arXiv subjects

Ranjeet Kumar

Publications and source records attributed to Ranjeet Kumar.

At least 19 recordsLinked to original sources

arXiv2026

From Kination to Detection: Scotogenic Scalar Dark Matter

We investigate scalar dark matter in the scotogenic model during an early kination-dominated epoch. The enhanced expansion rate causes earlier freeze-out and increases the relic abundance, opening viable dark matter masses from approximately $200~\mathrm{GeV}$ to $1~\mathrm{TeV}$ while reducing the need for co-annihilation with a nearly degenerate singlet fermion. Current indirect searches strongly constrain the $(100-200)~\mathrm{GeV}$ region, whereas much of the surviving parameter space can be probed by CTA and next-generation direct detection experiments. The viable spectra also lead to complementary collider signatures, including missing transverse energy, displaced leptons, and disappearing charged tracks, providing promising tests at the HL-LHC and future lepton colliders.

hep-ph↗

arXiv2026

Generalized Chiral $U(1)_{B-L}$ with Inelastic Scalar Dark Matter for the LZ 248 keV Event

The high-energy nuclear recoil event with $E_R\simeq248$ keV recently reported by the LZ collaboration, with a background-only significance of $2.6σ$, offers an intriguing window into dark matter scattering beyond the conventional elastic picture. We study an inelastic scalar dark matter scenario within a generalized chiral $U(1)_{B-L}$ framework. After symmetry breaking, the dark sector scalar gives rise to two nearly degenerate states with a small mass splitting. The lighter state serves as the dark matter candidate and couples off-diagonally to the $Z'$ boson, leading to endothermic inelastic scattering. The resulting kinematics suppress the contribution from low-velocity dark matter while making the high-velocity tail increasingly relevant for nuclear recoils at higher energies. We identify the parameter space consistent with the observed relic abundance and current experimental constraints, and discuss the inelastic scattering kinematics relevant to the recent LZ observation, together with the complementary collider prospects for the associated $Z'$ boson.

hep-ph↗

arXiv2026

From LUX-ZEPLIN to Colliders: Probing Higgsino Dark Matter

The high-energy nuclear recoil event with recoil energy $E_R \approx 248\text{ keV}$ observed by the LZ collaboration provides an exciting hint toward a model with a $1.1\text{ TeV}$ Higgsino inelastic dark matter. Such a recoil energy requires a mass splitting of order ($δ\approx 350\text{ keV}$) between the two nearly-degenerate neutral states. We show that within the framework of the MSSM, the model predicts a nearly-degenerate charged Higgsino state, chargino, whose mass splitting from the neutral states is of order $\mathcal{O}(350)$~MeV. The subsequent decays of such charginos once produced at colliders would lead to a sub-centimeter charged track or tracklet (with lifetime $τ\approx 0.025\text{ ns}$) at the detector. We show that such a scenario is not constrained by the current LHC bounds, and could be tested at future high-energy colliders, including HL-LHC, 100 TeV $pp$ colliders, and muon colliders. The most critical requirement is how short a track or tracklet can be reconstructed.

hep-ph↗

arXiv2026

Zooming in on `bi-large' neutrino mixing with the first JUNO results

The leptonic mixing matrix is examined within bi-large mixing patterns and confronted with the latest results announced by the Jiangmen Underground Neutrino Observatory (JUNO). We analyze the viability of bi large mixing schemes and assess JUNO's ability to test neutrino mixing and discriminate among different bi-large mixing patterns, some of which are strongly disfavored when compared with neutrino oscillation global-fit results. Specific octant and CP predictions emerge. Finally, we comment on the implications of JUNO's findings for neutrinoless double beta decay.

