $Z_4$ symmetry in Type I seesaw scenario, one could obtain mass-squared differences of light neutrinos, mixings and $CP$ violating phase within $3 σ$ confidence level based on neutrino oscillation data. This is possible with only three independent complex parameters for allowed Yukawa couplings and one real mass parameter for heavy right handed neutrino fields around electroweak scale. After considering only three more real parameters as coming from small soft-symmetry breaking terms, the lightest right handed neutrino could be considered as dark matter candidate via freeze-in mechanism and the other two heavier right handed neutrinos through their decays, could generate the baryonic asymmetry of the universe naturally via resonant leptogenesis.

hep-ph↗

arXiv2025

Leptogenesis in exponential $f(R)$ gravity model

We show that gravitational leptogenesis with dynamical $CPT$ breaking in an expanding universe can be reconciled with the exponential $f(R)$ gravity model, which introduces only one additional parameter $β$ compared to the standard $Λ$CDM cosmology. This model incorporated axions as cold dark matter. For $L$ violating interactions, we consider both a non-supersymmetric model with heavy right-handed neutrino decay and a supersymmetric model with sneutrino decay. For both cases, we have shown that the required baryonic asymmetry could be obtained. We have also shown the variation of decoupling temperature for lepton number violating interactions with the $β$ parameter in exponential $f(R)$ gravity. Lepton number-violating model parameters are constrained with the $β$ through the decoupling temperature. An upper bound on the $β$ parameter of the exponential $f(R)$ gravity is also obtained.

hep-ph↗

arXiv2025

Naturalness in Soft Leptogenesis and Gravitino Mass Bound due to Primordial Black Holes

If sneutrinos are produced through primordial black hole (PBH) evaporation, then some interesting features of soft leptogenesis in the Minimal Supersymmetric Standard Model with heavy right-handed neutrinos, are found. The required baryonic asymmetry could be possible from the decays of sneutrinos, for the soft SUSY breaking trilinear $A$ and bilinear $B$ parameters around the electroweak scale. The resonance condition in soft leptogenesis is not required. The allowed regions of different relevant parameters are discussed in detail. Using experimental constraints from collider searches on heavy leptons, the lower bound on the right-handed neutrino mass is found to be around $300$ GeV with Yukawa coupling lesser than about $0.4$. The allowed region of the typical mass scale of some supersymmetric particles and the $|A|$ parameter is also shown from the experimental constraint on the branching ratio for $μ\rightarrow e γ$ in MEG-II search. Depending on PBH mass, bounds on the mass of gravitinos, produced from PBH evaporation, is discussed and gravitino mass around the electroweak scale is found to be possible only for unstable gravitino.

hep-ph↗

arXiv2025

Explaining Fermions Mass and Mixing Hierarchies through $U(1)_X$ and $Z_2$ Symmetries

For understanding the hierarchies of fermion masses and mixing, we extend the Standard Model gauge group with $ U(1)_X $ and $ Z_2 $ symmetry. The field content of the Standard Model is augmented by three heavy right-handed neutrinos, two new scalar singlets, and a scalar doublet. $ U(1)_X $ charges of different fields are determined after satisfying anomaly cancellation conditions. In this scenario, the fermion masses are generated through higher-dimensional effective operators with $ O(1) $ Yukawa couplings. The small neutrino masses are obtained through type-1 seesaw mechanism using the heavy right-handed neutrino fields, whose masses are generated by the new scalar fields. We discuss the flavour-changing neutral current processes that arise due to the sequential nature of $ U(1)_X $ symmetry. We have written effective higher-dimensional operators in terms of renormalizable dimension-four operators by introducing vector-like fermions.

hep-ph↗

arXiv2025

The problem of flavour

We review the problem of flavour tracing back to the days when the standard model was just coming together. We focus on the recently discussed new solutions of this problem, namely the Froggatt and Nielsen mechanism based on a novel discrete $\mathcal{Z}_{\rm N} \times \mathcal{Z}_{\rm M}$ flavour symmetry, and the standard hierarchical VEVs model. The standard HVM, and the Froggatt and Nielsen mechanism based on the $\mathcal{Z}_{\rm N} \times \mathcal{Z}_{\rm M}$ flavour symmetry, can be recovered from a new dark-technicolour paradigm, where the hierarchical VEVs or the flavon VEV may appear as the chiral multifermion condensates. In particular, there appears a novel feature that the solution of the flavour problem based on the discrete flavour symmetry can provide the so-called flavonic dark matter. This predicts a specific relation between the mass and the symmetry-breaking scale, which can be contrasted with the standard QCD axion. Moreover, a possible direction towards the Grand Unified framework is also discussed.

