arXiv subjects

Jason Kumar

Publications and source records attributed to Jason Kumar.

At least 19 recordsLinked to original sources

arXiv2026

Astrophysical Neutrino Sources as Colliders

High-energy neutrinos arise from processes at large center-of-mass energies, offering a window to test physics at comparable scales or beyond those accessible in collider experiments on Earth. Here, we present a recipe for extracting two-sided bounds on the inelastic $pp$ and $p\gamma$ cross sections from neutrino point-source data, by independently constraining every astrophysical input (cosmic-ray luminosities and target densities) through electromagnetic observations or theoretical arguments. The cross section is then the only remaining free parameter. Applying this framework to the IceCube associations with TXS~0506+056, NGC~1068, and the Galactic Plane, to a stacked population of eleven X-ray bright Seyfert galaxies, to the ultra-high-energy KM3NeT event KM3-230213A, and to projected observations of ultra-high-energy neutrinos, we obtain constraints that span center-of-mass energies from $\sqrt{s}\sim 1$ GeV to $\sim 10^{5}$ GeV, some of which are well beyond the reach of the LHC and, for the $p\gamma$ channel, beyond HERA. Several of these bounds are more stringent than unitarity limits.

hep-ph↗

arXiv2026

Producing the GeV Galactic Center Excess via Cosmic Ray-Dark Matter Scattering

In this work, we propose a novel mechanism for generating gamma rays from the Galactic Center via scattering of cosmic-ray protons off dark matter in the Milky Way halo, in contrast to conventional explanations based on dark matter annihilation. We present two examples of this framework that produce an observable photon signal. In the inelastic dark matter model, cosmic rays up-scatter a lighter dark matter particle, with the subsequent decay of the heavier particle yielding two photons. In the elastic dark matter model, an energetic photon is directly produced in the final state of a 2-to-3 scattering process. We show that, for a range of viable model parameters, this framework provides a fit to the observed Galactic Center gamma-ray excess spectrum comparable to those obtained from dark matter annihilation and millisecond pulsar models. Our results open a new avenue for interpreting gamma-ray observations of the Galactic Center.

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arXiv2026

Constraints on the Injection of Radiation in the Early Universe

We consider the generic injection of radiation (both dark and electromagnetic) during the epoch between big bang nucleosynthesis (BBN) and recombination. The contribution of the additional radiation to the number of effective neutrinos may be quite small in this scenario, since dark radiation and electromagnetic radiation provide contributions of opposite sign. However, the injection of electromagnetic radiation dilutes the baryon-to-entropy ratio, which is measured both at BBN and at recombination. As a result, this scenario is expected to be tightly constrained. Indeed, performing a numerical study, we find that the allowed amount of extra radiation may be no more than $\sim 25\%$ greater than in the case where it is assumed to be entirely dark radiation.

astro-ph.CO↗

arXiv2026

A likelihood analysis for gamma-ray background models

Indirect searches for dark matter using dwarf spheroidal galaxies are limited by systematic uncertainties in modeling diffuse gamma-ray backgrounds. We present a likelihood-based comparison of locally constructed empirical background models and theoretically-motivated models that incorporate the Fermi-LAT diffuse background. The empirical models we study include both an independent-binning approach and a covariance-based approach that captures cross-energy correlations. Using ensembles of blank-sky regions and information criteria which account for model complexity, we find that empirical background descriptions provide a statistically competitive fit to gamma-ray data on degree scales in high-latitude regions.

astro-ph.HE↗

arXiv2025

Clustering with Light (but Massive) Relics

We consider the effect of Light (but Massive) Relics (LiMRs) on the clustering of matter in the early Universe. We account for the fact that LiMRs which are massive enough may cluster on large length scales at early times, and may thus impact weak lensing of the cosmic microwave background (CMB) even on small angular scales. In particular, we find that LiMRs in the $\gtrsim$ eV mass range (and even $> 10$ eV), can constitute a non-negligible component of dark matter. This opens up a class of scenarios in which energy is injected as dark radiation, but begins to redshift as matter before recombination, thus avoiding constraints on $\Delta N_{eff}$ while providing an eV-range dark matter component.

