arXiv subjects

Partha Kumar Paul

Publications and source records attributed to Partha Kumar Paul.

At least 19 recordsLinked to original sources

arXiv2026

Inelastic Self-interacting Dark Matter and LUX-ZEPLIN 248 keV Event in a Dirac Modular Inverse Seesaw

We propose a novel framework that simultaneously addresses the origin of Dirac neutrino masses and the nature of self-interacting dark matter (SIDM). The model is based on an $A_{4}$ modular symmetry to ensure the Diracness of neutrinos as well as the stability of the DM. The neutrino sector realizes a Dirac Inverse Seesaw mechanism where the smallness of the neutrino mass is governed by the vacuum expectation value (VEV) of a singlet scalar $\phi$. This same scalar couples to a vector-like fermion DM candidate, inducing a tiny Majorana mass splitting that renders the DM pseudo-Dirac and inelastic. Crucially, the scalar also acts as a light mediator for DM self-interactions, potentially solving the small-scale structure problems of Cold DM. In light of the recent 248 keV nuclear-recoil event, LZ230616, observed by LUX-ZEPLIN (LZ) DM direct detection experiment, we demonstrate that our inelastic SIDM parameter space naturally accommodates this signal via endothermic scattering kinematics. Furthermore, the spontaneous breaking of the dark parity required for this inelasticity produces a network of cosmological domain walls. We show that the explicit symmetry breaking needed to safely annihilate these walls generates a stochastic gravitational wave background. The non-holomorphic modular symmetry reduces the free parameters, correlating neutrino observables, addressing DM phenomenology, $\Delta N_{\rm eff}$, and gravitational-wave signatures.

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arXiv2026

Probing Dynamical Inverse Seesaw with Low-frequency Gravitational Waves

We study the possibility of probing the dynamical inverse seesaw mechanism for the origin of light neutrino masses via the detection of stochastic gravitational waves (GW) in the low-frequency regime currently being probed by pulsar timing arrays. As the lepton number-violating term in inverse seesaw typically remains in the sub-MeV ballpark, its dynamical origin naturally brings the possibility of a low-scale first-order phase transition, which can be probed at low-frequency GW experiments. We also find interesting complementarity with heavy neutral lepton searches, as GW experiments remain sensitive to parameter space with small active-sterile mixing, which is out of reach for most particle physics experiments.

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arXiv2026

Singlet-doublet dark matter induced radiative neutrino mass and TeV scale leptogenesis

The singlet-doublet dark matter (SDDM) model is a well-motivated WIMP framework that accommodates viable dark matter over a broad range of parameter space. In this work, we explore the possibility of TeV-scale leptogenesis within two realizations of the SDDM setup: Majorana SDDM scenario and Dirac SDDM scenario. The light neutrino mass, in either case, arises radiatively at one loop level. The particles running in the loop are responsible for Dark matter relic and TeV-scale leptogenesis while satisfying other phenomenological constraints. In the Majorana setup, the Standard Model is extended by three generations of singlet fermions $N_i$ and doublet fermions $\Psi_i$, and a singlet scalar $\phi$. The \textit{CP}-violating, out-of-equilibrium decays of the heavier singlets ($N_{2,3}$) generate baryon asymmetry via the leptogenesis route, while the first generation of singlet-doublet fermions give rise to the usual SD Majorana dark matter. In the Dirac setup, the standard model is extended by three generations of complex scalars ($\phi_i$) and right-handed Dirac partners ($\nu_{R_i}$) of SM neutrinos ($\nu_{L_i}$), along with a pair of singlet-doublet fermions $\chi$ and $\Psi$. The \textit{CP}-violating out-of-equilibrium decays of the scalar fields $\phi_i$ generate baryon asymmetry via the Dirac leptogenesis route. We show that in the Majorana setup, successful leptogenesis is possible even in the sub-TeV regime, while in the Dirac setup, the scale of leptogenesis is at a few TeV. With the particle mass at the TeV scale, the model remains promising for collider experiments, particularly through signatures such as prompt decays and displaced vertex searches. In addition, the presence of Dirac neutrinos can contribute to $\Delta N_{\rm eff}$, providing complementary cosmological signatures.

