arXiv subjects

Debasish Borah

Publications and source records attributed to Debasish Borah.

At least 19 recordsLinked to original sources

arXiv2026

Probing the dynamical inverse seesaw mechanism 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

Matter-Antimatter Asymmetry from Dirac-Majorana Flip-Flop

The observed matter-antimatter asymmetry can be generated dynamically via leptogenesis, where a non-zero lepton asymmetry is generated first and then gets converted into baryon asymmetry via sphalerons. Creation of non-zero lepton asymmetry typically requires lepton number violation (LNV) which, within the seesaw framework, results in the Majorana nature of light neutrinos. With no signs of LNV at terrestrial experiments, we propose a novel leptogenesis scenario where LNV occurs at high temperatures while zero-temperature physics is consistent with the pure Dirac nature of neutrinos. In a concrete model of this "Dirac-Majorana flip-flop", the right chiral parts of Dirac neutrinos initially acquire a large Majorana mass in the early Universe when a singlet scalar field with two units of lepton number acquires a non-zero vacuum expectation value (VEV), thereby facilitating leptogenesis from out-of-equilibrium decay of heavy Majorana fermions. At lower temperatures, the VEV of the scalar singlet gets restored to zero to be consistent with lepton number conservation. A first-order VEV-restoring phase transition can lead to observable gravitational waves, while light Dirac neutrinos can generate detectable dark radiation at cosmic microwave background experiments.

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arXiv2026

Neutrino texture-zeros after JUNO's first results: Implications for long-baseline neutrino experiments

The recent results from the JUNO reactor neutrino experiment have significantly improved our knowledge of the solar mixing angle $θ_{12}$ and the solar mass splitting $Δm^2_{21}$. We study the impact of these improved estimates on the validity of texture-zeros in the light neutrino mass matrix by assuming neutrinos to be of Majorana nature. Considering a diagonal charged lepton basis, we revisit the previously allowed one-zero and two-zero textures and check their validity by using updated neutrino data from JUNO. While JUNO data rule out one previously allowed two-zero texture, they also make predictions for other neutrino parameters more precise. We finally study the prospects of probing the currently allowed texture-zeros and their predicted correlations among neutrino parameters at the Deep Underground Neutrino Experiment (DUNE). The inclusion of JUNO and reactor experiments strengthens DUNE's ability to constrain the allowed parameter space of both one-zero and two-zero textures. We also observe that DUNE benefits substantially from the complementarity with the T2HK experiment.

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arXiv2026

QCD axion misalignment during reheating

We study the production of QCD axion dark matter (DM) via vacuum and kinetic misalignment mechanisms during an extended reheating period after the end of slow-roll inflation. With pre-inflationary Peccei-Quinn breaking and considering a monomial inflaton potential during reheating, we classify our study into three different perturbative reheating scenarios in which the inflaton decays into (i) a pair of Standard Model (SM)-like bosons, (ii) a pair of SM-like fermions, or (iii) exclusively into a pair of heavy right-handed neutrinos, which eventually decays into the SM final states after dominating the energy density of the Universe for an intermediate epoch. For each of these reheating scenarios, we study the details of QCD axion relic separately via vacuum and kinetic misalignment mechanisms. For different combinations of reheating, monomial inflaton potential and misalignment mechanisms, we identify the parameter space consistent with the observed DM relic density while satisfying other cosmological and laboratory constraints. We also check the scope of probing the currently allowed parameter space at axion-detection and gravitational wave (GW) experiments.

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arXiv2026

Charged scalar portal dark matter with $U(1)_Y$ non-restoration and gravitational waves

We explore the possibility of reviving scalar singlet dark matter (DM) with a minimal extension by a charged singlet scalar which also leads to hypercharge non-restoration in the early Universe. The minimal scalar singlet DM annihilating via the Higgs portal is tightly constrained by direct-detection experiments, leaving a desert between two narrow regions of the DM mass near the Higgs resonance and around a TeV. The addition of a charged scalar portal revives this desert region while keeping the one-loop direct-detection rate within reach of current and future experiments. A non-zero vacuum expectation value of the charged scalar at high temperature can also drive $U(1)_Y$ hypercharge symmetry non-restoration in the early Universe. In a subset of the parameter space, the charge-breaking minimum gets restored to the standard Higgs minimum via a strong first-order phase transition, making it a viable scenario for the generation of baryon asymmetry of the Universe via leptogenesis. A large part of the parameter space consistent with DM relic, direct-detection, symmetry non-restoration and other experimental bounds can be probed at upcoming GW detectors such as LISA, BBO and DECIGO.