astro-ph.CO↗

arXiv2025

Direct detection of electromagnetically interacting ultraheavy dark matter

At a level too faint for astronomy, particle dark matter may interact with Standard Model states via the photon. We derive limits from direct detection experiments on photon-mediated nuclear interactions up to operator dimension-6, viz., via a millicharge, charge radius, electric and magnetic dipole moment, and anapole moment, up to dark matter masses $\lesssim 10^{17}$ GeV, where detectors become flux-limited. We derive constraints for XENON1T, XENONnT, LZ, PANDAX-II, PANDAX-4T, DarkSide-50, DEAP-3600, PICO-60, and CDMS-II, and estimate future sensitivities for the multi-deca-tonne scale experiments DarkSide-20k, DARWIN/XLZD, PANDAX-xT, and Argo. Special attention is paid to millicharged dark matter, for which we constrain new parameter space by deriving the ceiling on sensitivity to its electric charge, arising from the nuclear-vs-electron recoil discriminants used in liquid argon and bubble chamber detectors. This result goes beyond ceilings previously identified in liquid xenon and semiconductor detectors from electron recoil vetoes. In particular, the ceiling from PICO-60 closes a large window between direct detection and neutrino experiment limits for dark matter masses below $10^{12}$ GeV.

hep-ph↗

arXiv2025

Gravitational Waves and Cosmological Observables from First-Order Phase Transitions: Thermal Corrections at Low Temperature

We consider the impact on cosmological first-order phase transitions (FOPTs) of low-temperature thermal corrections to the effective potential. These are corrections from degrees of freedom whose field-dependent masses in the true vacuum are much larger than the nucleation temperature, though in the false vacuum the field-dependent masses may be much smaller than the nucleation temperature. We focus on the regime in which the thin-wall approximation is valid, and in which these corrections are small enough that they do not alter the vacuum structure of the theory. Although the general form of these corrections to the thermal effective potential can be quite complicated, we argue that the net effect of all such corrections can be well-modeled in this limit with a single new parameter. We determine the shift in the parameters of the FOPT in terms of this new parameter, and the impact on gravitational wave signals and cosmological observables.

hep-ph↗

arXiv2025

Scaling Relations for Dark Matter Halos Hosting Ultra-Faint Dwarf Galaxies

We consider the extraction of parameters of dark matter halos hosting ultra-faint dwarf galaxies, in the case where there are only ${\cal O}(10)$ identified member stars with measured line-of-sight velocities. This scenario is likely to be increasingly common, as upcoming newly discovered dwarf galaxies in the Milky Way, by e.g. the Rubin Observatory, will likely (at least initially) have only a few identified members. Assuming an NFW dark matter profile, equilibrium modeling likely can only robustly extract one halo parameter ($\rho_s r_s$), but the scale radius itself will typically be unconstrained. In these cases, the results obtainable from Jeans modeling can be well replicated by a simple scaling relation motivated by the half-light mass estimator. As a application, we examine the recently discovered stellar system Ursa Major III, which has been optimistically assessed to have the largest $J$-factor of any known object. We suggest that, because of the presence of outlier stars, the $J$-factor obtained from modeling of Ursa Major III is likely inflated, as it is inconsistent with the half-light mass estimator, while removal of the outliers will leave the $J$-factor unconstrained from below.

astro-ph.CO↗

arXiv2025

Primordial black holes from Q-balls produced in a first-order phase transition

We consider the formation of Q-balls in false vacuum remnants during a cosmological first-order phase transition. We find that under certain circumstances Q-balls can collapse to form primordial black holes. This scenario can produce multimessenger signals that may be observed at upcoming experiments, including 1-100 nHz gravitational waves from the phase transition, and gamma-rays emitted from primordial black holes as Hawking radiation and as superradiance. These signals are quite distinctive, and differ markedly from signals expected from Fermi-balls. The reheating of the dark sector from the phase transition may address the Hubble tension.