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arXiv2026

Cogenesis of visible and dark matter in type-I Dirac seesaw

We propose a novel cogenesis framework based on the type-I Dirac seesaw mechanism. The minimal type-I Dirac seesaw with three heavy vector-like fermions $(N)$, one singlet scalar $(\eta)$, and the right-handed counterparts $(\nu_R)$ of the Standard Model (SM) neutrinos is extended to include a Dirac fermion dark matter (DM) $(\chi)$ and its heavier scalar companion ($\phi$). The out-of-equilibrium decays of the vector-like fermion generate asymmetries simultaneously in the visible sector, through decay channels involving $(\nu_R,\eta)$ or lepton, Higgs doublets in the SM, and in the dark sector via decaying into $(\chi,\phi)$. The resulting lepton asymmetry is partially converted into the observed baryon asymmetry by electroweak sphaleron processes, while the dark-sector asymmetry survives to constitute the present-day asymmetric DM relic. The generation of asymmetries in multiple sectors and their mutual washouts provide rich dynamics while also keeping the model testable at different observations involving DM, neutrinos, cosmic microwave background (CMB), as well as gravitational waves (GW). We find that successful cogenesis can be realized for DM masses in the range $100~\mathrm{MeV} \lesssim m_\chi \lesssim 39~\mathrm{TeV}$. The lower bound arises from the requirement that the symmetric component of DM annihilates efficiently before the big bang nucleosynthesis (BBN) epoch, while the upper bound is set by unitarity constraints on the asymmetric DM.

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arXiv2026

Can Dirac neutrinos destabilize $\mathcal{Z}_2$ domain wall network?

In particle physics model building, a discrete $\mathcal{Z}_2$ symmetry is often spontaneously broken for phenomenological reasons. When this breaking occurs dynamically in the early Universe, stable domain wall networks are formed, which can eventually dominate the cosmic energy density. To avoid this problem, explicit $\mathcal{Z}_2$-breaking terms in the scalar potential are usually introduced in an ad hoc manner. In this Letter, we show that if the same $\mathcal{Z}_2$ symmetry is also responsible for generating light Dirac neutrino masses, such explicit breaking terms can instead arise radiatively from the particles involved in the Dirac mass generation. We find that the resulting bias term scales inversely with the cube of the Dirac neutrino mass, leading to a gravitational wave spectrum proportional to the sixth power of the Dirac neutrino mass. This establishes a nontrivial connection between the Dirac seesaw scale, the domain wall annihilation epoch, and the resulting stochastic gravitational wave signal. We further demonstrate that a wide range of Dirac seesaw scales can be probed by upcoming gravitational wave and cosmic microwave background experiments, while part of the parameter space simultaneously explains the observed baryon asymmetry via Dirac leptogenesis.

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arXiv2026

Baryon-dark matter coincidence in Randall-Sundrum Model

Within the framework of the extra-dimensional Randall-Sundrum set-up, we investigate the freeze-in production of Standard Model (SM) gauge-singlet scalar, fermionic, and massive vector dark matter (DM). Assuming that both the DM and SM fields reside on the IR brane and interact solely through the graviton and radion portal, we demonstrate that the Planck-observed DM relic abundance can be achieved across a wide range of reheating temperatures, all while naturally addressing the hierarchy problem, satisfying constraints from collider and early Universe cosmology. We further show that the same set-up can accommodate TeV-scale leptogenesis capable of generating the observed baryon asymmetry of the Universe. Interestingly, we find that current graviton searches at the Large Hadron Collider (LHC) already impose strong constraints on the reheating temperature in this scenario, providing a complementarity between cosmological and collider probes.

