arXiv subjects

Sudhakantha Girmohanta

Publications and source records attributed to Sudhakantha Girmohanta.

At least 19 recordsLinked to original sources

arXiv2026

Neutron Portal and Dark Matter-Baryon Coincidence: from UV Completion to Phenomenology

We present a dynamical solution to the dark matter-baryon coincidence problem based on the neutron portal operator connecting the visible and dark sector asymmetries. This framework is motivated by the possibility that a strongly supercooled dark confinement phase transition accounts for the nano-Hz stochastic gravitational wave signal observed by pulsar timing arrays, while also generating the dark matter and baryon asymmetry in the Universe. We show that the GeV-scale mass of asymmetric dark matter can be naturally correlated with the (multi-)TeV scale cut-off for the neutron portal through its ultraviolet completion. The dark sector is governed by an approximate fixed point and confines once the heavy portal states are integrated out, dynamically generating a scale of $\mathcal{O} ({\rm GeV})$. We analyze both tree and loop-level ultraviolet completions and demonstrate how the resulting confinement scale is linked to the effective neutron portal scale. We also discuss cosmological constraints and experimental prospects in beam dump searches and colliders for probing the neutron portal.

hep-ph↗

arXiv2026

Implications of DESI for Dark Matter & Cosmic Birefringence

We explore an interacting dark matter (DM)-dark energy (DE) framework that naturally yields an effective dynamical DE equation of state crossing the phantom barrier at early times, as indicated by recent DESI data, while also accounting for the observed isotropic rotation of the cosmic microwave background (CMB) linear polarization. Within this unified framework, we also explain the DM relic abundance without introducing additional fields or couplings. Depending on the DE potential, we identify two viable scenarios: a superheavy freeze-in DM requiring a high reheating temperature, or a strongly interacting dark sector with a GeV-TeV scale thermal DM candidate.

hep-ph↗

arXiv2025

Quest for a phenomenologically consistent low cutoff theory

The Randall-Sundrum model with the Higgs localized on the IR brane solves the gauge hierarchy problem. However, the associated low cutoff ($Λ\sim 10$ TeV) generically leads to unacceptably rapid nucleon decay and excessively large Majorana neutrino masses. Achieving consistency while simultaneously explaining the Yukawa hierarchy requires either a horizontal symmetry or a discrete gauged symmetry. We demonstrate that eliminating all dangerous operators within a horizontal symmetry framework must come with large and unattractive charge assignments, if possible at all. Hence, we consider an exact discrete gauged $\mathbf{Z}_N$ symmetry, with fermion mass hierarchies generated via wave function overlap. We employ this to reproduce the current Cabibbo-Kobayashi-Maskawa and Pontecorvo-Maki-Nakagawa-Sakata structures. Assuming universal five-dimensional Yukawa couplings, generation-blind profile for right-handed neutrinos and flat profile for the third generation SM doublets, it predicts Dirac neutrinos with a total mass $\sim 66$ meV. Since the $\mathbf{Z}_N$ charges must be generation blind, flavor observables serve as key probes.

hep-ph↗

arXiv2025

Postsphaleron darkogenesis

A supercooled phase transition in a nearly conformal dark sector can provide a natural setting for darkogenesis via its out-of-equilibrium dynamics, where a particle-antiparticle number asymmetry in the dark sector can be reprocessed into the visible sector, yielding the observed baryon asymmetry and an asymmetric dark matter. We consider a scenario where the number asymmetry is generated from the decay of a mother particle produced via parametric resonance during the phase transition induced due to its coupling to the dilaton associated with spontaneous breaking of scale invariance. It is shown that the correct baryon asymmetry and dark matter abundance can be realized for a dark phase transition at $\mathcal{O}(1) \, \rm GeV$, which can also explain the nano-Hz gravitational wave signal reported by pulsar timing array experiments. The scenario will be tested further in neutron-antineutron oscillation experiments.

hep-ph↗

arXiv2025

Interplay of ALP Couplings at a Muon Collider

Axion-like particles can couple to Standard Model gluons, electroweak gauge bosons, and massive fermions. A future multi-TeV muon collider provides a favorable environment to probe axion-like particles through multiple production channels, including vector boson fusion via electroweak gauge boson couplings and the top-associated production mediated by direct fermionic couplings. Motivated by the quality issue of the QCD axion, we focus on axion-like particles with masses and decay constants around the TeV scale. We explore how different axion-like particle couplings shape its production and decay modes, revealing a rich and intricate phenomenological landscape.

