arXiv subjects

Rishav Roshan

Publications and source records attributed to Rishav Roshan.

At least 19 recordsLinked to original sources

arXiv2026

Minimal Scale-Invariant Dark Matter

We study the minimal classically scale-invariant extension of the Standard Model, containing a single $\mathbb Z_2$-stabilised real scalar singlet whose mass is not an independent input but is generated dynamically through the quantum effective potential and linked to radiative electroweak symmetry breaking through the Higgs portal. We construct the full two-field one-loop effective potential and introduce a modified on-shell renormalisation scheme that fixes the electroweak vacuum, the Higgs mass, the vanishing Higgs--singlet mixing and the singlet curvature at the physical point, yielding predictions stable under renormalisation-scale variation. Since thermal freeze-out is excluded by direct-detection limits, we identify a highly predictive freeze-in realisation of this model. Imposing perturbativity, vacuum stability and the observed relic abundance leads to a freeze-in solution with a dark-matter mass of around $2~{\rm MeV}$. The predicted electron-scattering cross section for this solution lies far below the current sensitivity of DAMIC-M. Current direct-detection experiments therefore do not constrain this scenario. The minimal scale-invariant singlet model thus provides a robust and highly predictive framework connecting radiative electroweak symmetry breaking and freeze-in dark-matter genesis.

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arXiv2026

Particle and Gravitational Wave Probes of Minimal Seesaw Neutrinos

Observable gravitational waves (GWs) from first-order phase transitions (FOPTs) can coexist with distinct particle physics signatures. These include same-sign dilepton plus four~jet events at colliders, such as $e^+ e^-/\mu^+\mu^- \to \ell^\pm \ell^\pm 4j$, neutrinoless double beta decay, as well as charged lepton flavor violating (cLFV) processes such as $\mu \to e \gamma$. We explore this synergy within the minimal low-scale linear seesaw model. This framework successfully reproduces neutrino oscillation data, providing a direct avenue to probe the neutrino mass ordering and Majorana nature at colliders. Crucially, the FOPT responsible for the GW background is driven by a leptophilic Higgs doublet, establishing a direct link between early-universe cosmology and terrestrial laboratory experiments.

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arXiv2026

Imprint of domain wall annihilation on induced gravitational waves

Domain wall annihilation can leave a distinctive imprint on the induced gravitational wave spectrum. During annihilation, most of the domain wall energy transforms into the scalar field responsible for the initial $\mathbb{Z}_2$ symmetry breaking that created the walls, along with any coupled species. If the produced scalar is sufficiently long-lived, its delayed decay drives an early matter-dominated phase following domain wall annihilation, significantly amplifying induced gravitational waves from primordial perturbations. The subsequent transition to radiation domination dilutes the domain wall contribution through entropy injection while preserving the enhanced induced signal. This creates a gravitational wave spectrum with two distinct peaks detectable across complementary frequency bands. We explore the observable parameter space and demonstrate how multi-band detection can probe early universe symmetry breaking.

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arXiv2026

Reheating Effects on Charged Lepton Yukawa Equilibration and Leptogenesis

We show that accounting for a non-instantaneous reheating phase after inflation can significantly modify the charged lepton Yukawa equilibration temperature in the early Universe. This finding calls for revisiting the role of lepton flavors in leptogenesis models where right-handed neutrinos are produced and decay during the extended reheating period. Our analysis reveals that this effect can induce shifts in the flavor regime(s) of leptogenesis relative to standard scenario.

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arXiv2026

Twin-peaked gravitational wave signal from a dark sector phase transition

We compute the gravitational wave spectrum from a dark sector phase transition driven by spontaneous $\ZDW$ breaking. If the transition is second-order, the only source of gravitational waves is the annihilation of domain walls (biased by quantum gravity). However, if the transition is first-order, this yields a twin-peaked signal from both the transition itself and the biased domain wall annihilation. Both scenarios originate when a scalar singlet odd under the $\ZDW$ obtains a non-zero vacuum expectation value. An additional $\ZDM$ odd scalar doublet strengthens the transition and produces fermionic dark matter via freeze-in, matching observed dark matter relic density.

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arXiv2025

Cosmological Consequences of Domain Walls Biased by Quantum Gravity

One of the simplest standard model extensions leading to a domain wall network is a real scalar $S$ with a $Z_2$ symmetry spontaneously broken during universe evolution. Motivated by the swampland program, we explore the possibility that quantum gravity effects are responsible for violation of the discrete symmetry, triggering the annihilation of the domain wall network. We explore the resulting cosmological implications in terms of dark radiation, dark matter, gravitational waves, primordial black holes, and wormholes connected to baby universes.

