arXiv subjects

Satyabrata Mahapatra

Publications and source records attributed to Satyabrata Mahapatra.

At least 19 recordsLinked to original sources

arXiv2026

Leptogenesis and Planck-scale black hole remnants in a Pati-Salam cosmology

We study leptogenesis and Planck-scale remnant dark matter from primordial black hole (PBH) evaporation in a minimal Pati-Salam cosmology, with the gauge symmetry broken before inflation so that magnetic monopoles are diluted away. A singlet inflaton with a near-inflection potential enhances the curvature power spectrum on small scales, producing a narrow black hole population that briefly dominates the energy density. The Pati-Salam embedding ties the right-handed neutrino masses to the SU(2)_R breaking scale, and cosmological consistency forces both the scalar-sector Yukawa coupling and the heavy neutrino mass well below that scale. Two regimes emerge, depending on whether the black holes are hot enough to emit the lightest right-handed neutrino. Lighter PBHs drive non-thermal leptogenesis, while heavier ones require a thermal asymmetry and also affect it through entropy dilution. If quantum gravitational backreaction halts evaporation at the Planck scale, each PBH leaves a stable remnant whose present abundance scales as the inverse five-halves power of the initial mass. Suppressing remnants requires heavier black holes, but the lighter PBHs needed for non-thermal leptogenesis overproduce remnant dark matter. Thus, remnant dark matter and non-thermal leptogenesis are mutually exclusive. When the thermal contribution is included, a single initial PBH mass near 10^6 g accommodates the observed dark matter abundance, the heavy neutrino mass required by the baryon asymmetry, and the Pati-Salam breaking scale. The scenario predicts a stochastic gravitational wave background from Poisson fluctuations of the black hole distribution, within reach of future experiments, alongside high-frequency graviton emission constrained by future measurements of the effective number of relativistic species.

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arXiv2026

Inelastic Dark Matter at LZ from Radiative Dirac Neutrino Mass Paradigm

The LUX-ZEPLIN (LZ) experiment has reported a high-energy nuclear recoil candidate, LZ230616, at $E_R = 248 \pm 23_{\rm stat} \pm 23_{\rm sys}\,$keV, which is difficult to reconcile with elastic scattering of halo dark matter (DM). Endothermic inelastic scattering offers a natural explanation, but it rests on a nucleon coupling that is off-diagonal rather than diagonal. We show that this feature arises automatically within a radiative Dirac neutrino mass framework. The Standard Model is extended by three right-handed neutrinos (RHNs), a pair of vector-like neutral fermions for each generations, and a scalar sector comprising an inert doublet and a real singlet, governed by a $\mathcal{Z}_2 \times \mathcal{Z}_4$ symmetry. The $\mathcal{Z}_2$ symmetry stabilizes the DM, while the $\mathcal{Z}_4$ symmetry forbids the tree-level Dirac Yukawa coupling together with all renormalizable Majorana mass terms, thereby allowing the generation of Dirac neutrino masses at the one-loop level through a softly broken scalar trilinear coupling `$κ$'. Since the neutral dark-sector fields can be expressed in terms of real scalar mass eigenstates, the $Z$ boson couples to them purely off-diagonally and elastic $Z$-mediated scattering is absent identically rather than simply suppressed. The singlet--doublet mixing induced by `$κ$' governs both the inelastic rate and the one-loop Dirac neutrino mass, which therefore vanish as $κ\to 0$. We find that DM masses in the few hundred GeV to TeV range, with splittings of $\mathcal{O}(340-360)\,$ keV, simultaneously reproduce the observed relic abundance, account for the LZ event and yield neutrino masses of the correct order, while respecting elastic direct-detection limits and all relevant theoretical and experimental constraints. A substantial part of the surviving parameter space lies within reach of DARWIN.

