We investigate the phenomenological aspects of a feebly interacting sterile neutrino dark matter candidate within a low-scale seesaw framework. The Type-I seesaw model is augmented by a second complex scalar doublet ($\Phi_{\nu}$), which couples exclusively to the heavy right-handed neutrinos and the lepton doublet, thereby generating the neutrino Dirac mass term while the first scalar doublet is responsible for giving mass to the remaining Standard Model particles.
The lightest sterile neutrino ($N_1$) acts as a feebly interacting massive particle (FIMP), produced via decays of $W^\pm$, $Z$ and extra scalars present in the setup. We point out that $W^\pm$ and $Z$ contributions were overlooked in the previous studies, which actually dominate the $N_1$ production by a factor of $\sim 10^{13}$ and solely determines the relic abundance. Incorporating them leads to several novel consequences for the DM phenomenology like a new non-thermal condition which leads to smaller Yukawa couplings. We thoroughly discuss the enhancement possibilities of $N_1$'s mass, which is controlled by the small vacuum expectation value ($v_{\nu}$) of the second Higgs doublet. After incorporating the latest Lyman-$\alpha$ forest observations, this setup can accommodate both warm and cold dark matter scenarios. We also discussed the dominant role of SM gauge bosons in dark matter production through heavy-light mixing ($V_{ij} = \frac{M_{D_{ij}}}{M_{N_{j}}}$), which leads to interactions between the heavy right-handed neutrinos with the $W$ and $Z$ bosons. In the context of the FIMP scenario, however, the dark matter couplings are inherently required to be extremely small to remain out-of-equilibrium. So, the out-of-equilibrium condition is the holy grail of the freeze-in mechanism.

This work presents a comprehensive investigation of the formation and cosmological implications of domain walls within the framework of Einstein–Gauss–Bonnet (EGB) gravity. A pivotal feature of this model is the capacity for the scalar field Lagrangian to undergo a spontaneous process of Z₂ symmetry breaking and restoration. This phase transition is a fundamental prerequisite for the formation of topological defects, specifically domain walls, which arise as solitonic solutions interpolating between the distinct vacua of the theory. We perform a detailed numerical analysis of the dynamics of a neutral scalar field non-minimally coupled to the Gauss–Bonnet invariant, exploring its behavior across different cosmological backgrounds. Our findings demonstrate that the coupling to the Gauss–Bonnet term facilitates the formation of the static domain wall in terms of proper distance in a de Sitter (inflationary) background. Furthermore, we extend our analysis to a radiation-dominated epoch, where we identify that expansion leads to the "melting" of these walls. To assess the potential observational signatures of this scenario, we calculate the predicted spectrum of stochastic gravitational waves generated by the network dynamics using the CosmoLattice package. Additionally, we study the production of primordial black holes, which could be associated with the collapse of domain wall structures. Regrettably, our calculations indicate that the direct observational detection of such domain walls from this model lies beyond the reach of foreseeable experiments.

We present a framework in which the origin of the baryon asymmetry of the universe is linked to the dynamics of an asymmetric and self-interacting dark sector, while neutrinos remain Dirac fermions. The model extends the Standard Model by introducing three right-handed neutrinos, a pair of dark fermions, and two heavy scalar doublets that mediate interactions between the visible and dark sectors. A global U(1)_B-L symmetry, consistent with a possible ultraviolet gauged completion, ensures the Dirac nature of neutrinos at the renormalisable level. The out-of-equilibrium and CP-violating decays of the heavy scalar doublets generate equal and opposite asymmetries in left-handed and right-handed neutrinos. Because the Yukawa couplings of Dirac neutrinos are tiny, the left–right equilibration occurs only after sphaleron freeze-out, allowing the asymmetry stored in left-handed neutrinos to be partially converted into baryon asymmetry. The same heavy-scalar decays that generate the lepton asymmetry also induce a dark-sector asymmetry, naturally linking the cosmic abundances of visible and dark matter. The dark sector contains a light MeV-scale gauge boson associated with a secluded U(1)_D symmetry. This mediator efficiently depletes the symmetric dark matter component and provides the self-interactions required to address small-scale structure anomalies, while interacting only feebly with the Standard Model through a highly suppressed kinetic portal. The framework offers a coherent and testable connection between Dirac leptogenesis, asymmetric dark matter, and self-interacting dark-sector dynamics.