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arXiv2026

Forbidden dark matter assisted by first-order phase transition and associated gravitational waves

We propose a simple yet testable framework for light fermion dark matter (DM) with mass in the MeV--GeV range, charged under a dark $U(1)_D$ gauge symmetry. Before symmetry breaking, DM annihilates excessively into massless dark gauge bosons, resulting in an under-abundant relic and hence creating severe tension with cosmic microwave background (CMB) and indirect detection constraints. This is naturally remedied by a strongly first-order phase transition (FOPT) in the dark sector, triggered by a scalar field $\Phi$, which gives rise to masses for the dark gauge boson ($X_D$) and the physical scalar ($\phi$). To achieve the correct relic abundance while evading indirect detection constraints, the $s$-wave annihilation channel $\chi\bar{\chi} \rightarrow X_D \phi$ is strictly maintained in the kinematically forbidden regime. Crucially, due to the $U(1)_D$ gauge structure, the usual $\chi\bar{\chi} \rightarrow \phi\phi$ annihilation is strictly absent at tree level and only arises at the one-loop level. Within this framework, we explore two distinct possibilities. When the loop-induced $\chi\bar{\chi} \rightarrow \phi\phi$ channel is also kinematically forbidden, it predominantly determines the relic density. Conversely, when the $\chi\bar{\chi} \rightarrow\phi\phi$ channel is kinematically allowed, a rich interplay emerges between the tree-level forbidden $\chi\bar{\chi} \rightarrow X_D \phi$ process and the loop-level allowed $\chi\bar{\chi} \rightarrow\phi\phi$ process, with the dominant contribution being dictated sensitively by the DM mass and gauge coupling. The late-time annihilations are highly suppressed in both of these channels, either by the energetic threshold of the forbidden channel or the $p$-wave velocity suppression of the loop-induced $\chi\bar{\chi} \rightarrow\phi\phi$ final state process, rendering the scenario entirely safe from CMB and indirect search bounds...

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arXiv2026

Naturally small Dirac neutrino mass and $B-L$ dark matter

In the conventional gauged ${B-L}$ extension of the standard model, the $B-L$ charge of the singlet scalar $\chi$, responsible for the breaking of $U(1)_{B-L}$ symmetry, is taken to be 2 such that it can anchor type-I seesaw by giving Majorana masses to the right-handed neutrinos, $\nu_R$. In this paper, we consider instead the cases $\chi \sim 3$ or 4 under $B-L$, so that $\nu_R$ may not acquire any Majorana mass and neutrinos are Dirac fermions. We then consider a vector-like fermion $S$ with 2 units of $B-L$ charge, which becomes a good candidate for dark matter, either Dirac for $\chi \sim 3$ or Majorana for $\chi \sim 4$. In both cases, spontaneous $B-L$ breaking can induce a strong first-order phase transition, producing stochastic gravitational waves (GW) which can be tested at GW experiments. Moreover, the presence of light $\nu_R$s gives rise to an additional contribution to the effective number of relativistic degrees of freedom, $\Delta{N}_{\rm eff}$, providing complementary constraints from current and upcoming CMB observations.

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arXiv2025

Cosmological Probes of Lepton Parity Freeze-in Dark Matter: $\Delta N_{\rm eff}$ & Gravitational Waves

In the canonical type-I seesaw mechanism for neutrino masses, a residual symmetry known as lepton parity: $(-1)^L$, remains preserved. Introducing a Majorana fermion $S$ with even lepton parity renders it naturally stable, making it a viable dark matter (DM) candidate. The addition of a lepton parity odd singlet scalar $\sigma$ allows for the coupling $N S \sigma$, where $N$ is the right-handed neutrino. If $S$ is not thermalized, then DM relic can be produced in two distinct ways: (i) for reheating temperature, $T_{\rm rh}>m_{N}$, dominantly through the decay of $N$ ($N\rightarrow S\sigma$), and (ii) for $T_{\rm EW}<T_{\rm rh}\ll m_{N}$, via standard model Higgs ($h$) decay ($h\rightarrow SS$ at one loop). If the $\sigma-h$ quartic coupling is large, then it can lead to a strong first-order electroweak phase transition even if $\langle\sigma\rangle=0$. Alternatively, if $\sigma-h$ coupling is small, then $\sigma$ can freeze out with a larger abundance, and hence its decay ($\sigma\rightarrow S\nu$) at late epochs can give rise to additional relativistic degrees of freedom ($\Delta{N}_{\rm eff}$). Thus, the framework gives a viable DM with mass range varying from MeV to TeV and leaves observable imprints, via gravitational waves and $\Delta{N}_{\rm eff}$, which offer complementary probes, potentially detectable in future gravitational wave and CMB experiments.