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arXiv2026

Boosted or Inelastic? Discriminating Interpretations of the LZ 248 keV Event

The LUX-ZEPLIN experiment has reported a single nuclear recoil candidate at $E_R = 248 \pm 23(\mathrm{stat}) \pm 23(\mathrm{sys})$~keV, disfavouring the background-only hypothesis at a global significance of $2.6σ$. The difficulty such an event poses is not the recoil energy itself but the absence of any accompanying excess at low energy as elastic scattering of halo dark matter yields a monotonically falling spectrum, and supplying the required momentum transfer $q \simeq 246$~MeV already demands $m_χ\gtrsim 79$~GeV. We confront the event with the two kinematically distinct mechanisms that evade this limitation, endothermic inelastic dark matter, in which a mass splitting $δ\sim \mathcal{O}(100)$~keV forbids low-energy recoils and boosted dark matter, in which a light relativistic flux supplies the momentum, treating both with the same model-independent scalar--scalar, pseudoscalar--scalar and pseudoscalar--pseudoscalar effective operators. The two scenarios prove spectrally distinguishable. The inelastic spectra sit near the observed energy for every operator, placing only $6$--$17\%$ of events below 150~keV, whereas the boosted spectra depend critically on the operator: the scalar and pseudoscalar--scalar interactions place $99\%$ and $92\%$ of their events below 150~keV, while the pseudoscalar--pseudoscalar interaction places $75\%$ above it. Momentum dependence is thus essential to the boosted interpretation, but in the inelastic case it trades against the splitting, the preferred $δ$ decreasing monotonically from $\mathcal{O}_{ss}$ to $\mathcal{O}_{ps}$ to $\mathcal{O}_{pp}$. Because the scenarios differ across the whole high-energy window, a handful of additional events would separate them, placing the question within reach of the full LZ exposure.

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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 $ϕ$. 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, $ΔN_{\rm eff}$, and gravitational-wave signatures.

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arXiv2026

Scalar Portal Verifiable Light Dark Matter and Correlated Gravitational Wave Signatures

The lack of signals in direct detection experiments has placed the canonical Weakly Interacting Massive Particle (WIMP) paradigm under severe tension, motivating a shift toward the sub-GeV Light Dark Matter (LDM) regime. However, realizing detectable LDM interaction rates typically requires large couplings to the visible sector, which leads to a severe thermal underabundance of the dark matter relic density within standard cosmology. Furthermore, LDM models featuring vector mediators face stringent constraints from the Cosmic Microwave Background (CMB) due to late-time energy injection. In this work, we propose a minimal scalar portal extension featuring a vector-like fermion dark matter candidate, which naturally evades CMB bounds via inherent p-wave annihilation suppression. To simultaneously achieve the correct relic density and large direct-detection couplings, we invoke a pre-Big Bang Nucleosynthesis (BBN) non-standard cosmology dominated by a stiff fluid ($w > 1/3$). The enhanced Hubble expansion during this epoch triggers an early dark matter freeze-out, successfully rescuing the asymptotic relic abundance. Crucially, this stiff pre-BBN phase heavily blue-shifts inflationary gravitational waves that re-enter the horizon prior to BBN, imprinting a distinct high-frequency tilt on the stochastic gravitational wave background. We establish a robust correlation between the non-standard expansion history, the particle physics parameters verifiable in future terrestrial direct detection experiments, and the unique gravitational wave signatures observable by forthcoming space-based interferometers like LISA and DECIGO. This framework highlights how multi-messenger observations can concurrently probe the dark sector and the pre-BBN thermal history of the Universe.

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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 $Φ$, which gives rise to masses for the dark gauge boson ($X_D$) and the physical scalar ($ϕ$). To achieve the correct relic abundance while evading indirect detection constraints, the $s$-wave annihilation channel $χ\barχ \rightarrow X_D ϕ$ is strictly maintained in the kinematically forbidden regime. Crucially, due to the $U(1)_D$ gauge structure, the usual $χ\barχ \rightarrow ϕϕ$ 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 $χ\barχ \rightarrow ϕϕ$ channel is also kinematically forbidden, it predominantly determines the relic density. Conversely, when the $χ\barχ \rightarrowϕϕ$ channel is kinematically allowed, a rich interplay emerges between the tree-level forbidden $χ\barχ \rightarrow X_D ϕ$ process and the loop-level allowed $χ\barχ \rightarrowϕϕ$ 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 $χ\barχ \rightarrowϕϕ$ final state process, rendering the scenario entirely safe from CMB and indirect search bounds...

