arXiv subjects

Lekhika Malhotra

Publications and source records attributed to Lekhika Malhotra.

6 recordsLinked to original sources

arXiv2026

Probing Leptophobic Dark Sectors via Gravitational Wave Signatures

We study a minimally extended version of the Standard Model where baryon number is gauged with a $U(1)_B$ symmetry. This model can be made anomaly-free by adding a set of additional fermions. The lightest component of these fermions behaves as a viable dark matter candidate. We show that the spontaneous breaking of $U(1)_B$ symmetry can produce gravitational waves via bubble dynamics resulting from a first-order phase transition, which can be detected in future gravitational wave experiments like LISA and ET. Such gravitational wave signatures can be used as a probe to constrain the model in future observations and complement dark matter and collider searches. We perform a random numerical scan of the parameter space and derive the viable region consistent with theoretical bounds from running of the coupling constants, current experimental bounds from dark matter experiments such as LUX-ZEPLIN and XENONnT and sensitive to future gravitational wave experiments. We find that dark matter with mass of 8 - 12 TeV is the most interesting to test in future gravitational wave as well as laboratory experiments. In the viable parameter space, the mass of the $Z'$ gauge boson associated with the $U(1)_B$ lies in the 16 - 24 TeV range, and the mass of the scalar associated with the symmetry breaking lies around 1 - 2.5 TeV scale. Recent results from LUX-ZEPLIN rules out mass scales below TeV in this model, while dark matter with mass larger than 12 TeV will not be sensitive to future GW experiments. Hence, the dark matter and mediator mass scales of interest are marginally accessible at current collider energies.

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arXiv2025

Inflationary Gravitational Wave Spectral Shapes as test for Low-Scale Leptogenesis

We study non-thermal resonant leptogenesis in a general setting where a heavy majoron $ϕ$ decays to right-handed neutrinos (RHNs) whose further out-of-equilibrium decay generates the required lepton asymmetry. Domination of the energy budget of the Universe by the $ϕ$ or the RHNs alters the evolution history of the primordial gravitational waves (PGW) of inflationary origin, which re-enter the horizon after inflation, modifying the spectral shape. The decays of $ϕ$ and RHNs release entropy into the early Universe while nearly degenerate RHNs facilitate low and intermediate-scale leptogenesis. A characteristic damping of the GW spectrum resulting in knee-like features would provide evidence for low-scale non-thermal leptogenesis. We explore the parameter space for the lightest right-handed neutrino mass $M_1\in[10^2,10^{14}]$ GeV and washout parameter $K$ that depends on the light-heavy neutrino Yukawa couplings $λ$, in the weak ($K < 1$) and strong ($K > 1$) washout regimes. The resulting novel features compatible with observed baryon asymmetry are detectable by future experiments like LISA and ET. By estimating signal-to-noise ratio (SNR) for upcoming GW experiments, we investigate the effect of the majoron mass $M_ϕ$ and reheating temperature $T_ϕ$, which depends on the $ϕ-N$ Yukawa couplings $y_N$.

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arXiv2025

Inflationary Gravitational Waves and Laboratory Searches as Complementary Probes of Right-handed Neutrinos

We analyze the damping of inflationary gravitational waves (GW) that re-enter the Hubble horizon before or during a post-inflationary era dominated by a meta-stable, right-handed neutrino (RHN), whose out-of-equilibrium decay releases entropy. Within a minimal type-I seesaw extension of the Standard Model (SM), we explore the conditions under which the population of thermally produced RHNs remain long-lived and cause a period of matter-domination. We find that the suppression of the GW spectrum occurs above a characteristic frequency determined by the RHN mass and active-sterile mixing. For RHN masses in the range $0.1$-$10$ GeV and mixing $10^{-12} \lesssim |V_{eN}|^2 \lesssim 10^{-5}$, we estimate such characteristic frequencies and the signal-to-noise ratio to assess the detection prospects in GW observatories such as THEIA, $μ$-ARES, LISA, BBO and ET. Additionally we use LIGO data to put upper bounds on the reheating temperature after inflation, for a given blue-tilted GW spectrum. We find complementarity between GW signals and laboratory searches in SHiP, DUNE and LEGEND-1000. Notably, RHN masses of $0.2$-$2$ GeV and mixing $10^{-10} \lesssim |V_{eN}|^2 \lesssim 10^{-7}$ are testable in both laboratory experiments and GW observations. Additionally, GW experiments can probe the canonical seesaw regime of light neutrino mass generation, a region largely inaccessible to laboratory searches.

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arXiv2025

Minimal Dirac seesaw dark matter

We propose a minimal Type-I Dirac seesaw which accommodates a thermal scalar dark matter (DM) candidate protected by a charge conjugation symmetry in dark sector $C_{\rm dark}$, without introducing any additional field beyond the ones taking part in the seesaw. A $Z_4$ symmetry is introduced to realise the tree level Dirac seesaw while the Majorana mass terms are prevented by an unbroken global lepton number symmetry. While the spontaneous $Z_4$ breaking together with electroweak symmetry breaking lead to the generation of light Dirac neutrino mass, it also results in the formation of domain walls. These cosmologically catastrophic walls can be made to annihilate away by introducing bias terms while also generating stochastic gravitational waves (GW) within reach of near future experiments like \texttt{LISA}, \texttt{BBO}, $μ$-\texttt{ARES} etc. The scalar DM parameter space can be probed at direct and indirect search experiments. Light Dirac neutrinos also enhance the relativistic degrees of freedom $N_{\rm eff}$ within reach of future cosmic microwave background (CMB) experiments. The model can also explain the observed baryon asymmetry via Dirac leptogenesis.

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arXiv2025

Left-Right Symmetric Neutrino Mass Model without Scalar Bi-doublet

We consider a left-right symmetric model with an $SU(2)_L$ and an $SU(2)_R$ scalar doublet but without the scalar bidoublet. The charged fermion masses in this model are generated via a universal seesaw mechanism. We add a set of three gauge singlet neutral fermions with Majorana masses of the order of a TeV. The masses for the left-handed neutrinos are naturally small in this model because they occur only at one-loop and are generated through a see-saw mechanism. By an appropriate choice of the Yukawa couplings of the $SU(2)_L$ doublet and the masses of the gauge singlet fermions, it is possible to implement resonant leptogenesis at TeV scale. The right-handed sector of the model, through appropriate values of the Yukawa couplings of the $SU(2)_R$ doublet, leads to a warm dark matter candidate in the lightest right-handed neutrino with a mass of a few keV and an observable effective electron mass for neutrinoless double beta decay $m_{ββ}$.

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arXiv2023

Cogenesis of visible and dark sector asymmetry in a minimal seesaw framework

Recently there is a renewed interest in exploring the Dark sector of the universe in a more constrained way. Particularly in [1], the FIMP ( Feebly Interacting Massive Particle) scenario was shown to be realized with a minimal extension of the SM with three sterile neutrinos in the spirit of νMSM. In this paper, we show that without invoking any additional symmetries of the model, we can realize the idea of ADM (Asymmetric Dark Matter) signaling a common origin of the matter-anti-matter asymmetries in visible as well as Dark sectors. The model allows for a range of dark matter masses, 1 - 100keV, with correct active neutrino masses through the Type-I seesaw mechanism. Thus, the model explains the neutrino masses, Dark Matter abundance and replicates matter-anti-matter asymmetry of the visible sector in the Dark sector, all in a version of the νMSM. The asymmetry in the dark sector is manifested in the predominance of one parity of the heavy neutrinos in the comoving frame, which should be determinable in the ongoing experiments.

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