hep-ph↗

arXiv2024

Type I seesaw mechanism at TeV scale or below with minimal fields

A novel scenario is presented within the Type-I seesaw mechanism in which no other beyond Standard Model fields except three heavy right handed neutrinos, have been considered. Light neutrino masses around sub eV scale, could be possible at low seesaw scale around TeV or even below that. At the leading order, 6x6 seesaw mass matrix reproduces three massless neutrinos. The Dirac mass matrix with one loop corrections, breaks that massless texture and it is possible to get massive neutrinos. We have obtained the expression of mixing and Dirac CP violating phase for light neutrino mass matrix with one loop corrections. Only unknown parameters are the Yukawa couplings related to right handed neutrinos and their masses. These parameters satisfy the ATLAS, CMS experimental constraints.

hep-ph↗

arXiv2023

Flavonic dark matter

We first time show that a common solution to dark matter and the flavor problem of the standard model can be obtained in the framework of the $\mathcal{Z}_{\rm N} \times \mathcal{Z}_{\rm M}$ flavor symmetry where the flavonic Goldstone boson of this flavor symmetry acts as a good dark matter candidate through the misalignment mechanism. Hierarchical mass pattern of quarks and charged leptons naturally follows from the discrete symmetry. For light active neutrinos, we construct the Dirac-type mass matrix which is preferred to fit the observed neutrino oscillation data with normal hierarchy. Our model predicts the axion-like photon coupling characteristically different from the standard QCD axion, and could be probed by the future X-ray or radio observations.

hep-ph↗

arXiv2022

Lower tensor to scalar ratio in a SUGRA motivated inflationary potential

A scalar potential obtained from the $D$-term in the Supergravity models, which dominates over $F$ term and is mainly responsible for the inflationary phase in the early universe, is studied. The potential with canonical kinetic terms for scalar fields in the Lagrangian, has a very slow roll feature in comparison to various other plateau type inflationary potentials. In this case, a much lower tensor-to-scalar ratio ($r$) of $\mathcal{O}(10^{-3})$ is achievable. The requirement of slow roll condition for the inflation potential implies that the up type neutral scalar and the down type neutral scalar in Supergravity models are with equal field strength at the time of inflation. If this relationship holds down to the electroweak scale for the corresponding $vev$ values of these fields, then it will indicate a higher SUSY breaking scale around 100 TeV. The predicted values of the inflationary observables are well within the 1-$σ$ bounds of the recent constraints from {\it Planck'18} observations. The era of reheating after the inflationary phase, is also studied and the bounds on the reheating temperature ($T_{re}$) is calculated for a different equation of states during reheating ($w_{re}$) for the {\it Planck'18} allowed values of the scalar spectral index ($n_s$). For our model with $w_{re}=2/3$ and $w_{re}=1$, after satisfying all the bounds due to gravitino overproduction, we can have big parameter space for $T_{re}$ which is well inside {\it Planck'18} 1-$σ$ bound on $n_s$.

astro-ph.CO↗

arXiv2022

Anomalous Magnetic Moment and Higgs Coupling of the Muon in a Sequential U(1) Gauge Model with Dark Matter

We study an Abelian gauge extension of the standard model with fermion families having non-universal gauge charges. The gauge charges and scalar content are chosen in such an anomaly-free way that only the third generation fermions receive Dirac masses via renormalisable couplings with the Higgs boson. Incorporating additional vector like fermions and scalars with appropriate $U(1)$ charges can lead to radiative Dirac masses of first two generations with neutral fermions going in the loop being dark matter candidates. Focusing on radiative muon mass, we constrain the model from the requirement of satisfying muon mass, recently measured muon anomalous magnetic moment by the E989 experiment at Fermilab along with other experimental bounds including the large hadron collider (LHC) limits. The anomalous Higgs coupling to muon is constrained from the LHC measurements of Higgs to dimuon decay. The singlet fermion dark matter phenomenology is discussed showing the importance of both annihilation and coannihilation effects. Incorporating all bounds lead to a constrained parameter space which can be probed at different experiments.