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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

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

Light thermal dark matter models in the light of DAMIC-M 2025 constraints

We study the viability of light thermal dark matter (DM) in sub-GeV mass range in view of the stringent new DAMIC-M limits on DM-electron scattering. Considering a Dirac fermion singlet DM charged under a new Abelian gauge symmetry $U(1)$, we outline two possibilities: (i) family non-universal $U(1)$ gauge coupling with resonantly enhanced DM annihilation into standard model (SM) fermions and (ii) family universal dark $U(1)$ gauge symmetry where relic is set by DM annihilation into light gauge bosons. As an illustrative example of the first class of models, we consider a gauged $L_μ-L_τ$ extension of the SM having interesting detection prospects at several experiments. While both of these class of models lead to observed DM relic and consistency with DAMIC-M together with other experimental limits, the second class of models also lead to strong DM self-interactions, potentially solving the small-scale structure issues of cold dark matter. While a vast part of the parameter space in both the models is already ruled out, the current allowed region of parameter space can be further probed at ongoing or future experiments keeping the models testable.

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arXiv2025

Cosmological Origin of the KM3-230213A event and associated Gravitational Waves

We propose a novel cosmological scenario to explain the exceptional KM3-230213A neutrino event reported at an energy scale of $\mathcal{O}(100)$~PeV by the KM3NeT collaboration, along with its associated gravitational wave (GW) signatures. In our framework, ultra high energy neutrinos originate from the decay of a super-heavy sterile neutrino produced via the Hawking evaporation of primordial black holes (PBHs) in the early Universe. Employing an ultraviolet complete type-I seesaw model, we demonstrate that while two sterile neutrinos are responsible for light neutrino masses as required by oscillation data, one sterile neutrino can have an exceedingly feeble coupling, allowing its lifetime to be tuned so that its decay yields a neutrino flux consistent with the observed event. Furthermore, our scenario predicts two distinct GW signatures: one arising from gravitons emitted during PBH evaporation and another from the Bremsstrahlung process during the decay of the sterile neutrino. These complementary signals provide a multi-messenger probe of the underlying physics. Our results thus offer a compelling explanation for the KM3-230213A event and open new avenues for investigating the interplay between high-energy neutrino astronomy and gravitational wave cosmology.

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arXiv2025

Effective theory of light Dirac neutrino portal dark matter with observable ${ΔN_{\rm eff}}$

We study the possibility of light Dirac neutrino portal dark matter (DM) in an effective field theory setup. Dirac nature of light neutrino automatically includes its right chiral part $ν_R$ which, in our setup, also acts like a portal between DM and the standard model (SM) particles. Considering a Dirac fermion singlet DM stabilized by an unbroken $Z_2$ symmetry, we write down all possible dimension-6 effective operators involving DM-$ν_R$ as well as $ν_R$-SM which conserve $Z_2$, global lepton number and SM gauge symmetries. DM thermalization also ensures the thermalization of $ν_R$, leading to enhanced effective relativistic degrees of freedom $N_{\rm eff}$, within reach of future cosmic microwave background (CMB) experiments. We study the complementarity among DM and CMB related observations for different Lorentz structures of effective operators. We also propose a UV complete gauged $\rm B-L$ symmetric model with Dirac neutrino portal dark matter.

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arXiv2025

Light thermal dark matter via type-I seesaw portal

We propose a minimal scenario for light thermal dark matter (DM) in sub-GeV to GeV range by incorporating a scalar singlet DM in a type-I seesaw scenario extended by an additional Higgs doublet $ϕ_2$. The latter permits efficient annihilation of light scalar DM into leptonic final states including right-handed neutrinos (RHN). We keep the charged fermion as well as neutral fermion final states in forbidden regime to avoid bounds from indirect search as well as cosmic microwave background (CMB) data. After studying the purely forbidden DM scenario with neutral and charged fermion final states separately, we discuss the interplay of forbidden and non-forbidden channels in generating light thermal DM relic by considering neutral fermions in non-forbidden mode due to relatively weaker constraints. The model can also explain the anomalous magnetic moment of muon, W-mass anomaly and saturate experimental bounds on charged lepton flavour violation and DM direct detection while offering tantalising detection prospects of RHN, the mass of which is kept approximately in the same range as DM.