The Standard Model (SM) of particle physics, despite its tremendous success, fails to explain two key observations: the nonzero masses of neutrinos and the existence of dark matter. These fundamental shortcomings clearly indicate the need for new physics beyond the Standard Model (BSM). Among the viable frameworks addressing these challenges, the Scotogenic mechanism proposed by Ernest Ma in 2006 provides a compelling setup in which neutrino masses are generated radiatively at one loop, while a discrete Z₂ symmetry simultaneously guarantees the stability of a dark matter candidate. This framework naturally links neutrino mass generation and dark matter within a unified setup. This idea has since been generalised to the Dirac scotogenic framework, where small Dirac neutrino masses are generated radiatively through the introduction of new fields together with the additional discrete symmetries or the global U(1)_B-L symmetry already present in the SM. The Dirac scotogenic model offers an elegant framework for generating small Dirac neutrino masses radiatively at the one-loop level. A single abelian discrete symmetry, Z_6, simultaneously preserves the Dirac nature of neutrinos and ensures the stability of the dark matter candidate, emerging as an unbroken subgroup of the 445 U(1)_B-L symmetry. In this work, we present a comprehensive study of the phenomenological consequences of this construction, focusing on electroweak vacuum stability, charged lepton flavor violation, and dark matter constraints. After incorporating current theoretical and experimental bounds, we find that the model not only remains viable but also permits novel low-mass scalar and fermionic dark matter regimes—distinct from those in the canonical Majorana scotogenic scenario. These features position the framework as a compelling bridge between neutrino physics, dark matter, and BSM cosmology.

Long-lived or stable heavy multiply-charged particles X, predicted in various Beyond-the-Standard-Model (BSM) scenarios, can significantly affect Big Bang Nucleosynthesis (BBN). The neutralization process of such states via the capture of light primordial nuclei N (e.g., p, d, t, 3He, 4He) leads to the formation of bound states XN (electrically neutral dark atoms, negatively charged dark ions or positively charged anomalous isotopes) and a shift in the ratios of the primordial abundances of ordinary light elements. The dependence of reaction rates on the charge of the heavy particle and the deviations from the standard BBN predictions are studied using a combination of analytic and numerical estimates. The cross-section of the first stage of the dark recombination may be calculated numerically in the dipole approximation. The finite size of the nucleus in the shell of the dark atom is taken into account. The rates of further reactions are strongly affected by strong nuclear forces and can be estimated by scaling the experimental data for the corresponding ordinary nuclear fusion processes. The reduced Coulomb barrier and modified reduced masses are considered. Changes in the reaction network are predicted for high charges. To avoid contradictions with the observed ratios of the primordial abundances, the fine-tuning of the model parameters (charge, mass, and baryon-to-photon ratio) may be required.

The origin of the large-scale magnetic fields permeating the Universe and the nature of dark matter are two of the most enduring open questions in modern cosmology. In this work, we investigate a unified early-universe scenario in which both phenomena arise naturally from a single primordial magnetogenesis mechanism. We show that the generation of primordial magnetic fields during the post-inflationary era not only provides seeds for the coherent magnetic fields observed today on cosmological scales, but also enhances curvature perturbations at intermediate wavelengths. These amplified perturbations become sufficiently large to trigger the formation of primordial black holes (PBHs), which can contribute substantially—if not dominantly—to the present dark-matter abundance. The same enhanced scalar fluctuations responsible for PBH formation inevitably induce a stochastic background of gravitational waves (GWs). We analyze the spectral features of this induced GW background and demonstrate that it carries a detailed imprint of the underlying magnetogenesis model and the expansion history of the Universe, particularly during non-standard reheating phases. By combining constraints from large-scale magnetic fields, PBH abundances, and induced GWs, we establish a powerful multi-messenger approach for probing the physics of the early Universe.
Furthermore, we highlight how upcoming and next-generation gravitational-wave detectors—such as LISA, DECIGO, BBO, and the SKA pulsar-timing array—can measure or tightly constrain the predicted GW signal. Such observations offer a unique opportunity to reconstruct the spectral behavior of the induced GWs and thereby uncover information about the reheating dynamics and the primordial magnetic field spectrum. Our results emphasize that jointly analyzing electromagnetic, gravitational-wave, and early-universe cosmological signatures provides an exceptionally sensitive pathway to uncovering the mechanisms that shaped cosmic magnetism and the origin of dark matter.