arXiv subjects

Kousik Loho

Publications and source records attributed to Kousik Loho.

9 recordsLinked to original sources

arXiv2026

Impact of Bubble Nucleation History and Friction on Primordial Black Hole Formation

Cosmological first-order phase transitions (FOPTs) in the early Universe are an exciting prediction of many beyond the Standard Model scenarios. Owing to the stochastic nature of bubble nucleation, some causal patches may remain trapped in the false vacuum long after the surrounding regions have completed the transition and started to redshift. Under suitable conditions, these delayed patches can become sufficiently overdense to collapse into primordial black holes (PBHs). One therefore expects characteristic parameters, such as the PBH mass and PBH formation time to be sensitive to the underlying phase-transition dynamics. Here, we adopt a model-independent framework with a fixed mean bubble separation scale ($R_*$) to directly compare the impact of exponential and Gaussian nucleation profiles on PBH formation. We further incorporate the effects of friction on the bubble walls arising from interactions with the surrounding plasma. We find that friction significantly modifies the bubble dynamics, leading to significant changes in the PBH mass and formation time for both nucleation histories. Moreover, the stochastic gravitational-wave spectrum generated by the FOPT can provide a complementary probe of the underlying dynamics, allowing the effects of friction on the bubble wall evolution to be distinguished from the frictionless case.

hep-ph↗

arXiv2026

Primordial black hole evaporation in a thermal bath and gravitational waves

Primordial black holes (PBHs) formed in the early Universe evaporate via Hawking radiation and constitute a generic source of stochastic gravitational waves. Existing studies of gravitational wave production from evaporating PBHs typically assume vacuum evaporation, neglecting the fact that PBHs in the early Universe are embedded in a hot thermal plasma. In this work, we investigate gravitational wave production from primordial black holes whose evaporation is thermally influenced by their surrounding environment. We adopt a thermal evaporation framework in which interactions with the ambient plasma modify the effective decay rate of the black hole, leading to enhanced mass loss at early times and a redistribution of the evaporation history compared to the standard non-thermal vacuum case. Since graviton emission is intrinsically tied to the evaporation history of PBHs, these thermal effects play a crucial role in determining the timing and spectral properties of the resulting stochastic gravitational wave background. Our results provide a consistent framework for incorporating thermal effects into gravitational wave production from evaporating primordial black holes and set the stage for a detailed analysis of their observational signatures.

hep-ph↗

arXiv2025

From $U(1) \times U(1)$ Symmetry Breaking to Majoron Cosmology: Insights from NANOGrav 15-year Data

We study the cosmology of a modified majoron model motivated by the need to protect a global $U(1)$ symmetry from gravity-induced hard explicit breaking (by $d \leq 4$ operators) at the Planck scale. The model extends the Standard Model by introducing a gauged $U(1)_{B-L}$ and an approximate global $U(1)$ symmetry, each spontaneously broken by a corresponding complex scalar singlet. This setup gives rise to a network of effectively global and local cosmic strings, whose stochastic gravitational wave signals can jointly account for the spectrum observed by the NANOGrav collaboration, particularly for majoron masses $m_{\chi} < 10^{-23}$ eV. Although the fit is not as strong as that from supermassive black hole mergers, the model still provides an alternative explanation rooted in high-energy physics. The model also generates light neutrino masses via the seesaw mechanism and avoids cosmological constraints from $\Delta N_{\text{eff}}$, CMB anisotropies, and isocurvature fluctuations. Although the majoron can contribute to dark matter through thermal, coherent oscillation, and string-induced production mechanisms, its relic abundance remains subdominant in the NANOGrav-compatible region. In contrast, the measured dark matter relic density is achievable at higher $m_\chi$, though at the cost of tension with cosmological bounds. If the NANOGrav fits are viewed as constraints, given their comparatively lower Bayes factors, they yield bounds that are significantly stronger than those imposed by the CMB and other cosmological data.

hep-ph↗

arXiv2024

Asymmetries from a charged memory-burdened PBH

We explore a purely gravitational origin of observed baryon asymmetry and dark matter (DM) abundance from asymmetric Hawking radiation of light primordial black holes (PBH) in presence of a non-zero chemical potential, originating from the space-time curvature. Considering the PBHs are described by a Reissner-Nordstr\"{o}m metric, and are produced in a radiation dominated Universe, we show, it is possible to simultaneously explain the matter-antimatter asymmetry along with right DM abundance satisfying bounds from big bang nucleosynthesis, cosmic microwave background and gravitational wave energy density due to PBH density fluctuation. We also obtain the parameter space beyond the semiclassical approximation, taking into account the quantum effects on charged PBH dynamics due to memory burden.