hep-ph↗

arXiv2026

Generalized Chiral $U(1)_{B-L}$: Towards a Less Constrained $Z'$ and Dark Matter

Extensions of the Standard Model by a $U(1)_{X}$ gauge symmetry predict a new $Z'$ boson, whose mass is severely constrained by high-mass dilepton resonance searches at the LHC. We show that these bounds can be relaxed by generalizing the chiral $(B-L)$ charge assignment while preserving gauge anomaly cancellation. The resulting $U(1)_X$ charges are parameterized by two independent parameters, and an appropriate choice of these parameters suppresses the $Z'$ branching fraction into charged leptons, substantially weakening the dilepton constraints. The framework accommodates Dirac neutrino masses through a Dirac type-I seesaw mechanism and a stable singlet scalar DM candidate without requiring an additional discrete symmetry. We find that the modified charge assignment, although beneficial for collider phenomenology, introduces a tension between obtaining the observed relic abundance and satisfying direct detection limits when DM annihilation proceeds solely through the $Z'$ portal. The inclusion of scalar-mediated annihilation channels resolves this tension and opens up a broad viable parameter space, with DM masses ranging from $M_{\rm DM}\simeq 60~{\rm GeV}$ to $10~{\rm TeV}$. The resulting parameter space is consistent with the observed relic density, direct detection, collider, and electroweak precision constraints, demonstrating that a generalized chiral charge structure can simultaneously accommodate a less constrained $Z'$ sector, neutrino masses, and viable dark matter phenomenology.

hep-ph↗

arXiv2026

The Simplest Dirac Scoto-Seesaw Realization

We present a simple Dirac scoto-seesaw framework based on the anomaly-free $U(1)_{B-L}$ charge assignment $(-4,-4,5)$ for $ν_R$. This chiral charge assignment naturally accounts for the observed neutrino mass-squared differences, with $Δm^2_{\rm atm}$ generated at tree level and $Δm^2_{\rm sol}$ arising radiatively. After the spontaneous breaking of gauged $U(1)_{B-L}$, a residual $Z_6$ symmetry stabilizes the dark matter candidate. We investigate two minimal realizations of the framework, finding that both normal and inverted orderings are viable in one case, whereas only normal ordering survives in the other, with distinctive features for neutrino observables. Moreover, the chiral nature of the $U(1)_{B-L}$ charges suppresses the dilepton branching fraction of $Z'$, resulting in weaker ATLAS mass bounds than in the conventional vector $B-L$ scenario, thereby easing constraints on the dark sector. We explore the dark matter phenomenology of the singlet scalar and fermionic dark matter candidates. While singlet scalar DM is often severely constrained, the presence of the $Z'$ portal together with annihilation and co-annihilation channels substantially broadens the allowed parameter space. Thus, the framework offers a predictive scenario for neutrino and dark matter phenomenology that can be probed in future experiments.

hep-ph↗

arXiv2026

Fate of $θ_{12}$ under $μ-τ$ Reflection Symmetry in Light of the First JUNO Results

The recent Jiangmen Underground Neutrino Observatory (JUNO) measurements of $θ_{12}$ and $Δm^2_{21}$ open a new avenue for probing flavor symmetric structures in the lepton sector. Motivated by this, we study a model in which $μ-τ$ reflection symmetry naturally emerges from an underlying $A_4$ flavor symmetry within a type-II seesaw framework. Beyond its standard predictions of $θ_{23}=45^{\circ}$ and $δ_{\rm CP}=\pm π/2$, the framework yields testable predictions for $θ_{12}$ that can be probed by JUNO. Two viable scenarios arise, one predicting $\sin^2θ_{12} \gsim 0.335$, which is strongly disfavored by the latest JUNO results. Correlations between $θ_{12}$ and model parameters further enhance the model's predictivity. Future measurements at DUNE and T2HK will provide complementary tests of this scenario.

hep-ph↗

arXiv2026

Implications of the First JUNO Results for Dirac Neutrino Texture Zeros

Motivated by the first oscillation results from JUNO, we study the phenomenological viability of texture zeros in the Dirac neutrino mass matrix. The improved precision on the solar mixing angle $\sin^2{θ_{12}}$ and the solar mass-squared difference $Δm_{21}^2$ provide a stringent probe for scrutinizing predictive texture zero frameworks. We perform a systematic scan of the allowed parameter space for two-zero textures, identifying sharp correlations among oscillation observables arising from the reduced parameter space. Our analysis reveals that current JUNO measurements impose stringent constraints on the viable texture structures. In particular, although textures $C$, $A_2$, and $A_1$ were previously viable, current JUNO data strongly disfavor $C$, leaving only textures $A_2$ and $A_1$ compatible with the data. These findings underscore the remarkable sensitivity of Dirac texture zero scenarios to the solar sector.