hep-ph↗

arXiv2024

Tools for probing new physics with newly discovered gamma-ray targets

We present a computational tool, TweedleDEE, for empirically modeling diffuse gamma-ray background emission in a 1 degree region of the sky, using publicly available gamma-ray data off-axis from the region of interest. This background model allows a user to perform a purely data-driven search for anomalous localized sources of gamma-ray emission, including new physics. A major application of this tool would be in searching for dark matter annihilation in newly discovered astrophysical targets. For this purpose, we derive a scaling relation for determining velocity-dependent $J$-factors using only the stellar parameters, which can be broadly applied to obtain dark matter constraints from new targets. As an application of these tools, we use TweedleDEE and MADHATv2 to perform the first search for dark matter annihilation in the newly discovered Leo VI dwarf spheroidal galaxy, and present model constraints for a variety of choices of the annihilation channel and velocity dependence of the cross section.

astro-ph.HE↗

arXiv2024

Applying Simulation-Based Inference to Spectral and Spatial Information from the Galactic Center Gamma-Ray Excess

The two most favored explanations of the Fermi Galactic Center gamma-ray excess (GCE) are millisecond pulsars and self annihilation of the smooth dark matter halo of the galaxy. In order to distinguish between these possibilities, we would like to optimally use all information in the available data, including photon direction and energy information. To date, analyses of the GCE have generally treated directional and energy information separately, or have ignored one or the other completely. Here, we develop a method for analyzing the GCE that relies on simulation-based inference with neural posterior models to jointly analyze photon directional and spectral information while correctly accounting for the spatial and energy resolution of the telescope, here assumed to be the Fermi Large Area Telescope (LAT). Our results also have implications for analyses of the diffuse gamma-ray background, which we discuss.

astro-ph.HE↗

arXiv2024

New dark matter analysis of Milky Way dwarf satellite galaxies with MADHATv2

We obtain bounds on dark matter annihilation using 14 years of publicly available Fermi-LAT data from a set of 54 dwarf spheroidal galaxies, using spectral information from 16 energy bins. We perform this analysis using our updated and publicly available code MADHATv2, which can be used to test a variety of models for dark matter particle physics and astrophysics in an accessible manner. In particular, we note that including Carina III in the analysis strengthens constraints on $s$-wave annihilation into two-body Standard Model final states by a factor of $\sim 3$ but broadens the error on the constraint due to the large uncertainty of its $J$-factor. Our findings illustrate the importance of verifying if Carina III is in fact a dwarf spheroidal galaxy and measuring more precisely its $J$-factor. More generally, they highlight the significance of forthcoming discoveries of nearby ultra-faint dwarfs for dark matter indirect detection.

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arXiv2023

Are There Correlations in the HAWC and IceCube High Energy Skymaps Outside the Galactic Plane?

We use publicly-available data to perform a search for correlations of high energy neutrino candidate events detected by IceCube and high-energy photons seen by the HAWC collaboration. Our search is focused on unveiling such correlations outside of the Galactic plane. This search is sensitive to correlations in the neutrino candidate and photon skymaps which would arise from a population of unidentified point sources. We find no evidence for such a correlation, but suggest strategies for improvements with new data sets.

hep-ph↗

arXiv2023

Opening windows with Isospin-Violating Dark Matter

We consider the effect of isospin-violating dark matter-nucleon interactions on direct detection constraints in the regime of small dark matter mass and large scattering cross section. Isospin-violation can lead to both reductions in sensitivity (due to a reduced cross section for scattering with nuclei in the detector) and enhancements in sensitivity (due to a reduced cross section for scattering in the overburden). Isospin-violating effects can thus open up some closed regions of parameter space, while closing off other regions.

hep-ph↗

arXiv2023

A Strategy for Identifying Periodic Sources Contributing to the Galactic Center Excess

