arXiv subjects

Hemant Kumar Prajapati

Publications and source records attributed to Hemant Kumar Prajapati.

6 recordsLinked to original sources

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 $2.6\sigma$, 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.

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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.

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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 $\nu_R$. This chiral charge assignment naturally accounts for the observed neutrino mass-squared differences, with $\Delta m^2_{\rm atm}$ generated at tree level and $\Delta m^2_{\rm sol}$ 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.

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arXiv2026

Light and Heavy $Z'$ from Flavored Chiral $U(1)_X$ Gauge Symmetries: Purely Axial and Mixed Vector-Axial Couplings

Model independent phenomenological studies, ranging from neutrino to B-physics, often consider effective interactions involving either purely vector (V), purely axial vector (A), or mixed vector and axial vector (V, A) couplings. While pure vector $Z'$ interactions can naturally emerge in gauged $U(1)_X$ extensions of the Standard Model, such as the $B-L$ model, generating other coupling structures from a UV complete theory is highly nontrivial. To realize such couplings, we propose a new class of flavor specific chiral $U(1)_X$ gauge symmetries. Gauge anomaly cancellation is achieved by introducing three right-handed neutrinos charged under the $U(1)_X$ symmetry. We systematically classify anomaly free charge assignments and analyze viable ultraviolet completions with minimal scalar content, requiring no additional fermions beyond the three necessary for anomaly cancellation. We present several benchmark models illustrating the range of possible charge assignments, under which the quark and lepton flavor structures can differ substantially, leading to distinct phenomenological signatures. In particular, such non universal charge configurations naturally give rise to $Z'$ mediated flavor changing neutral currents in both the quark and lepton sectors. We also demonstrate that, within this framework, the $Z'$ boson can naturally acquire purely axial vector or mixed vector-axial couplings to the SM fermions, both in the heavy and light $Z'$ regimes.

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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.

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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.

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