hep-ph↗

arXiv2021

Leptogenesis in supersymmetry with one $L$ violating coupling

We have shown a new scenario of successful leptogenesis with one $L$ violating coupling and a relative Majorana phase playing the role of $CP$ violation. This is in contrast to the usual consideration of Feynman diagram with at least two $L$ violating couplings. We have considered $R$-parity violating Minimal Supersymmetric Standard Model (MSSM) for leptogenesis at TeV scale. This scenario is also consistent with generating light neutrino mass if asymmetry is generated through semileptonic $λ^{\prime}$ coupling.

hep-ph↗

arXiv2021

Spectro-Timing Analysis of a highly variable narrow-line Seyfert 1 galaxy NGC 4748 with AstroSat and XMM-Newton

We present a detailed timing and spectral study of an extremely variable narrow-line Seyfert~1 galaxy NGC 4748 using observations in the year 2017 and 2014 performed with AstroSat and XMM-Newton, respectively. Both observations show extremely variable soft and hard X-ray emission that are correlated with each other. In the 2014 data set, the source retains its general behaviour of "softer when brighter" while the 2017 observation exhibits a "harder when brighter" nature. Such changing behaviour is rare in AGNs and is usually observed in the black hole binary systems. The "harder when brighter" is confirmed with the anti-correlation between the photon index and the 0.3-10 keV power-law flux. This suggests a possible change in the accretion mode from standard to the advection-dominated flow. Additionally, both the observations show soft X-ray excess below 2 keV over the power-law continuum. This excess was fitted with a single or multiple blackbody component(s). The origin of soft excess during the 2017 observation is likely due to the cool Comptonization as the photon index changes with time. On the other hand, the broad iron line and delayed UV emission during the 2014 observation strongly suggest that X-ray illumination onto the accretion disk and reflection and reprocessing play a significant role in this AGN.

astro-ph.HE↗

arXiv2020

Inflation, reheating, leptogenesis and bounds on soft supersymmetry breaking parameters

In the no-scale supergravity with Type-I Seesaw model of Non-minimal supersymmetric standard model (NMSSM), we have analysed inflation, reheating and leptogenesis. A no-scale supergravity realization of Starobinsky model of inflation in simple Wess-Zumino model have been shown earlier by Ellis et al. Here we show a no-scale supergravity realization of Starobinsky model of inflation in Type-I Seesaw framework of NMSSM. In this framework an appropriate choice of no-scale Kähler potential results in Starobinsky like plateau inflation along a Higgs-sneutrino $D$-flat direction consistent with the CMB observations. In leptogenesis, the soft-breaking trilinear and bilinear terms play important role. Using conditions for non-thermal contribution to $CP$ asymmetry and successful leptogenesis together with the appropriate reheating at the end of inflation, we have obtained important constraints on the soft supersymmetry breaking parameters.

hep-ph↗

arXiv2020

Power Law Plateau Inflation Potential In The RS $II$ Braneworld Evading Swampland Conjecture

In the recent time, inflationary cosmology is facing an existential crisis due to the proposed Swampland criterion which aims to evade any (meta-)stable de Sitter construction within the String landscape. It is been realised that a single field slow roll inflation is inconsistent with the Swampland criterion unless the inflationary model in realised in some non standard scenario such as Warm inflation or the Braneworld scenario. In \cite{Dimopoulos}, Dimopoulos and Owen introduced a new class of model of inflation dubbed as the power law plateau inflation in the standard cold inflationary scenario. But to realise this model in the standard scenario consistent with observation, they had to introduce a phase of thermal inflation. In this paper we have analysed this model in the braneworld scenario to show that for some choice of the parameters defining the model class, one can have an observationally consistent power law plateau without any phase of thermal inflation. We have also shown that, for the correct choice of model parameters, one can easily satisfy the Swampland criterion. Besides, for a particular choice of the potential one can also satisfy the recently proposed Trans-Planckian Censorship Conjecture.

astro-ph.CO↗

arXiv2020

Dark matter mass from relic abundance, an extra $U(1)$ gauge boson, and active-sterile neutrino mixing

In a model with an extra $U(1)$ gauge to SM gauge group, we have shown the allowed region of masses of extra gauge boson and the dark matter which is the lightest one among other right-handed Majorana fermions present in the model. To obtain this region, we have used bounds coming from constraints on active-sterile neutrino masses and mixing from various oscillation experiments, constraint on dark matter relic density obtained by PLANCK together with the constraint on the extra gauge boson mass and its gauge coupling recently obtained by ATLAS Collaboration at LHC. From the allowed regions, it is possible to get some lower bounds on the masses of the extra gauge boson and the dark matter and considering those values it is possible to infer what could be the spontaneous symmetry breaking scale of an extra $U(1)$ gauge symmetry.

