arXiv subjects

Kenny C. Y. Ng

Publications and source records attributed to Kenny C. Y. Ng.

At least 19 recordsLinked to original sources

arXiv2026

New Physics Opportunities at Neutrino Facilities: BSM Physics at Accelerator, Atmospheric, and Reactor Neutrino Experiments

Since the discovery of the Higgs boson, the long-standing task at hand in particle physics is the search for new physics beyond the Standard Model, which accounts for only about 5\% of the Universe. In light of this situation, the neutrino sector has drawn significant attention due to neutrino oscillations, which require physics beyond the Standard Model and have prompted a wide array of active and planned experimental programs. Notably, neutrino facilities offer substantial potential to search for new physics beyond neutrino oscillations, owing to their precision measurement capabilities, diverse experimental configurations, and various neutrino sources. This white paper summarizes the landscape of new physics that can be probed at current and future neutrino experiments, categorized into laboratory-produced and cosmogenic signals. We discuss recent experimental results interpreted through the lens of new physics, as well as detailed plans and projected sensitivities of next-generation facilities. This summary is based on presentations from the 4th Workshop on New Physics Opportunities in Neutrino Facilities (NPN 2024), held at IBS in Daejeon, Korea, on June 3-5, 2024. Particular emphasis is placed on accelerator-based neutrino experiments and a range of neutrino programs in East Asia. We also outline key tasks necessary to realize the promising new physics opportunities ahead.

hep-ph↗

arXiv2026

Improving the Angular Resolution of IBD Events Using Neutron Capture Information in Super-Kamiokande

One of the most important neutrino interactions is the Inverse Beta Decay (IBD). However, the IBD events typically carry no directional information in water Cherenkov detectors as the positron directions are mostly isotropic at low energies, such as those in supernova studies. As Gadolinium is being added to Super-Kamiokande, the improved neutron capture efficiency not only allows better background rejection, but the neutron capture information could potentially provide additional information that allows better event reconstruction. Due to neutron diffusion in water, event-by-event reconstruction is difficult. However, if the final neutron capture position is correlated with the initial neutrino momentum, it may be possible that neutrino directionality could be reconstructed statistically, with or without using the positron information. In this work, we use Geant4 to simulate neutron propagation in water. We show that in a wide range of neutrino energies from about 10 MeV to several hundred MeV, neutron capture information could statistically enhance the neutrino directionality, compared to positron-only inference, even with neutron diffusion considered. However, practical application of this technique depends crucially on detection effects, especially the vertex reconstruction resolutions. Our work therefore motivates developments of better reconstruction algorithms and techniques, as well as detector upgrades.

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arXiv2025

Searching for Particle Dark Matter with eROSITA Early Data

Many well motivated dark matter (DM) particle candidates can decay into detectable X-ray photons. We analyze the Final Equatorial Depth Survey (eFEDS) data from eROSITA early data release to search for unidentified X-ray lines that could indicate DM signals. Having discovered no anomalous signal, we set limits on DM decay rate in mass range between 1.8-18 keV, and constrain the parameter space of two DM particles: sterile neutrino and axion-like particles. Finally we also study the projected sensitivity of eROSITA full sky search, showing that eROSITA all-sky survey is expected to set the most stringent limits in the soft X-ray band.

hep-ph↗

arXiv2024

Sensitivity floor for primordial black holes with neutrino searches

Primordial black holes~(PBHs) formed in the early Universe are well-motivated dark matter~(DM) candidates over a wide range of masses. These PBHs could emit detectable signals in the form of photons, electrons, and neutrinos through Hawking radiation. We consider the null observations of astrophysical $\barν_{e}$ flux from several neutrino detectors and set new constraints on the PBHs as the dominant DM component to be above $6.4\times10^{15}\,{\rm g}$. We also estimate the expected constraints with JUNO for the prospects in the near future. Lastly, we note that the diffuse supernova neutrino background~(DSNB) is an unavoidable isotropic background. We thus estimate the sensitivity floor for PBH parameter space due to the DSNB and show that it is challenging for neutrino detectors to identify PBHs as they constitute 100\% of the DM above a mass of 9$\times10^{15}$g.