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arXiv2026

LZ nuclear recoil event from inelastic singlet-doublet scalar dark matter

We study the possibility of explaining the recently reported high-energy nuclear recoil event by the LUX-ZEPLIN (LZ) collaboration within the framework of singlet-doublet scalar dark matter (DM). Considering DM to be the CP even mass eigenstate formed out of a scalar doublet and a real scalar singlet, both being odd under an unbroken $Z_2$ symmetry, we find the parameter space of the model consistent with correct relic abundance and direct-detection limits on elastic scattering rates. A large part of the parameter space can also lead to inelastic up-scattering of DM with a rate and recoil energy consistent with the recent LZ event. Depending upon the singlet-doublet mixing, the model allows a much wider range of currently allowed parameter space compared to the pure scalar doublet DM limit, which is disfavored due to non-observations of high-energy neutrinos from solar-captured DM annihilation by the IceCube experiment. Presence of $Z_2$-odd right-handed neutrinos also leads to other interesting phenomenology related to the origin of light neutrino masses and leptogenesis.

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arXiv2026

Inelastic Singlet-Doublet Fermion Dark Matter in light of the 248 keV LZ event

Recently, the LUX-ZEPLIN (LZ) collaboration reported the observation of a single dark matter (DM)-nucleus scattering event at a nuclear recoil energy of $248\pm23_{\rm stat}\pm23_{\rm sys}$ keV, corresponding to an exposure of 2.84 tonne-year. The absence of events at lower nuclear recoil energies in the predicted spectrum is naturally explained if the underlying process is inelastic DM-nucleus scattering. Motivated by this, we investigate the singlet-doublet fermion DM model, in which the DM consists of two pseudo-Dirac states: the Majorana nature of the lighter state forbids tree level $Z$-mediated elastic scattering identically, while the same states enable inelastic DM-nucleus scattering via $Z$ exchange, with any residual elastic scattering proceeding only through a suppressed Higgs-mediated channel. We further extend the model with a $Z_2$-even scalar triplet, which is responsible both for generating the pseudo-Dirac splitting and for realizing Majorana neutrino masses via the Type-II seesaw mechanism.

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arXiv2026

Singlet-Doublet fermion origin of dark matter, neutrino mass and inverse first-order electroweak phase transition

We study the possibility of an inverse first-order electroweak phase transition (IFOEWPT) and observable gravitational waves (GW) in a radiative neutrino mass model of scotogenic type where singlet-doublet (SD) fermions, the lightest of whom is the dark matter (DM) candidate, generate the necessary seesaw at one-loop level. Considering the possibility of light neutrinos being Dirac for simplicity and additional detection prospects, we extend the standard model (SM) with two generations of $SU(2)_L$ singlet and doublet fermions, one singlet scalar, and three right-handed neutrinos (RHNs). While RHNs provide the right chiral parts of light Dirac neutrinos, the SD fermions and the scalar singlet facilitate the one-loop neutrino mass diagram. The neutral component of the lighter SD fermion, stabilized under a residual $Z_2$ symmetry plays the role of DM while the heavier SD fermions strongly couple to the Higgs leading to an IFOEWPT where the Universe undergoes two different first-order phase transition as it goes from the symmetric to the final broken Higgs phase. We constrain the parameter space from the requirements of generating the correct neutrino mass, DM relic as well as IFOEWPT while incorporating the existing constraints from different experiments. The final allowed parameter space of the model can be probed at collider, direct-detection, GW and cosmic microwave background (CMB) experiments in near future.

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arXiv2026

Leptogenesis with sub-electroweak-scale reheating temperature

We study the generation of the baryon asymmetry of the Universe via leptogenesis during the post-inflationary reheating epoch, considering reheating temperatures below the temperature of sphaleron freeze-out. Within the framework of a monomial inflaton potential during reheating, we analyze three perturbative reheating scenarios in which the inflaton decays into (i) a pair of Standard Model (SM)-like bosons, (ii) a pair of SM-like fermions, or (iii) exclusively into a pair of heavy right-handed neutrinos, which eventually decays into the SM final states after briefly dominating the energy density of the Universe. For each case, we identify the regions of parameter space that successfully reproduce the observed baryon asymmetry consistently tracking the sphaleron interaction rate during reheating, while satisfying existing cosmological constraints. We also highlight the potential of future primordial gravitational wave observations to probe this class of scenarios.