hep-ph↗

arXiv2026

Dark Matter Induced Proton Decays

We propose a novel theoretical framework in which proton decay is induced by the dark matter. While proton decay requires violation of the $B+L$ symmetry, dark matter stability often relies on the presence of an unbroken symmetry. These seemingly distinct phenomena are unified through the global $U(1)_{B+L}$ symmetry inherent in the Standard Model. Its spontaneous breaking leads to a residual $Z_4$ symmetry, which ensures dark matter stability and forbids proton decay at tree level. Consequently, proton decay occurs at the one-loop level, mediated by dark sector particles. The proton lifetime is linked with the dark matter, the heavier dark matter mass enhancing proton stability, and vice versa. The $\mathcal{O}$(TeV) masses of the mediators remain consistent with current proton lifetime limits, making them accessible to experimental searches. In particular, the leptoquark mediating proton decay, carrying exotic $B+L$ charges, leads to a distinctive signature in collider searches. By intertwining proton decay, dark matter stability, and collider phenomenology, this framework offers distinctive signatures that can be probed in current and future experiments.

hep-ph↗

arXiv2026

Minimal A4 Type-II Seesaw Realization of Testable Neutrino Mass Sum Rules

We propose a flavour model based on an $A_4$ symmetry combined with a type-II seesaw mechanism for neutrino mass generation. The resulting neutrino mass matrix obeys a sum rule that, together with the measured mass-squared differences, fully determines the absolute neutrino mass spectrum. The constrained flavour structure yields correlated predictions for lepton mixing parameters, leads to inverted ordering after imposing mixing constraints, restricts the Majorana phases and implies a neutrinoless double beta decay rate close to its maximal value for inverted ordering. In the charged lepton sector an approximate triality symmetry arises in the seesaw limit, suppressing muon flavour-violating processes and allowing only specific $τ$ decay channels. The model provides a tightly constrained and experimentally testable framework linking neutrino masses, lepton mixing and lepton-number-violating observables.

hep-ph↗

arXiv2026

Radiative Dirac Neutrino Masses from Modular $S_3$ Symmetry in an Axion Model

We present a unified axion model framework that simultaneously addresses the origin of neutrino masses, leptonic flavor structure, the strong CP problem, and dark matter. The model is based on a global $U(1)_{\rm PQ}$ symmetry combined with a modular $S_3$ symmetry and is realized within a novel class of KSVZ-type axion model. Exotic colored fermions and scalars mediate radiative neutrino mass generation at the one loop-level. The PQ charge assignment forbids tree-level neutrino masses and leaves a residual $Z_3$ symmetry that ensures the Dirac nature of neutrinos. In the minimal realization, the neutrino mass matrix is of rank two, predicting one massless neutrino. Consequently, the sum of neutrino masses is constrained for both the normal and inverted hierarchies. We analyze the implications for charged lepton flavor violation and the lepton $g-2$. The axion emerging from this framework dynamically resolves the strong CP problem and accounts for the observed dark matter abundance. Notably, the predicted axion-photon coupling is within reach of upcoming experiments and consistent with existing astrophysical and cosmological bounds.