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arXiv2025

Self-interacting dark matter with observable $ΔN_{\rm eff}$

We propose a GeV-scale self-interacting dark matter (SIDM) candidate within a dark $U(1)_D$ gauged extension of the Standard Model (SM), addressing small-scale structure issues in $Λ$CDM while predicting an observable contribution to $ΔN_{\rm eff}$ in the form of dark radiation. The model introduces a fermionic DM candidate $χ$ and a scalar $ϕ$, both charged under an unbroken $U(1)_D$ gauge symmetry. The self-interactions of $χ$ are mediated by a light vector boson $X^μ$, whose mass is generated via the Stueckelberg mechanism. The relic abundance of $χ$ is determined by thermal freeze-out through annihilations into $X^μ$, supplemented by a non-thermal component from the late decay of $ϕ$. Crucially, $ϕ$ decays after the Big Bang Nucleosynthesis (BBN) but before the Cosmic Microwave Background (CMB) epoch, producing additional $χ$ and a dark radiation species ($ν_S$). This late-time production compensates for thermal underabundance due to efficient annihilation into light mediators, while remaining consistent with structure formation constraints. The accompanying dark radiation yields a detectable $ΔN_{\rm eff}$, compatible with Planck 2018 bounds and within reach of next-generation experiments such as SPT-3G, CMB-S4, and CMB-HD.

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arXiv2025

Discrete dark matter with light Dirac neutrinos

We propose a new realisation of light Dirac neutrino mass and dark matter (DM) within the framework of a non-Abelian discrete flavour symmetry based on $A_4$ group. In addition to $A_4$, we also consider a $Z_2$ and an unbroken global lepton number symmetry $U(1)_L$ to keep unwanted terms away while guaranteeing the Dirac nature of light neutrinos. The field content, their transformations and flavon vacuum alignments are chosen in such a way that the type-I Dirac seesaw generates only one light Dirac neutrino mass while the other two masses arise from scotogenic contributions at one-loop. This leads to the Dirac scoto-seesaw framework, a generalisation of the widely studied scoto-seesaw model to Dirac neutrinos. The symmetry breaking of $A_4$ leaves a remnant $\mathcal{Z}_2$ symmetry responsible for stabilising DM. Dirac nature of light neutrinos introduces additional relativistic degrees of freedom $ΔN_{\rm eff}$ within reach of cosmic microwave background experiments.

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arXiv2025

Asymmetric long-lived dark matter and leptogenesis from type-III seesaw framework

We propose a simple model in the type-III seesaw framework to explain the neutrino mass, asymmetric dark matter (ADM), and baryon asymmetry of the Universe. We extend the standard model with a vector-like singlet lepton ($χ$) and a hypercharge zero scalar triplet ($Δ$) in addition to three hypercharge zero triplet fermions($Σ_i~,i=1,2,3$). A $Z_2$ symmetry is imposed under which $χ$ and $Δ$ are odd, while all other particles are even. As a result, the lightest $Z_2$ odd particle $χ$ behaves as a candidate of DM. In the early Universe, the $CP$-violating out-of-equilibrium decay of heavy triplet fermions to the Standard Model lepton ($L$) and Higgs ($H$) generate a net lepton asymmetry, while that of triplet fermions to $χ$ and $Δ$ generate a net asymmetric DM. The lepton asymmetry is converted to the required baryon asymmetry of the Universe via the electroweak sphalerons, while the asymmetry in $χ$ remains as a DM relic that we observe today. We introduce a singlet scalar $Φ$, with mass $M_ϕ< M_χ$, which not only assists to deplete the symmetric component of $χ$ through the annihilation process: $\barχ χ\to ΦΦ$ but also paves a path to detect DM $χ$ at direct search experiments through $Φ-H$ mixing. The electro-weak symmetry breaking induces a non-zero vacuum expectation value to $Δ$, which leads to an unstable asymmetric DM ranging from a few MeV to hundreds of GeV. We then explore the displaced vertex signatures of the charged components of the scalar triplet $Δ$ at colliders.