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arXiv2026

Dark Phoenix: dark matter relic from its own decay

We propose a novel mechanism for generating correct relic of dark matter (DM) which otherwise gets thermally overproduced from the conventional freeze-out mechanism. The mechanism, dubbed as {\it Dark Phoenix}, relies on a transient decay window for DM after its freeze-out which brings its relic within observed limits. Due to finite-temperature effects on masses, DM $ψ$ becomes heavier than its dark sector partner $ϕ$ in this window allowing it to decay. The decay of DM then stops after $ϕ$ gets a sudden jump in its mass from a first-order phase transition (FOPT) driven by another scalar $η$. The dark sector partner $ϕ$, assumed to be a charged scalar, undergoes sufficient pair annihilation during this epoch such that its late decay into DM does not overproduce the latter again. While direct-detection rates of DM remains suppressed due to small couplings, the charged scalar $ϕ$ can have interesting signatures like long-lived charged track at colliders. The associated FOPT can also lead to observable gravitational waves at future experiments like LISA.

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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 $(η)$, and the right-handed counterparts $(ν_R)$ of the Standard Model (SM) neutrinos is extended to include a Dirac fermion dark matter (DM) $(χ)$ and its heavier scalar companion ($ϕ$). The out-of-equilibrium decays of the vector-like fermion generate asymmetries simultaneously in the visible sector, through decay channels involving $(ν_R,η)$ or lepton, Higgs doublets in the SM, and in the dark sector via decaying into $(χ,ϕ)$. 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 $100~\mathrm{MeV} \lesssim m_χ\lesssim 39~\mathrm{TeV}$. 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

Multi-peaked high-frequency gravitational waves from PBH-assisted leptogenesis

We study the possibility of probing non-thermal leptogenesis with multi-peaked high-frequency gravitational waves (GW) by considering heavy right-handed neutrino (RHN) produced from primordial black hole (PBH) evaporation to be responsible for generating the required lepton asymmetry. The decay of RHN also produces a GW spectrum due to graviton bremsstrahlung with the corresponding amplitude being enhanced for heavier RHN. The presence of an ultra-light PBH dominated epoch not only ensures sufficient production of RHNs, but also keeps the leptogenesis scenario free from strong washout problem of thermal leptogenesis at very high scale. In addition, the PBH dominated epoch also helps in generating a gravitational bremsstrahlung spectrum distinct from the stochastic GW background from the thermal bath. Finally, PBH evaporation also brings two separate sources of GW via density perturbation and graviton emission via Hawking evaporation. For the most optimistic scenario with very high scale seesaw consistent with neutrino mass and leptogenesis, this leads to a multi-peaked GW spectrum with peak frequencies lying in the MHz-EHz range.

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arXiv2026

Gravitational waves from seesaw assisted collapsing domain walls

Spontaneous breaking of discrete symmetries like $Z_2$ leads to the formation of stable topological defects such as domain walls which, if allowed to dominate, can potentially be in conflict with cosmological observations. Incorporating explicit $Z_2$-breaking bias terms can lead to annihilation of such walls while also emitting stochastic gravitational waves (GW). We study the role of heavy right-handed neutrinos present in the type-I seesaw framework to generate such a bias term via quantum corrections. This offers interesting correlations among the seesaw scale, GW peak amplitude and peak frequency which can be probed at present and future experiments related to GW as well as precision measurements of the cosmic microwave background (CMB). In flavor symmetric UV complete scenarios with degenerate RHNs at leading order, such tiny coupling of RHNs to a $Z_2$-odd scalar can also lead to small mass splittings suitable for explaining the observed baryon asymmetry of the universe via resonant leptogenesis.

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arXiv2026

Reviving WIMP dark matter with temperature-dependent couplings

The persistent null results at dark matter (DM) direct-detection experiments have pushed the popular weakly interacting massive particle (WIMP) DM to tight corners. Generic WIMP models with direct-detection rate below the current upper limits often lead to a thermally overproduced relic abundance after freeze-out. To resolve this conundrum, we propose a novel scenario where DM has temperature-dependent couplings with the standard model (SM) bath. A scalar field having a large vacuum expectation value (VEV) at high temperatures generates sizeable DM-SM interactions leading to efficient DM annihilations responsible for generating the desired thermal relic. At lower temperatures, the scalar field VEV settles down to a small value as a result of a phase transition which can generically be of first order, effectively leading to suppressed DM-SM interaction rate at low temperature, consistent with null results at direct-detection experiments. Upper bound on thermal DM mass forces the first-order phase transition (FOPT) to occur at scales such that the corresponding gravitational wave signal remains within reach of future experiments like LISA.