hep-ph↗

arXiv2025

Flavor Imprints on Novel Low Mass Dark Matter

We present a Majorana scotogenic-like loop framework in which neutrino mass generation and dark matter stability are intrinsically connected to the breaking of the discrete flavor symmetry $A_4$. This breaking leads to the emergence of the scoto-seesaw mechanism and a $Z_2$ symmetry. This naturally explains the solar and atmospheric mass-squared differences, $Δm_{sol}^{2}$ and $Δm_{atm}^{2}$, while simultaneously ensuring dark matter stability. Our model accommodates normal ordering of neutrino masses, with a generalized $μ$-$τ$ reflection symmetry shaping the structure of leptonic mixing and a lower limit on the lightest neutrino mass. Moreover, the model provides predictions for the octant of $θ_{23}$ and a strong correlation between $Δm_{sol}^{2}$ and $Δm_{atm}^{2}$. This correlation puts a lower bound on the fermionic DM mass. In contrast, scalar dark matter remains viable over a broad mass spectrum. A notable feature is that the low mass regime ($\sim 15$ GeV onwards) survives owing to the presence of efficient co-annihilation channels, which are typically absent in the Majorana scotogenic scenario. Additionally, the model aligns with current and future limits from lepton flavor violation experiments.

hep-ph↗

arXiv2025

Dirac Scoto Inverse-Seesaw from $A_4$ Flavor Symmetry

We present a Dirac scotogenic-like one loop radiative model where the stability of dark matter is intricately linked to the breaking of $A_4$ flavor symmetry. This breaking induces a $Z_2$ dark symmetry, stabilizing the dark matter candidate. The breaking of $A_4 \to Z_2$ leads to cutting the loop and facilitating a "scoto inverse-seesaw" mass mechanism responsible for neutrino mass generation. This elucidates the explicit explanation of two mass-squared differences, $Δm^2_{\rm{atm}}$ and $Δm^2_{\rm{sol}}$ observed in neutrino oscillations. Our model accounts for normal and inverted ordering of neutrino masses, revealing sharp correlations between $\sum m_i$ and $\langle m_β \rangle$. It also shows strong compatibility with current data in the $δ_{CP}$-$θ_{23}$ plane. Moreover, stringent constraints on scalar masses narrow down the viable dark matter mass regions, accommodating $SU(2)_L$ singlet and doublet scalar dark matter as well as fermionic dark matter. Additionally, our model presents a viable avenue for addressing lepton flavor violating decays while remaining consistent with current experimental constraints.

hep-ph↗

arXiv2024

Dark Matter Escaping Direct Detection Runs into Higgs Mass Hierarchy Problem

The current generation of Dark Matter Direct Detection Experiments has ruled out a large region of parameter space for dark matter, particularly in the ($10 - 1000$) GeV mass range. However, due to very low event rates, searching for dark matter in the heavy mass range, $\mathcal{O}$(TeV), is a daunting task requiring even larger volume detectors and long exposure times. We show that for a broad class of dark matter models of the type that these experiments are searching, including some of the most popular candidates, the heavy dark matter mass range can be ruled out in its entirety once we take into account the large corrections to Higgs mass imparted by such heavy dark matter. We show that such a limit is applicable to all types of dark matter i.e. scalar, vector, and fermionic, provided they couple directly with Higgs. By taking some simple and well studied dark matter models we show that the latest LZ limits can completely rule out such a dark matter except in a narrow range around $M_h/2$ mass.

hep-ph↗

arXiv2024

Cutting the Scotogenic loop: Adding flavor to Dark Matter

We introduce a framework for hybrid neutrino mass generation, wherein scotogenic dark sector particles, including dark matter, are charged non-trivially under the $A_4$ flavor symmetry. The spontaneous breaking of the $A_4$ group to residual $\mathcal{Z}_2$ subgroup results in the ``cutting" of the radiative loop. As a consequence the neutrinos acquire mass through the hybrid ``scoto-seesaw" mass mechanism, combining aspects of both the tree-level seesaw and one-loop scotogenic mechanisms, with the residual $\mathcal{Z}_2$ subgroup ensuring the stability of the dark matter. The flavor symmetry also leads to several predictions including the normal ordering of neutrino masses and ``generalized $μ-τ$ reflection symmetry" in leptonic mixing. Additionally, it gives testable predictions for neutrinoless double beta decay and a lower limit on the lightest neutrino mass. Finally, $A_4 \to \mathcal{Z}_2$ breaking also leaves its imprint on the dark sector and ties it with the neutrino masses and mixing. The model allows only scalar dark matter, whose mass has a theoretical upper limit of $\lesssim$ 600 GeV, with viable parameter space satisfying all dark matter constraints, available only up to about 80 GeV. Conversely, fermionic dark matter is excluded due to constraints from the neutrino sector. Various aspects of this highly predictive framework can be tested in both current and upcoming neutrino and dark matter experiments.