hep-ph↗

arXiv2024

TeV Solar Gamma Rays as a probe for the Solar Internetwork Magnetic Fields

The magnetic fields that emerge from beneath the solar surface and permeate the solar atmosphere are the key drivers of space weather and, thus, understanding them is important to human society. Direct observations, used to measure magnetic fields, can only probe the magnetic fields in the photosphere and above, far from the regions the magnetic fields are being enhanced by the solar dynamo. Solar gamma rays produced by cosmic rays interacting with the solar atmosphere have been detected from GeV to TeV energy range, and revealed that they are significantly affected by solar magnetic fields. However, much of the observations are yet to be explained by a physical model. Using a semi-analytic model, we show that magnetic fields at and below the photosphere with a large horizontal component could explain the $\sim$1 TeV solar gamma rays observed by HAWC. This could allow high-energy solar gamma rays to be a novel probe for magnetic fields below the photosphere.

astro-ph.SR↗

arXiv2024

Simulating gamma-ray production from cosmic rays interacting with the solar atmosphere in the presence of coronal magnetic fields

Cosmic rays can interact with the solar atmosphere and produce a slew of secondary messengers, making the Sun a bright gamma-ray source in the sky. Detailed observations with Fermi-LAT have shown that these interactions must be strongly affected by solar magnetic fields in order to produce the wide range of observational features, such as high flux and hard spectrum. However, the detailed mechanisms behind these features are still a mystery. In this work, we tackle this problem by performing particle interaction simulations in the solar atmosphere in the presence of coronal magnetic fields modelled using the potential field source surface (PFSS) model. We find that the low-energy (~GeV) gamma-ray production is significantly enhanced by the coronal magnetic fields, but the enhancement decreases rapidly with energy. The enhancement is directly correlated with the production of gamma rays with large deviation angles relative to the input cosmic-ray direction. We conclude that coronal magnetic fields are essential for correctly modeling solar disk gamma rays below 10GeV, but above that the effect of coronal magnetic fields diminishes. Other magnetic field structures are needed to explain the high-energy disk emission.

astro-ph.HE↗

arXiv2023

Improving HAWC dark matter constraints with Inverse-Compton Emission

The particle nature of dark matter (DM) has been a long-lasting mystery. Many models suggest that DM could decay or self annihilate into standard model particles, and thus could be a source of gamma rays in the sky. The High Altitude Water Cherenkov (HAWC) observatory has yielded some of the strongest limits in searches of DM decay or annihilation. Building on the flux limits provided by the HAWC collaboration in 2018, we consider the effects of additional components from Galactic secondary Inverse-Compton scatterings and extragalactic DM distributions. We find that these effects can significantly improve the DM constraints, up to an order of magnitude in some cases. This highlights the importance of considering secondary effects in detail in LHAASO-WCDA and SWGO in the future.

astro-ph.HE↗

arXiv2023

Warm Dark Matter Constraints Using Milky-Way Satellite Observations and Subhalo Evolution Modeling

Warm dark matter (WDM) can potentially explain small-scale observations that currently challenge the cold dark matter (CDM) model, as warm particles suppress structure formation due to free-streaming effects. Observing small-scale matter distribution provides a valuable way to distinguish between CDM and WDM. In this work, we use observations from the Dark Energy Survey and PanSTARRS1, which observe 270 Milky-Way satellites after completeness corrections. We test WDM models by comparing the number of satellites in the Milky Way with predictions derived from the Semi-Analytical SubHalo Inference ModelIng (SASHIMI) code, which we develop based on the extended Press-Schechter formalism and subhalos' tidal evolution prescription. We robustly rule out WDM with masses lighter than 4.4 keV at 95% confidence level for the Milky-Way halo mass of $10^{12} M_\odot$. The limits are a weak function of the (yet uncertain) Milky-Way halo mass, and vary as $m_{\rm WDM}>3.6$-$5.1$ keV for $(0.6$-$2.0) \times 10^{12} M_\odot$. For the sterile neutrinos that form a subclass of WDM, we obtain the constraints of $m_{ν_s}>11.6$ keV for the Milky-Way halo mass of $10^{12} M_{\odot}$. These results based on SASHIMI do not rely on any assumptions of galaxy formation physics or are not limited by numerical resolution. The models, therefore, offer a robust and fast way to constrain the WDM models. By applying a satellite forming condition, however, we can rule out the WDM mass lighter than 9.0 keV for the Milky-Way halo mass of $10^{12} M_\odot$.