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arXiv2026

High-Quality Axion Dark Matter at Gravitational Wave Interferometers

Gravitational effects are known to violate global symmetries, threatening the Peccei-Quinn (PQ) solution to the strong CP problem. Ultraviolet completions featuring a gauged $U(1)$ symmetry, where $U(1)_{\rm PQ}$ arises as an accidental global symmetry, can suppress Planck-suppressed operators, enabling high-quality axions in a mass window where it can also account for the observed dark matter (DM) in the Universe. We show that in such models, the spontaneous breaking of the $U(1)$ gauge symmetry generates a strong stochastic gravitational wave background (SGWB) from gauge cosmic string loops. Even in the most conservative scenario, for breaking scales $\gtrsim 10^{14}$ GeV, the SGWB signal strength can exceed astrophysical foregrounds across a broad frequency range. Such quality axion models have a characteristic IR break frequency originating from the dynamics of the string-wall network collapse. We propose this characteristic SGWB frequency-amplitude region, identified as \textit{Signature-Window-Axion-Gravitational waves} (SWAG), to be a novel probe of high-quality axion DM at future space and ground-based interferometers.

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arXiv2026

Leptogenesis and neutrino mass with one right-handed neutrino and Higgs inflaton

We propose a novel and minimal setup where the observed baryon asymmetry of the Universe and neutrino oscillation data can be satisfied with only one right-handed neutrino (RHN) and a second Higgs doublet with the latter being also responsible for driving cosmic inflation. While inflation is realised via non-minimal coupling of the Higgs to gravity, baryon asymmetry is generated via Affleck-Dine leptogenesis. Due to the presence of only two new fields beyond the standard model (BSM), the proposed setup remains very predictive with only a small allowed parameter space consistent with the PLANCK 2018 and ACT 2025 data simultaneously. The preferred mass spectrum of the BSM particles also keeps the detection prospects alive at terrestrial experiments.

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arXiv2026

Imprint of matter-antimatter asymmetry on collapsing domain walls

Spontaneous breaking of discrete symmetries play non-trivial role in many well-motivated particle physics models. However, it leads to a network of cosmologically unwanted domain walls (DWs) which can be made unstable by introducing a bias term in the scalar potential. In this letter, we provide a novel origin of such bias terms at finite temperature due to radiative corrections from a Dirac fermion with large asymmetry $\sim \mathcal{O}(0.1)$ in its number density. In addition to getting a new viable region of parameter space for collapsing DWs not explored previously and resulting gravitational waves (GWs) accessible at future experiments, the viability of the scenario crucially depends on the temperature of asymmetry generation too. This provides a unique way of probing both the amount of asymmetry and the corresponding temperature via future observations of GWs from collapsing DWs. The large asymmetry in the Dirac fermion can also have interesting implications for the observed baryon asymmetry as well as dark matter and large neutrino asymmetry.

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arXiv2026

Collider probes of baryogenesis with maximal CP asymmetry

We propose a novel collider probe of baryogenesis at TeV scale by measuring decay asymmetries into particle and anti-particle final states. Motivated by the idea of Dirac leptogenesis, we consider an extension of the standard model with new colored and $SU(2)_L$ singlet particles in such a way that the out-of-equilibrium decay of heavy colored fermions creates equal and opposite CP asymmetries in two sectors, prevented from equilibrating with each other. While the TeV scale viability of this mechanism requires a resonantly enhanced CP asymmetry, the latter also plays a crucial role leading to observable decay asymmetries in colliders. In addition to discussing conventional signatures of such heavy colored particles, namely, mono-jet plus missing transverse energy, displaced vertex, colored track at hadron colliders, we also show the unique possibility of measuring decay asymmetries via forward-backward and charge asymmetries at future muon colliders. In addition to being a verifiable TeV-scale baryogenesis scenario, the model also predicts a singlet scalar dark matter candidate consistent with the required thermal dark matter properties near the Higgs resonance.

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