hep-ph↗

arXiv2025

Wiggly dilaton: a landscape of spontaneously broken scale invariance

The dilaton emerges as a pseudo-Nambu-Goldstone boson (pNGB) associated with the spontaneous breaking of scale invariance in a nearly conformal field theory (CFT). We show the existence of a wiggly dilaton potential that contains multiple vacuum solutions in a five-dimensional (5D) holographic formulation. The wiggly feature originates from boundary potentials of a 5D axion-like scalar field, whose naturally small bulk mass parameter corresponds to a marginally-relevant deformation of the dual CFT. Depending on the energy density of a boundary 3-brane, our model can provide a relaxion potential or generate a light dilaton. However, an extremely light dilaton requires fine-tuning.

hep-th↗

arXiv2025

Multi-Messenger and Cosmological Constraints on Dark Matter through Two-Fluid Neutron Star Modeling

In this study, we investigate the impact of dark matter (DM) on neutron stars (NSs) using a two-fluid formalism that treats nuclear matter (NM) and DM as gravitationally coupled components. Employing NM equations of state spanning a wide range of stiffness and a self-interacting asymmetric fermionic DM framework, we explore the emergence of DM core- and halo-dominated structures and their observational implications. Constraints from gravitational waves (GW170817), NICER X-ray measurements (PSR J0030+0451), and pulsar mass limits (PSR J0740+6620) delineate a consistent parameter space for DM properties derived from these multi-messenger observations. DM halo-dominated configurations, while consistent with PSR J0740+6620's mass limits and NICER's radius measurements for PSR J0030+0451, are ruled out by the tidal deformability bounds inferred from the GW170817 event. Consequently, the combined limits inferred from the observational data of GW170817, PSR J0030+0451, and PSR J0740+6620 support the plausibility of DM core-dominated configurations. Constraints on the DM self-interaction strength from galaxy cluster dynamics further refine the DM parameter space permitted by NS observations. This work bridges multi-messenger astrophysics and cosmology, providing insights into DM interactions and their implications for NS structure, evolution, and observational signatures.

astro-ph.HE↗

arXiv2024

Note on Warped Compactification -- Finite Brane Potentials and Non-Hermiticity --

We study radius stabilization in the Randall-Sundrum model without assuming any unnaturally large stabilizing scalar potential parameter at the boundary branes ($γ$) by the frequently used superpotential method. Employing a perturbative expansion in $1/γ^2$ and the backreaction parameter, we obtain approximate analytical expressions for the radion mass and wavefunction. We validate them through a dedicated numerical analysis, which solves the linearized coupled scalar and metric field equations exactly. It is observed that the radion mass decreases with decreasing $γ$. Below a critical value of $γ$, the radion becomes tachyonic, suggesting destabilization of the extra dimension. We also address the issue of non-Hermiticity of the differential operator that determines the radion and Kaluza-Klein (KK) mode wavefunctions in the finite $γ$ limit. It is accomplished by finding an explicit form of the general scalar product that re-establishes the orthogonality in the KK decomposition.

hep-th↗

arXiv2024

How Viable Is a QCD Axion near 10 MeV?

There has been an attempt to revive the visible QCD axion at the 10 MeV scale assuming that it exclusively couples to the first-generation quarks and the electron. This variant of the QCD axion is claimed to remain phenomenologically viable, partly due to a clever model construction that induces tree-level pion-phobia and exploits uncertainties inherent in the chiral perturbation theory. We confront this model with the cosmological domain wall problem, the quality issue and constraints arising from the electron electric dipole moment. It is also pointed out that the gluon loop-generated axion-top coupling can provide a very large contribution to rare $B$-meson decays, such that the present LHCb data for $B^0 \to K^{*0} e^+ e^-$ rule out the model for the axion mass larger than 30 MeV. There is a strong motivation for pushing the experimental analysis of $B \to K^{(*)} e^+ e^-$ to a lower $e^+ e^-$ invariant mass window, which will conclusively determine the fate of the model, as its contribution to this branching ratio significantly exceeds the Standard Model prediction.