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arXiv2025

Delayed Charged Lepton Yukawa Equilibration in Minimal Seesaw

While most studies of the \textit{minimal type-I seesaw} neglect the heaviest decoupled right-handed neutrino, assuming negligible contributions to neutrino masses and leptogenesis, we demonstrate that its cosmological role can be significant. When long-lived, the presence of this particle can substantially modify the charged lepton Yukawa equilibration temperature in the early universe, necessitating reassessment of lepton flavor effects in thermal leptogenesis and potentially shifting flavor regime boundaries. Additionally, we identify experimental probes, including neutrino oscillation measurements and gravitational wave observations to investigate this scenario. More broadly, the resulting modification of the cosmic expansion history carries implications beyond leptogenesis, impacting dark matter production and evolution, the matter-antimatter asymmetry generation, and the primordial gravitational wave spectrum.

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arXiv2025

Boomerang mechanism explaining the excess radio background

We propose a boomerang mechanism for the explanation of the excess radio background detected by ARCADE 2. In an early stage of the Universe, at a temperature $T$ in the range $\sim 0.1\,{\rm keV}$--$1\,{\rm MeV}$, a fraction of relic neutrinos is resonantly converted into dark neutrinos by mixing induced by a pre-existing lepton asymmetry. Dark neutrinos decay much later into a dark-standard photon state and a dark fermion, with a lifetime longer than the age of the Universe, as required by a solution to the excess radio background. This scenario circumvents the upper bound on the neutrino magnetic moment but still implies a testable lower bound.

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arXiv2025

Freeze-In Dark Matter and Leptogenesis: a $\psi'$SM route

We investigate the possibility of \emph{freeze-in} dark matter production and baryogenesis via leptogenesis in a $\psi'$SM model, which is an $E_6$ extension of the Standard Model, featuring a residual $U(1)_{\psi'}$ gauge symmetry. This symmetry arises from a linear combination of $U(1)_\chi$ and $U(1)_{\psi}$, both of which are subgroups of the $E_6$. The spontaneous breaking of $U(1)_{\psi'}$ symmetry governs the dynamics of a singlet fermion, which serves as a freeze-in dark matter candidate. The dark matter mass arises from dimension-five operators, and a discrete symmetry ensures its stability. We show that freeze-in production from scalar decay can yield the correct relic abundance for dark matter masses between a few MeV to a few hundred GeV. Simultaneously, heavy right-handed neutrinos generate light neutrino masses via the type-I seesaw and produce the observed baryon asymmetry via leptogenesis.

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arXiv2025

Leptogenesis with Majoron Dark Matter

We discuss a model of neutrino mass based on the type I seesaw mechanism embedded in a spontaneously broken global lepton number framework with a $Z_2$ symmetry. We show that the resulting Majoron is a viable freeze-in dark matter candidate. Two right-handed neutrinos are assumed to have dominant off-diagonal masses suggesting resonant leptogenesis as the origin of baryon asymmetry of the Universe. Explicit higher dimensional lepton number violating operators, are shown to play a crucial role in simultaneously controlling both the Majoron production in the early Universe and the right handed neutrino mass splitting relevant for resonant leptogenesis. We perform a combined analysis of Majoron dark matter and leptogenesis, discussing the relative importance of self energy and vertex contributions to CP asymmetry, and explore the parameter space, leading to an intricate relation between neutrino mass, dark matter and baryon asymmetry.

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arXiv2025

Testing leptogenesis and dark matter production during reheating with primordial gravitational waves

We study the generation of baryon asymmetry as well as dark matter (DM) in an extended reheating period after the end of slow-roll inflation. Within the regime of perturbative reheating, we consider different monomial potential of the inflaton field during reheating era. The inflaton condensate reheats the Universe by decaying into the Standard Model (SM) bath either via fermionic or bosonic decay modes. Assuming the leptogenesis route to baryogenesis in a canonical seesaw framework, we consider both the radiation bath and perturbative inflaton decay to produce such RHNs during the period of reheating when the maximum temperature of the SM bath is well above the reheating temperature. The DM, assumed to be a SM gauge singlet field, also gets produced from the bath during the reheating period via UV freeze-in. In addition to obtaining different parameter space for such non-thermal leptogenesis and DM for both bosonic and fermionic reheating modes and the type of monomial potential, we discuss the possibility of probing such scenarios via spectral shape of primordial gravitational waves.

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arXiv2024

Gravitational Wave Production During Reheating: From the Inflaton to Primordial Black Holes

We calculate the gravitational waves (GWs) produced by primordial black holes (PBHs) in the presence of the inflaton condensate in the early Universe. Combining the GW production from the evaporation process, the gravitational scattering of the inflaton itself, and the density fluctuations due to the inhomogeneous distribution of PBHs, we propose for the first time a complete coherent analysis of the spectrum, revealing three peaks, one for each source. Three frequency ranges ($\sim$ kHz, GHz, and PHz, respectively) are expected, each giving rise to a similar GW peak amplitude $Ω_{\rm GW}$. We also compare our predictions with current and future GWs detection experiments.