The origin of the Galactic Center gamma-ray excess has not been conclusively determined after over a decade of careful study. The two most widely discussed possibilities are a population of millisecond pulsars (MSPs), and annihilation of dark matter particles. In contrast with annihilating dark matter, MSPs are expected to produce periodic emission. We show that even though the number of photons contributing to the excess is small, there is potentially sufficient information in the data from Fermi to detect a periodic MSP signal. Such a detection would definitively prove that at least some fraction of the excess is due to MSPs. We argue that this conclusion is robust to potential timing perturbations of the gamma-ray photons, such as those due to Earth's orbit, even if the number of parameters that must be used to model the perturbations is $\sim 7$.

astro-ph.HE↗

arXiv2023

Machine Learning Techniques for Intermediate Mass Gap Lepton Partner Searches at the Large Hadron Collider

We consider machine learning techniques associated with the application of a Boosted Decision Tree (BDT) to searches at the Large Hadron Collider (LHC) for pair-produced lepton partners which decay to leptons and invisible particles. This scenario can arise in the Minimal Supersymmetric Standard Model (MSSM), but can be realized in many other extensions of the Standard Model (SM). We focus on the case of intermediate mass splitting ($\sim 30~{\rm GeV}$) between the dark matter (DM) and the scalar. For these mass splittings, the LHC has made little improvement over LEP due to large electroweak backgrounds. We find that the use of machine learning techniques can push the LHC well past discovery sensitivity for a benchmark model with a lepton partner mass of $\sim 110~{\rm GeV}$, for an integrated luminosity of $300~{\rm fb}^{-1}$, with a signal-to-background ratio of $\sim 0.3$. The LHC could exclude models with a lepton partner mass as large as $\sim 160~{\rm GeV}$ with the same luminosity. The use of machine learning techniques in searches for scalar lepton partners at the LHC could thus definitively probe the parameter space of the MSSM in which scalar muon mediated interactions between SM muons and Majorana singlet DM can both deplete the relic density through dark matter annihilation and satisfy the recently measured anomalous magnetic moment of the muon. We identify several machine learning techniques which can be useful in other LHC searches involving large and complex backgrounds.

hep-ph↗

arXiv2023

Dark Matter Velocity Distributions: Comparing Numerical Simulations to Analytic Results

We test the consistency of dark matter velocity distributions obtained from dark matter-only numerical simulations with analytic predictions, using the publicly available Via Lactea 2 dataset as an example. We find that, well inside the scale radius, the velocity distribution obtained from numerical simulation is consistent with a function of a single integral of motion -- the energy -- and moreover is consistent with the result obtained from Eddington inversion. This indicates that the assumptions underlying the analytic result, namely, spherical symmetry, isotropy, and a static potential, are sufficiently accurate to govern the coarse properties of the velocity distribution in the inner regions of the halo. We discuss implications for the behavior of the high-velocity tail of the distribution, which can dominate dark matter annihilation from a $p$- or $d$-wave state.

astro-ph.CO↗

arXiv2023

Constraining $p$-wave Dark Matter Annihilation with Gamma-ray Observations of M87

We consider constraints on $p$-wave dark matter in a dark matter spike surrounding the supermassive black hole at the center of M87. Owing to the large mass of the black hole, and resulting large velocity dispersion for the dark matter particles in the spike, it is possible for Fermi-LAT and MAGIC data to place tight constraints on $p$-wave annihilation, which would be far more stringent than those placed by observations of dwarf spheroidal galaxies. Indeed, for optimistic choices of the spike parameters, gamma-ray data would exclude thermal $p$-wave dark matter models with a particle mass $\lesssim {10}~\rm TeV$. But there is significant uncertainty in the properties and parameters of the spike, and for less optimistic scenarios, thermal dark matter candidates would be completely unconstrained. In addition to better understanding the spike parameters, a second key to improving constraints on dark matter annihilation is an accurate astrophysical background model.

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