hep-ph↗

arXiv2024

Cold Darkogenesis: Dark Matter and Baryon Asymmetry in Light of the PTA Signal

We build upon the intriguing possibility that the recently reported nano-Hz gravitational wave signal by Pulsar Timing Array (PTA) experiments is sourced by a strong first-order phase transition from a nearly conformal dark sector. The phase transition has to be strongly supercooled to explain the signal amplitude, while the critical temperature has to be in the $\cal{O}$(GeV) range, as dictated by the peak frequency of the gravitational wave spectrum. However, the resulting strong supercooling exponentially dilutes away any pre-existing baryon asymmetry and dark matter, calling for a new paradigm of their productions. We then develop a mechanism of cold darkogenesis that generates a dark asymmetry during the phase transition from the textured dark $SU(2)_{\rm D}$ Higgs field. This dark asymmetry is transferred to the visible sector via neutron portal interactions, resulting in the observed baryon asymmetry. Furthermore, the mechanism naturally leads to the correct abundance of asymmetric dark matter, with self-interaction of the scale that is of the right order to solve the diversity problem in galactic rotation curves. Collider searches for mono-jets and dark matter direct detection experiments can dictate the viability of the model.

hep-ph↗

arXiv2023

Light Dilaton in Rare Meson Decays and Extraction of its CP Property

The dilaton $ϕ$ is a pseudo-Nambu-Goldstone boson associated with the spontaneous breaking of scale invariance in a nearly conformal theory, and couples to the trace of the stress-energy tensor. We analyze experimental constraints on a light dilaton with mass in the MeV-GeV range from rare meson decays. New model-independent inclusive bounds for the $b \to s ϕ$ transition largely exclude the parameter space of a light dilaton that could explain the muon $g-2$ anomaly. Despite similarities between a dilaton and a Higgs-portal scalar, the dilaton-photon coupling is enhanced compared to the Higgs-portal scalar due to contributions from loops of the conformal sector. Consequently, the shortened lifetime of the dilaton relaxes bounds from $K \to π$ + invisible searches at the NA62 experiment and constraints from the Big Bang Nucleosynthesis. We utilize this fact to search for the dilaton signature at a lepton collider such as the ongoing Belle II experiment. Further, we demonstrate how to extract the CP property of the dilaton using the variation of the differential cross-section of $e^+ e^- \to e^+ e^- ϕ$ with the azimuthal angle between the outgoing leptons.

hep-ph↗

arXiv2023

Aiming for Tops of ALPs with a Muon Collider

Future muon colliders with center-of-mass energy of $\mathcal{O}(1-10)$ TeV can provide a clean high-energy environment with advantages in searches for TeV-scale axion-like particles (ALPs), pseudo-Nambu-Goldstone bosons associated with spontaneously broken global symmetries, which are widely predicted in physics beyond the Standard Model (SM). We exploit ALP couplings to SM fermions, and guided by unitarity constraints, build a search strategy focusing on the ALP decay to top quark pairs at muon colliders. It is found that a large parameter space of TeV-scale ALPs with TeV-scale decay constants can be probed by utilizing the ALP-top quark coupling.

hep-ph↗

arXiv2023

Multi-brane cosmology

5D warped extra dimension models with multiple 3-branes can naturally realize multiple hierarchical mass scales which are ubiquitous in physics beyond the Standard Model. We discuss cosmological consequences of such multi-brane models with stabilized radions. It is confirmed that for temperatures below the scale of the IR brane at the end of the extra dimension, we recover the ordinary expansion of the Universe, with the Hubble expansion rate determined by sum of the physical energy densities on all 3-branes where they are localized. In addition, we explore the cosmology for temperatures above the scales of the intermediate and IR branes where the Universe is described by a spacetime with the 3-branes replaced by an event horizon. As the temperature of the Universe cools down, phase transitions are expected to take place, and the intermediate and IR branes come out from behind the event horizon. The Goldberger-Wise mechanism for radion stabilization has a well-known problem of having a supercooled phase transition, which typically does not get completed in time. This problem is even more severe when an intermediate brane is introduced, whose scale is well above TeV, as the corresponding Hubble rate is much larger. We circumvent the problem by employing an alternative mechanism for radion stabilization with dark Yang-Mills fields, which prevents a long supercooling epoch, but still allows the strong first order phase transitions. As a result, the phase transitions in our multi-brane Universe predict a stochastic gravitational wave background with a unique multi-peak signature, which is within the sensitivity reach of future space-based gravitational wave observers. We also show that there are $N-1$ radions for an $N$ 3-brane set-up, unlike a recent claim that there exists only one radion.