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arXiv2025

Gravitational Wave Probe of Singlet-Doublet Dark Matter Induced Radiative Neutrino Mass

We investigate an one loop radiative neutrino mass model, where the loop particles, notably a singlet fermion ($\chi$), a doublet fermion ($\Psi$) and three generations of singlet scalars ($\phi_i, i=\{1,2,3\}$) are assumed to be odd under an additional $\mathcal{Z}_2$-symmetry. In this setup, the singlet fermion mixes with the neutral component of the doublet to give rise singlet-doublet Majorana dark matter. The addition of $\mathcal{Z}_2$ odd scalars in the model provides rich phenomenological implications. We find that the quartic interaction terms between the SM Higgs and $\phi_i$s play a significant role in modifying the scalar potential to have a first-order phase transition (FOPT) leading to observable gravitational waves (GWs) spectra. We also examine the non-trivial role played by the singlet-doublet fermion DM and the scalars in loop-induced neutrino mass, $(g-2)_\mu$, and lepton flavor violation. We find that the model is predictive due to the combined constraints and can be verified at different terrestrial experiments.

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arXiv2025

Impact of conversion-driven processes on singlet-doublet Majorana dark matter relic

The singlet-doublet dark matter model offers a rich framework for exploring the nature of dark matter (DM) through its unique fermion structure. In this model, the important parameters are the singlet-doublet mass splitting $\Delta{M}$, the singlet-doublet mixing angle $\sin\theta$, and the DM mass $M_{\rm DM}$. If the DM is assumed to be of Dirac nature, then the annihilation, co-annihilation, and conversion driven processes combined allow a range of parameter space: $100~{\rm GeV} \lesssim M_{\rm DM}\lesssim750$ GeV and $10^{-6}\lesssim\sin\theta\lesssim0.04$ for $\Delta{M}>1$ GeV. While the nature of DM, either Dirac or Majorana, is not known, in this work, we assume the singlet-doublet DM to be of Majorana type and find that the relic density and direct detection can be satisfied over a larger parameter space. In particular, the allowed ranges of DM mass and $\sin\theta$ are $100~{\rm GeV}\lesssim M_{\rm DM}\lesssim1750$ GeV and $2\times10^{-7}\lesssim\sin\theta\lesssim0.45$ for $\Delta{M}>1$ GeV.

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arXiv2025

Revisiting the limits on dark matter annihilation cross-section and decay lifetime in light of electron and positron fluxes

We revisit the upper bound on the annihilation cross-section, $\langle\sigma v\rangle$ of a stable dark matter (DM) of mass $500-10^{14}$ GeV by considering five different channels: $W^+W^-$, $b\bar{b}$, $\mu^+\mu^-$, $\tau^+\tau^-$, and $e^+e^-$. We use the observed electron and positron fluxes from CALET, DAMPE, HESS, positron flux from AMS-02, and gamma-ray flux from HAWC, GRAPES-3, CASA-MIA to constrain the annihilation cross-section. We also consider unstable DM of mass $10^3-10^{16}$~GeV decaying to $W^+W^-$, $b\bar{b}$, $\mu^+\mu^-$, $\tau^+\tau^-$, and $e^+e^-$ and derive the corresponding lower bound on the DM lifetime, $\tau_{\rm DM}$. We find that the latest AMS-02 data provide the most stringent constraints on $\langle\sigma v\rangle$ for DM masses below 2 TeV, while HESS yields the strongest limits for $M_{\rm DM}\gtrsim2$ TeV. The HESS gives a much more stringent limit on the DM lifetime, excluding $\tau_{\rm DM\rightarrow\mu^+\mu^-}\lesssim\mathcal{O}(10^{30})$ s for a 10 TeV mass of DM. The limits on $\langle\sigma v\rangle$ derived from the $e^+e^-$ flux are competitive with those from $\gamma$-ray and neutrino observations for DM masses in the range $10^5$--$10^{11}$ GeV, and become the most stringent beyond this range. For decaying DM, the $e^+e^-$ flux provides the strongest constraints on the DM lifetime over the mass range $10^3$--$10^9$ GeV.