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arXiv2024

New realisation of light thermal dark matter with enhanced detection prospects

Light dark matter (DM) with mass around the GeV scale faces weaker bounds from direct detection experiments. If DM couples strongly to a light mediator, it is possible to have observable direct detection rate. However, this also leads to a thermally under-abundant DM relic due to efficient annihilation into light mediators. We propose a novel scenario where a first-order phase transition (FOPT) occurring at MeV scale can restore GeV scale DM relic by changing the mediator mass sharply at the nucleation temperature. The MeV scale FOPT predicts stochastic gravitational waves with nano-Hz frequencies within reach of pulsar timing array (PTA) based experiments like NANOGrav. In addition to enhancing direct detection rate, the light mediator can also give rise to the required DM self-interactions necessary to solve the small scale structure issues of cold dark matter. The existence of light scalar mediator and its mixing with the Higgs keep the scenario verifiable at different particle physics experiments.

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arXiv2024

Reconciling Cosmological Tensions with Inelastic Dark Matter and Dark Radiation in a $\boldsymbol{U(1)_D}$ Framework

We propose a novel and comprehensive particle physics framework that addresses multiple cosmological tensions observed in recent measurements of the Hubble parameter, $S_8$, and Lyman-$α$ forest data. Our model, termed `{\bf SIDR$+\boldsymbol{z_t}$}' (Self Interacting Dark Radiation with transition redshift), is based on an inelastic dark matter (IDM) scenario coupled with dark radiation, governed by a $U(1)_D$ gauge symmetry. This framework naturally incorporates cold dark matter (DM), strongly interacting dark radiation (SIDR), and the interactions between these components. The fluid-like behavior of the dark radiation component which originates from the self-quartic coupling of the $U(1)_D$ breaking scalar can suppress the free-streaming effects. Simultaneously, the interacting DM-DR system can attenuate the matter power spectrum at small scales. The inelastic nature of DM provides a distinct temperature dependence for the DM-DR interaction rate determined by the mass-splitting between the inelastic dark fermions which is crucial for resolving the Ly-$α$ discrepancies. We present a cosmologically consistent analysis of the model by solving the relevant Boltzmann equations to obtain the energy density and number density evolution of different species of the model. The DR undergoes two ``steps" of increased energy density when the heavier dark species freeze out and become non-relativistic, transferring their entropy to the dark radiation and enhancing $ΔN_{\rm eff}$. The analysis showcases the model's potential to uphold the Big Bang Nucleosynthesis (BBN) prediction of $ΔN_{\rm eff}$ but dominantly producing additional contributions prior to recombination, while simultaneously achieving correct relic density of DM though an hybrid of freeze-in and non-thermal production.

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arXiv2024

Asymmetric self-interacting dark matter with a canonical seesaw model

We study the possibility of generating dark matter (DM) and baryon asymmetry of the Universe (BAU) simultaneously in an asymmetric DM framework, which also alleviates the small-scale structure issues of cold DM. While the thermal relic of such self-interacting DM remains under-abundant due to efficient annihilation into light mediators, a nonzero asymmetry in the dark sector can lead to the survival of the required DM in the Universe. The existence of a light mediator leads to the required self-interactions of DM at small scales while keeping DM properties similar to cold DM at large scales. It also ensures that the symmetric DM component annihilates away, leaving the asymmetric part in the spirit of cogenesis. The particle physics implementation is done in canonical seesaw models of light neutrino mass, connecting it to the origin of DM and BAU. In particular, we consider type-I and type-III seesaw origin of neutrino mass for simplicity and minimality of the field content. We show that the desired self-interactions and relic of DM together with BAU while satisfying relevant constraints lead to strict limits on DM mass $\mathcal{O}({\rm GeV}) \lesssim M_{\rm DM} \lesssim460 $ GeV. In spite of being a high-scale seesaw, the models remain verifiable in different experiments, including direct and indirect DM searches as well as colliders.

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