hep-ph↗

arXiv2023

Model of the quintessence axion

We construct a model of the quintessence axion based on a gauged chiral $U(1)$ symmetry and an additional flat fifth dimension. The required high qualities are guaranteed by the brane separation. The observed cosmological constant (i.e., the potential energy of the quintessence axion) is determined by the size of the extra dimension and the axion decay constant $F_a$ is fixed almost at $F_a\simeq10^{17}\,{\rm GeV}$, which is sufficiently large for the stability of the axion field near the hilltop of its potential. Furthermore, the movement of the axion can also easily explain the recently reported isotropic cosmic birefringence of the cosmic microwave background photon.

hep-ph↗

arXiv2023

NANOGrav Signal from a Dark Conformal Phase Transition

We explore the possibility that a confining first-order phase transition of a nearly-conformal dark sector generates the reported NANOGrav signal of a stochastic gravitational wave background. The visible Standard Model (SM) sector and the dark sector are initially thermally decoupled so that their temperatures are different. The nearly conformal phase transition is described by the shallow potential of a dilaton (or a radion in the 5D holographic perspective) generated by a new dark Yang-Mills field coupled to the conformal sector. For a dark sector only gravitationally connected with the visible sector, the NANOGrav signal is explained by the phase transition without contradicting the $ΔN_{\rm eff}$ constraint, together with a contribution from supermassive black hole binaries. While the dilaton and dark glueballs can be produced after the phase transition, they immediately decay into dark radiation, which can help ameliorate the Hubble tension and be tested by the future CMB-S4 experiment. Alternatively, for a dark conformal sector decaying into the visible sector after the phase transition, the $ΔN_{\rm eff}$ constraint is not applied and the phase transition can solely explain the NANOGrav signal.

hep-ph↗

arXiv2023

Fitting a Self-Interacting Dark Matter Model to Data Ranging From Satellite Galaxies to Galaxy Clusters

We present a fit to observational data in an asymmetric self-interacting dark matter model using our recently calculated cross sections that incorporate both $t$-channel and $u$-channel exchanges in the scattering of identical particles. We find good fits to the data ranging from dwarf galaxies to galaxy clusters, and equivalent relative velocities from $\sim 20$ km/sec to $\gtrsim 10^3$ km/s. We compare our results with previous fits that used only $t$-channel exchange contributions to the scattering.

hep-ph↗

arXiv2022

A Natural Model of Spontaneous CP Violation

We examine the possibility of building a natural non-supersymmetric model of spontaneous CP violation equipped with the Nelson-Barr (NB) mechanism to address the strong CP problem. Our approach is to utilize a doubly composite dynamics where the first confinement of the CFT occurs at the scale of spontaneous CP violation (SCPV) and the second confinement at the TeV scale. A holographic dual description of this 4D set-up via a warped extra dimension with three 3-branes provides an explicit realization of this idea.In this model, radiative corrections to the strong CP phase are well under control, and the coincidence of mass scales, which we generally encounter in NB models, is addressed. Our model also provides an explanation to the quark Yukawa hierarchies, and a solution to the gauge hierarchy problem just as in the usual Randall-Sundrum model with the Higgs being localized on the TeV brane.

hep-ph↗

arXiv2022

Cross Section Calculations in Theories of Self-Interacting Dark Matter

We study an asymmetric dark matter model with self-interacting dark matter consisting of a Dirac fermion $χ$ coupled to a scalar or vector mediator, such that the reaction $χ+ χ\to χ+ χ$ is well described by perturbation theory. We compute the scattering cross section $σ$, the transfer cross section $σ_T$, and the viscosity cross section $σ_V$ for this reaction. As one part of our study, we give analytic and numerical comparisons of results obtained with the inclusion of both $t$-channel and $u$-channel exchanges and results obtained in an approximation that has often been used in the literature that includes only the $t$-channel contribution. The velocity dependences of these cross sections are studied in detail and shown to be in accord with observational data.

hep-ph↗