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arXiv2025

Lepton parity dark matter and naturally unstable domain walls

We propose a simple and predictive setup that connects neutrino masses, dark matter (DM), and gravitational waves. A minimal lepton parity DM scenario is considered where the residual symmetry $(-1)^L$ from the type I seesaw acts as the dark parity $D=(-1)^{L+2j}$, ensuring DM stability without imposing any new symmetry. A singlet Majorana fermion $S$ with even lepton parity serves as the DM candidate, interacting via a real scalar $\sigma$ which is also even lepton parity. The scalar potential possesses an accidental $\mathcal{Z}_2$ symmetry, whose spontaneous breaking gives rise to unstable domain walls (DW) in the presence of explicit $\mathcal{Z}_2$ breaking terms allowed by the lepton parity. The subsequent DW annihilation generates a stochastic gravitational wave (GW) background potentially observable at different GW experiments.

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arXiv2025

Verifiable type-III seesaw and dark matter in a gauged $\boldsymbol{U(1)_{\rm B-L}}$ symmetric model

We propose a new extension of the Standard Model that incorporates a gauged $ U(1)_{\rm B-L} $ symmetry and the type-III seesaw mechanism to explain neutrino mass generation and provide a viable dark matter (DM) candidate. Unlike the type-I seesaw, the type-III seesaw extension under $ U(1)_{\rm B-L} $ is not automatically anomaly-free. We show that these anomalies can be canceled by introducing additional chiral fermions, which naturally emerge as DM candidates in the model. We thoroughly analyze the DM phenomenology, including relic density, direct and indirect detection prospects, and constraints from current experimental data. Furthermore, we explore the collider signatures of the model, highlighting the enhanced production cross-section of the triplet fermions mediated by the $ \rm B-L $ gauge boson, as well as the potential disappearing track signatures. Additionally, we investigate the gravitational wave signals arising from the first-order phase transition during $ \rm B-L $ symmetry breaking, offering a complementary cosmological probe of the framework.

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arXiv2025

Does thermal leptogenesis in a canonical seesaw rely on initial memory?

It is a common lore that in thermal leptogenesis within the type-I seesaw framework and a hierarchical spectrum of heavy right-handed neutrinos (RHNs), the CP-violating, out-of-equilibrium decay of the lightest RHN ($N_1$) is the only relevant source of the final $B-L$ asymmetry, since any asymmetry produced by the heavier RHNs is expected to be erased by subsequent $N_1$-mediated washout processes. In this work, we revisit this assumption by solving the density-matrix equations, including decay, inverse decay, and relevant scattering processes, and by fully accounting for flavor-projection effects induced by the Yukawa coupling structure. We show that the asymmetries generated by the heavier RHNs ($N_2$ and $N_3$) generally possess components that are misaligned in flavor space with respect to $N_1$, resulting in a partially protected contribution that survives the $N_1$ washout. Unlike the conventional picture of $N_2$-dominated leptogenesis, this memory effect arises even when $N_1$ remains dynamically relevant and cannot be captured within the classical Boltzmann framework. Furthermore, imposing consistency with low-energy neutrino mass and mixing data, we find that at most one RHN can lie in the weak washout regime, which naturally divides the parameter space into four distinct dynamical regimes. We systematically quantify the memory effect in each regime and demonstrate that it can significantly modify the final $B-L$ asymmetry. We find that including projection effects can indeed extend the viable parameter space into the sensitivity range of neutrinoless double beta decay experiments.