hep-ph↗

arXiv2016

New U(1) Gauge Model of Radiative Lepton Masses with Sterile Neutrino and Dark Matter

An anomaly-free U(1) gauge extension of the standard model (SM) is presented. Only one Higgs doublet with a nonzero vacuum expectation is required as in the SM. New fermions and scalars as well as all SM particles transform nontrivially under this U(1), resulting in a model of three active neutrinos and one sterile neutrino, all acquiring radiative masses. Charged-lepton masses are also radiative as well as the mixing between active and sterile neutrinos. At the same time, a residual $Z_2$ symmetry of the U(1) gauge symmetry remains exact, allowing for the existence of dark matter.

hep-ph↗

arXiv2015

Common Origin of 3.55 keV X-Ray Line and Galactic Center Gamma Ray Excess in a Radiative Neutrino Mass Model

We attempt to simultaneously explain the recently observed 3.55 keV X-ray line in the analysis of XMM-Newton telescope data and the galactic center gamma ray excess observed by the Fermi gamma ray space telescope within an abelian gauge extension of standard model. We consider a two component dark matter scenario with tree level mass difference 3.55 keV such that the heavier one can decay into the lighter one and a photon with energy 3.55 keV. The lighter dark matter candidate is protected from decaying into the standard model particles by a remnant $Z_2$ symmetry into which the abelian gauge symmetry gets spontaneously broken. If the mass of the dark matter particle is chosen to be within $31-40$ GeV, then this model can also explain the galactic center gamma ray excess if the dark matter annihilation into $b\bar{b}$ pairs has a cross section of $\langle σv \rangle \simeq (1.4-2.0) \times 10^{-26} \; \text{cm}^3/\text{s}$. We constrain the model from the requirement of producing correct dark matter relic density, 3.55 keV X-ray line flux and galactic center gamma ray excess. We also impose the bounds coming from dark matter direct detection experiments as well as collider limits on additional gauge boson mass and gauge coupling. We also briefly discuss how this model can give rise to sub-eV neutrino masses at tree level as well as one-loop level while keeping the dark matter mass at few tens of GeV. We also constrain the model parameters from the requirement of keeping the one-loop mass difference between two dark matter particles below a keV. We find that the constraints from light neutrino mass and keV mass splitting between two dark matter components show more preference for opposite $CP$ eigenvalues of the two fermion singlet dark matter candidates in the model

hep-ph↗

arXiv2015

The cosmological constant from the zero point energy of compact dimensions

We consider extra compact dimensions as the origin of a cosmological universal energy density in the regular dimensions, with only graviton fields propagating in the compact space dimensions. The quantum zero point energy originating from the finite size boundary condition in the compact dimensions can produce a constant energy density in regular $3$ space which is homogeneous and isotropic. It then makes a natural identification with the cosmological constant in conformity with the Einstein equation. It turns out that for the emergent energy density to agree with the observed value of the cosmological constant, the size/radius of compact dimension is to be of order of $10^{-2}$ cm.

gr-qc↗

arXiv2015

Nonthermal $CP$ violation in soft leptogenesis

Soft leptogenesis is a mechanism which generates the matter-antimatter asymmetry of the Universe via the out-of-equilibrium decays of heavy sneutrinos in which soft supersymmetry breaking terms play two important roles: they provide the required $CP$ violation and give rise to the mass splitting between otherwise degenerate sneutrino mass eigenstates within a single generation. This mechanism is interesting because it can be successful at the lower temperature regime $T \lesssim 10^9$ GeV in which the conflict with the overproduction of gravitinos can possibly be avoided. In earlier works, the leading $CP$ violation is found to be nonzero only if finite temperature effects are included. By considering generic soft trilinear couplings, we find two interesting consequences: (1) the leading $CP$ violation can be nonzero even at zero temperature realizing nonthermal $CP$ violation, and (2) the $CP$ violation is sufficient even far away from the resonant regime allowing soft supersymmetry breaking parameters to assume natural values at around the TeV scale. We discuss phenomenological constraints on such scenarios and conclude that the contributions to charged lepton flavor violating processes are close to the sensitivities of present and future experiments.

hep-ph↗