hep-ph↗

arXiv2024

Predictions from scoto-seesaw with $A_4$ modular symmetry

This paper's novelty lies in introducing a hybrid scoto-seesaw model rooted in $A_4$ discrete modular symmetry leading to several interesting phenomenological implications. The scoto-seesaw framework leads to generation of one mass square difference $( Δm^2_{\rm atm}$) using the type-I seesaw mechanism at the tree level. Additionally, the scotogenic contribution is vital in obtaining the other mass square difference ($Δm^2 _{\rm sol}$) at the loop level, thus providing a clear interpretation of the two different mass square differences. The non-trivial transformation of Yukawa couplings under the $A_4$ modular symmetry helps to explore neutrino phenomenology with a specific flavor structure of the mass matrix. In addition to predictions for neutrino mass ordering, mixing angles and CP phases, this setup leads to precise predictions for $\sum m_i$ as well as $|m_{ee}|$. In particular, the model predicts $\sum m_i \in (0.073,0.097)$ eV and $\left| m_{ee}\right| \in (3.15,6.66)\times 10^{-3} $ eV range; within reach of upcoming experiments. Furthermore, our model is also promising for addressing lepton flavor violations, i.e., $\ell_α\to \ell_βγ$, $\ell_α\to 3\ell_β$ and $μ- e $ conversion rates while staying within the realm of current experimental limits.

hep-ph↗

arXiv2024

Neutrino Mass Sum Rules from Modular $\mathcal{A}_4$ Symmetry

Modular symmetries offer a dynamic approach to understanding the flavour structure of leptonic mixing. Using the modular $\mathcal{A}_4$ flavour symmetry integrated in a type-II seesaw, we propose a simple and minimalistic model that restricts the neutrino oscillation parameter space and, most importantly, introduces a sum rule in the physical neutrino masses. When combined with the mass squared differences observed in neutrino oscillations, this sum rule determines the absolute neutrino mass scale. This has significant implications for cosmology, neutrinoless double beta decay experiments and direct neutrino mass measurements. In particular, the model predicts $\sum_i m_i \approx 0.1$ eV for both normal and inverted ordering, and thus can be fully probed by the current generation of cosmological probes in the upcoming years.

hep-ph↗

arXiv2023

Sub-Volt High-Speed Silicon MOSCAP Microring Modulator Driven by High Mobility Conductive Oxide

Low driving voltage (Vpp), high-speed silicon microring modulator plays a critical role in energy-efficient optical interconnect and optical computing systems owing to its ultra-compact footprint and capability for on-chip wavelength-division multiplexing. However, existing silicon microring modulators usually require more than 2 V of Vpp, which is limited by the relatively weak plasma dispersion effect of silicon and the small capacitance density of the reversed PN-junction. Here we present a highly efficient metal-oxide semiconductor capacitor (MOSCAP) microring modulator through heterogeneous integration between silicon photonics and titanium-doped indium oxide, which is a high-mobility transparent conductive oxide (TCO) material with a strong plasma dispersion effect. The device is co-fabricated by Intel's photonics fab and TCO patterning processes at Oregon State University, which exhibits a high electro-optic modulation efficiency of 117 pm/V with a low VpiL of 0.12 Vcm, and consequently can be driven by an extremely low Vpp of 0.8 V. At a 11 GHz modulation bandwidth where the modulator is limited by the high parasitic capacitance, we obtained 25 Gb/s clear eye diagrams with energy efficiency of 53 fJ/bit and demonstrated 35 Gb/s open eyes with a higher driving voltage. Further optimization of the device is expected to increase the modulation bandwidth up to 52 GHz that can encode data at 100 Gb/s for next-generation, energy-efficient optical communication and computation with sub-volt driving voltage without using any high voltage swing amplifier.

physics.optics↗