arXiv subjects

Halim Shaikh

Publications and source records attributed to Halim Shaikh.

3 recordsLinked to original sources

arXiv2026

Solar Capture Tests of Inelastic Dark Matter after the LZ High-Recoil Event

The LUX-ZEPLIN (LZ) Collaboration has reported a $248$ keV nuclear-recoil candidate for which endothermic dark matter (DM) gives some of the largest local significances. We study Solar-capture constraints on three interpretations: a thermal Higgsino, a thermal pseudo-Dirac fermion with off-diagonal vector interactions, and neutron-philic endothermic spin-dependent scattering through $\mathcal{O}_4$. The canonical full-density thermal Higgsino is excluded: its LZ-preferred splitting near $0.38\,\mathrm{MeV}$ lies well below the splitting required to suppress Solar capture sufficiently to satisfy the IceCube upper limits on DM annihilation in the Sun, $\delta\gtrsim0.51$-$0.56\,\mathrm{MeV}$. For the pseudo-Dirac benchmark parameters that fit the LZ event, we find $C_\odot=1.53\times10^{20}\,\mathrm{s}^{-1}$, while two-state kinetics limits the fixed-orbit annihilation rate to $\Gamma_A\lesssim4.4\times10^{14}\,\mathrm{s}^{-1}$, about $1.7\times10^5$ below the IceCube upper limit for the $b\bar b$ channel. A semi-analytic treatment indicates that re-excitation cycles further cool the captured population, although a full phase-space calculation is required for its final distribution. For neutron-philic $\mathcal{O}_4$ scattering at $m_\chi=1$ TeV and $\delta=300$ keV, finite-temperature capture gives $C_\odot=3.98\times10^{17}\,\mathrm{s}^{-1}$, with nuclear-structure uncertainties giving an envelope $(1.77$-$9.72)\times10^{17}\,\mathrm{s}^{-1}$. Under the equilibrium assumption, the upper edge remains about a factor $154$ below the IceCube upper limit for the $b\bar b$ channel. We use $b\bar b$ only as a soft-hadronic proxy because an exact constraint requires the model-specific annihilation spectrum. Solar capture therefore excludes the thermal-Higgsino interpretation but not endothermic explanations generically.

hep-ph↗

arXiv2026

Vector Higgs-Portal Dark Matter: How UV Completion Reopens Viable Parameter Space

The particle nature of dark matter (DM) remains one of the central open problems in modern physics. Among the most extensively studied candidates are weakly interacting massive particles, whose parameter space is now under strong pressure from direct detection, indirect detection, and collider searches. In this work we revisit the Higgs-portal scenario with vector DM, first in an effective-field-theory description and then in a renormalizable UV-complete realization. We show that the effective Higgs-portal model with a Proca vector coupled quadratically to the Standard Model Higgs is essentially excluded over almost all of its parameter space by current direct-detection limits, with only a narrow region near the Higgs resonance surviving with a required fine tuning of the DM to Higgs mass that should at the permille level. We then consider a UV completion based on an additional gauged $U(1)_X$ symmetry, in which the DM candidate is a massive vector boson $V$ and the scalar sector is extended by a dark Higgs that mixes with the Standard Model Higgs. In this framework, the presence of a second scalar mediator opens an additional resonant annihilation channel and can substantially weaken the direct-detection constraints. In particular, when the DM mass lies sufficiently close to the heavy-scalar resonance, $m_V \simeq m_{H_2}/2$, the coupling required to reproduce the observed relic abundance can lie up to about two orders of magnitude below current direct-detection bounds, thereby opening viable parameter space that is absent in the effective description. Our results highlight the importance of going beyond the effective-field-theory approximation in Higgs-portal vector DM models and show that UV-complete realizations can qualitatively change the phenomenological conclusions.

hep-ph↗

arXiv2025

Indirect searches for realistic sub-GeV Dark Matter models

Indirect searches for Dark Matter (DM) particles with mass in the MeV -- GeV scale have received significant attention lately. Pair-annihilations of such DM particles in the Galaxy can give rise to (at the same time) MeV to GeV $\gamma$-rays via prompt emission, sub-GeV $e^\pm$ in cosmic-rays, as well as a broad photon spectrum ranging from $X$-rays to soft $\gamma$-rays, produced by the DM induced $e^\pm$ via inverse Compton scattering, bremsstrahlung and in-flight annihilation processes (collectively called `secondary emissions'). We focus on two representative realistic sub-GeV DM models, namely, the vector-portal kinetic-mixing model and the higgs-portal model, and perform a detailed study of the indirect detection constraints from existing $X$-rays, $\gamma$-rays and cosmic-ray observations, based on all of the above-mentioned signals. We also estimate the future prospects from the upcoming MeV photon telescope COSI, including all possible types of prompt and secondary emission signals. We compare our results with the constraints and (or) projections from cosmological and terrestrial observations. We find that, for both the sub-GeV DM models, the current observations constrain the annihilation cross-section at the level of $\langle \sigma v \rangle \lesssim 10^{-27} {\rm cm}^3/{\rm s}$, or lower for some specific mass ranges or under optimistic assumptions. Moreover, new unconstrained DM parameter space can be probed at the upcoming instruments like COSI, thanks to the inclusion of secondary photons which in many cases provide the dominant signal.

hep-ph↗