hep-ph↗

arXiv2024

Constraining burdened PBHs with gravitational waves

We investigate the implications of memory burden on the gravitational wave (GW) spectrum arising from the Hawking evaporation of light primordial black holes (PBHs). By considering both rotating (Kerr) and non-rotating (Schwarzschild) PBHs, we demonstrate that the overproduction of primordial GWs from burdened PBHs could impose stringent constraints on the parameters governing backreaction effects. These constraints, derived from $\Delta N_{\rm eff}$ measurements by Planck and prospective experiments such as CMB-S4 and CMB-HD, offer novel insights into the impact of memory burden on PBH dynamics.

gr-qc↗

arXiv2023

Multicomponent Scalar Dark Matter with an Extended Gauge Sector

We consider an extension of the Standard Model of particle physics with an additional $SU(2)$ gauge sector along with an additional scalar bidoublet and a non-linear sigma field. The neutral components of the bidoublet serve as dark matter candidates by virtue of the bidoublet being odd under a $Z_2$ symmetry. Generic beyond Standard Model constraints like vacuum stability, invisible decay of higgs, Higgs alignment limit and collider constraints on heavy gauge bosons restrict the parameter space of this model. In this multicomponent dark matter scenario, we investigate the interplay between the annihilation and co-annihilation channels originating from the new gauge sector as those contribute to the relic abundance. We also inspect the direct detection constraints on scattering cross-sections of the dark matter particles with the detector nucleons and present our observations.

hep-ph↗

arXiv2023

Dark matter production from two evaporating PBH distributions

Particulate Dark Matter (DM), completely isolated from the Standard Model particle sector, can be produced in the early universe from Primordial Black Hole (PBH) evaporation. However, Big Bang Nucleosynthesis (BBN) observations put an upper bound on the initial mass of PBH requiring the PBH to evaporate completely before the advent of BBN. DM particles in the mass range $\sim(1-10^9)$ GeV can not explain the observed relic abundance for an early matter dominated universe due to this BBN constraint. However, this assumes the presence of only one monochromatic PBH mass distribution in the early universe. In this work, we explore the simple possibility of achieving the observed relic with DM masses from the above mentioned range for an early matter dominated era with two monochromatic evaporating PBH mass distributions and demonstrate that the fermionic DM masses consistent with BBN change slightly.

hep-ph↗

arXiv2022

Dark Sector extensions of the Littlest Seesaw in the presence of Primordial Black Holes

The Littlest Seesaw model is a very well motivated dark matter model. Here we consider an extension of that model with an additional scalar and an additional fermionic particle under the freeze-in scenario. Formation of black hole of a certain mass range at primordial times can act as an alternate production mechanism for the dark matter particles as it evaporates via Hawking radiation. Furthermore, the presence of primordial black holes with substantial energy density gives rise to non-standard cosmology which also modifies the freeze-in production. In this paper, we have investigated the extended Littlest Seesaw model under the freeze-in scenario in the presence of a primordial black hole for various interesting cases and constrained the parameter space accordingly. If the universe is primordial black hole dominated at any point in the evolution of the universe, we find that the final relic in that case is dominated mostly by the evaporation component for a high dark matter mass and by the freeze-in component for a low dark matter mass.

hep-ph↗

arXiv2021

An update on the two singlet Dark Matter model

We revisit the two real singlet extension of the Standard Model with a $Z_2\times Z_2^\prime$ symmetry. One of the singlet scalars $S_2$, by virtue of an unbroken $Z_2^\prime$ symmetry, plays the role of a stable dark matter candidate. The other scalar $S_1$, with spontaneously broken $Z_2$-symmetry, mixes with the SM Higgs boson and acts as the scalar mediator. We analyze the model by putting in the entire set of theoretical and recent experimental constraints. The latest bounds from direct detection Xenon1T experiment severely restricts the allowed region of parameter space of couplings. To ensure the dark matter satisfies the relic abundance criterion, we rely on the Breit-Wigner enhanced annihilation cross-section. Further, we study the viability of explaining the observed gamma-ray excess in the galactic center in this model with a dark matter of mass in the $\sim 36-51$ GeV window and present our conclusions.

hep-ph↗