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arXiv2024

Observable gravitational waves and $ΔN_{\rm eff}$ with global lepton number symmetry and dark matter

We study the possibility of testing a dark matter (DM) scenario embedded in a global lepton number symmetry $U(1)_L$ via gravitational waves (GW) and cosmic microwave background (CMB) observations. The spontaneous breaking of $U(1)_L$ symmetry generates the seesaw scale as well as DM mass dynamically. The (pseudo) Nambu-Goldstone boson, known as majoron, acquires non-zero mass due to soft symmetry breaking terms of quadratic type in the scalar potential, which eventually breaks $U(1)_L$ to its $Z_2$ subgroup. The spontaneous symmetry breaking, which effectively breaks $Z_2$, leads to the formation of domain walls (DW), posing a threat to successful cosmology, if allowed to dominate. As gravity does not respect any global symmetries, we consider higher dimensional operators suppressed by the scale of quantum gravity (QG) namely, $Λ_{\rm QG}$ which introduces the required bias leading to DW annihilation and emission of stochastic gravitational waves (GW) observable at near future experiments. The same operators also lead to decay of DM bringing interesting indirect detection aspects. While DM is produced non-thermally via scalar portal interactions, light majoron can give rise to additional $ΔN_{\rm eff}$ within reach of future CMB experiments.

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arXiv2024

Using gravitational waves to see the first second of the Universe

Gravitational waves are a unique probe of the early Universe, as the Universe is transparent to gravitational radiation right back to the end of inflation. In this article, we summarise detection prospects and the wide scope of primordial events that could lead to a detectable stochastic gravitational wave background. Any such background would shed light on what lies beyond the Standard Model, sometimes at remarkably high scales. We overview the range of strategies for detecting a stochastic gravitational wave background before delving deep into three major primordial events that can source such a background. Finally, we summarize the landscape of other sources of primordial backgrounds.

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arXiv2024

Leptogenesis, Dark Matter and Gravitational Waves from Discrete Symmetry Breaking

We analyse a model that connects the neutrino sector and the dark sector of the universe via a mediator $Φ$, stabilised by a discrete $Z_4$ symmetry that breaks to a remnant $Z_2$ upon $Φ$ acquiring a non-zero vacuum expectation value ($v_ϕ$). The model accounts for the observed baryon asymmetry of the universe via additional contributions to the canonical Type-I leptogenesis. The $Z_4$ symmetry breaking scale ($v_ϕ$) in the model not only establishes a connection between the neutrino sector and the dark sector, but could also lead to gravitational wave signals that are within the reach of current and future experimental sensitivities.

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arXiv2024

Relic neutrino decay: a solution to the excess radio background

The detection of excess radio background detected by ARCADE 2 suggests the presence of new physics as there exists no clear astrophysical solution. We find that the radiative decay of a relic neutrino into a sterile neutrino, assumed to be quasi-degenerate, provides a very good fit to ARCADE 2 data. The solution also predicts a stronger 21 cm absorption global signal than the predicted one from the $Λ$CDM model, with a contrast brightness temperature. The solution obtained is in mild tension with a much stronger signal claimed by the EDGES collaboration.

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arXiv2024

Quantum Gravity Effects on Fermionic Dark Matter and Gravitational Waves

We explore the phenomenological consequences of breaking discrete global symmetries in quantum gravity (QG). We extend a previous scenario where discrete global symmetries are responsible for scalar dark matter (DM) and domain walls (DWs), to the case of fermionic DM, considered as a feebly interacting massive particle, which achieves the correct DM relic density via the freeze-in mechanism. Due to the mixing between DM and the standard model neutrinos, various indirect DM detection methods can be employed to constrain the QG scale, the scale of freeze-in, and the reheating temperature simultaneously. Since such QG symmetry breaking leads to DW annihilation, this may generate the characteristic gravitational wave background, and hence explain the recent observations of the gravitational wave spectrum by pulsar timing arrays. This work therefore highlights a tantalizing possibility of probing the effective scale of QG from observations.

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arXiv2024

Probing high scale seesaw and PBH generated dark matter via gravitational waves with multiple tilts

We propose a scenario where a high scale seesaw origin of light neutrino mass and gravitational dark matter (DM) in MeV-TeV ballpark originating from primordial black hole (PBH) evaporation can be simultaneously probed by future observations of stochastic gravitational wave (GW) background with multiple tilts or spectral breaks. A high scale breaking of an Abelian gauge symmetry ensures the dynamical origin of the seesaw scale while also leading to the formation of cosmic strings responsible for generating stochastic GW background. The requirement of a correct DM relic in this ballpark necessitates the inclusion of a diluter as PBH typically leads to DM overproduction. This leads to a second early matter dominated epoch after PBH evaporation due to the long-lived diluter. These two early matter dominated epochs, crucially connected to the DM relic, lead to multiple spectral breaks in the otherwise scale-invariant GW spectrum formed by cosmic strings. We find interesting correlations between DM mass and turning point frequencies of GW spectrum which are within reach of several near future experiments like LISA, BBO, ET, CE, etc.

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