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arXiv2025

Super heavy dark matter origin of the PeV neutrino event: KM3-230213A

The recent observation of the ultrahigh-energy neutrino event KM3-230213A by the KM3NeT experiment offers a compelling avenue to explore physics beyond the Standard Model (SM). In this paper, we explore a simplest possibility that this event originates from the decay of a super-heavy dark matter (SHDM). We consider a minimal scenario where the SHDM decays to neutrino and SM Higgs. We derive constraints on the DM lifetime as a function of DM mass, ensuring consistency with IceCube, Auger upper limits, and the observed KM3-230213A event, along with the gamma-ray constraints. We find that KM3-230213A gives stringent constraint on the DM mass ranging from $1.5\times10^8$ GeV to $5.2\times10^9$ GeV with lifetime in the range: $1.42\times10^{30}$ s to $5.4\times10^{29}$ s. Remarkably, in our SHDM scenario, the apparent tension between the KM3NeT observation and the nonobservation of this event by IceCube and Auger can be reduced to below $1.2\sigma$. Our results are applicable to any neutrinophilic SHDM models while evading gamma-ray constraints.

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arXiv2024

Anatomy of singlet-doublet dark matter relic: annihilation, co-annihilation, co-scattering, and freeze-in

The singlet-doublet vector-like fermion dark matter model has been extensively studied in the literature over the past decade. An important parameter in this model is the singlet-doublet mixing angle ($\sin\theta$). All the previous studies have primarily focused on annihilation and co-annihilation processes for obtaining the correct dark matter relic density, assuming that the singlet and doublet components decouple at the same epoch. In this work, we demonstrate that this assumption holds only for larger mixing angles with a dependency on the mass of the dark matter. However, it badly fails for the mixing angle $\sin\theta<0.05$. We present a systematic study of the parameter space of the singlet-doublet dark matter relic, incorporating annihilation, co-annihilation, and, for the first time, co-scattering processes. Additionally, non-thermal productions via the freeze-in and SuperWIMP mechanism are also explored. We found that due to the inclusion of co-scattering processes, the correct relic density parameter space is shifted towards the detection sensitivity range of the LHC and MATHUSLA via displaced vertex signatures.

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arXiv2024

Thermal leptogenesis, dark matter, and gravitational waves from an extended canonical seesaw scenario

In a canonical type-I seesaw scenario, the Standard Model is extended with three singlet right-handed neutrinos (RHNs) $N_i, i=1,2,3$ with masses $M_i, i=1,2,3$ to simultaneously explain sub-eV masses of light neutrinos and baryon asymmetry of the Universe at high scales. In this paper, we show that a relatively low-scale thermal leptogenesis accompanied by gravitational wave signatures is possible when the type-I seesaw is extended with a singlet fermion ($S$) and a singlet scalar ($\rho$), where $S$ and $\rho$ are odd under a discrete $Z_2$ symmetry. We also add a vectorlike fermion doublet $\Psi$ and impose a $Z^\prime_2$ symmetry under which both $N_1$ and $\Psi$ are odd while all other particles are even. This gives rise to a singlet-doublet Majorana fermion dark matter in our setup. At a high scale, the $Z_2$ symmetry is broken spontaneously by the vacuum expectation value of $\rho$ and leads to (i) mixing between RHNs ($N_2, N_3$) and $S$, and (ii) formation of Domain walls (DWs). In the former case, the final lepton asymmetry is generated by the out-of-equilibrium decay of $S$, which dominantly mixes with $N_2$. We show that the scale of thermal leptogenesis can be lowered to $M_S \sim 2 \times 10^6$ GeV, which is \textit{3} orders of magnitude lower than the thermal leptogenesis in canonical type-I seesaw. In the latter case, the disappearance of the DWs gives observable gravitational wave signatures, which can be probed at LISA, DECIGO, ${\rm \mu ARES}$ etc.

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