astro-ph.CO↗

arXiv2023

Long-Exposure NuSTAR Constraints on Decaying Dark Matter in the Galactic Halo

We present two complementary NuSTAR x-ray searches for keV-scale dark matter decaying to mono-energetic photons in the Milky Way halo. In the first, we utilize the known intensity pattern of unfocused stray light across the detector planes -- the dominant source of photons from diffuse sources -- to separate astrophysical emission from internal instrument backgrounds using ${\sim}$7-Ms/detector deep blank-sky exposures. In the second, we present an updated parametric model of the full NuSTAR instrument background, allowing us to leverage the statistical power of an independent ${\sim}$20-Ms/detector stacked exposures spread across the sky. Finding no evidence of anomalous x-ray lines using either method, we set limits on the active-sterile mixing angle $\sin^2(2θ)$ for sterile-neutrino masses 6--40 keV. The first key result is that we strongly disfavor a ${\sim}$7-keV sterile neutrino decaying into a 3.5-keV photon. The second is that we derive leading limits on sterile neutrinos with masses ${\sim}$15--18 keV and ${\sim}$25--40 keV, reaching or extending below the Big Bang Nucleosynthesis limit. In combination with previous results, the parameter space for the Neutrino Minimal Standard Model ($ν$MSM) is now nearly closed.

astro-ph.HE↗

arXiv2022

Anisotropic Photon and Electron Scattering without Ultrarelativistic Approximation

Interactions between photons and electrons are ubiquitous in astrophysics. Photons can be down scattered (Compton scattering) or up scattered (inverse Compton scattering) by moving electrons. Inverse Compton scattering, in particular, is an essential process for the production of astrophysical gamma rays. Computations of inverse Compton emission typically adopts an isotropic or an ultrarelativistic assumption to simplify the calculation, which makes them unable to broadcast the formula to the whole phase space of source particles. In view of this, we develop a numerical scheme to compute the interactions between anisotropic photons and electrons without taking ultrarelativistic approximations. Compared to the ultrarelativistic limit, our exact results show major deviations when target photons are down scattered or when they possess energy comparable to source electrons. We also consider two test cases of high-energy inverse Compton emission to validate our results in the ultrarelativistic limit. In general, our formalism can be applied to cases of anisotropic electron-photon scattering in various energy regimes, and for computing the polarizations of the scattered photons.

astro-ph.HE↗

arXiv2021

Searches for sterile neutrinos and axionlike particles from the Galactic halo with eROSITA

Dark matter might be made of "warm" particles, such as sterile neutrinos in the keV mass range, which can decay into photons through mixing and are consequently detectable by X-ray telescopes. Axionlike particles (ALPs) are detectable by X-ray telescopes too when coupled to standard model particles and decay into photons in the keV range. Both particles could explain the unidentified 3.5 keV line and, interestingly, XENON1T observed an excess of electron recoil events most prominent at 2-3 keV. One explanation could be an ALPs origin, which is not yet excluded by X-ray constraints in an anomaly-free symmetry model in which the photon production is suppressed. We study the diffuse emission coming from the Galactic halo, and calculate the sensitivity of all-sky X-ray survey performed by eROSITA to identify a sterile neutrino or ALP dark matter. We estimate bounds on the mixing angle of the sterile neutrinos and coupling strength of the ALPs. After four years of data-taking by eROSITA, we expect to set stringent constraints, and in particular, we expect to firmly probe mixing angle $\sin^2(2θ)$ up to nearly two orders magnitude below the best-fit value for explaining the unidentified 3.5 keV line. Moreover, with eROSITA, we will be able to probe the ALP parameter space of couplings to photons and electrons, and potentially confirm an ALP origin of the XENON1T excess.

astro-ph.HE↗

arXiv2021

Decaying dark matter in dwarf spheroidal galaxies: Prospects for X-ray and gamma-ray telescopes

Dwarf spheroidal galaxies are dark matter dominated systems, and as such, ideal for indirect dark matter searches. If dark matter decays into high-energy photons in the dwarf galaxies, they will be a good target for current and future generations of X-ray and gamma-ray telescopes. By adopting the latest estimates of density profiles of dwarf galaxies in the Milky Way, we revise the estimates dark matter decay rates in dwarf galaxies; our results are more robust, but weaker than previous estimates. Applying these results, we study the detectability of dark matter decays with X-ray and very-high-energy gamma-ray telescopes, such as eROSITA, XRISM, Athena, HAWC, and CTA. Our projection shows that all of these X-ray telescopes will be able to critically assess the claim of the 7 keV sterile neutrino decays from stacked galaxy clusters and nearby galaxies. For TeV decaying dark matter, we can constrain its lifetime to be longer than $\sim$10$^{27}$-10$^{28}$ s. We also make projections for future dwarf galaxies that would be newly discovered with the Vera Rubin Observatory Legacy Survey of Space and Time, which will further improve the expected sensitivity to dark matter decays both in the keV and PeV mass ranges.

astro-ph.HE↗

arXiv2020

Sensitivities of KM3NeT on decaying dark matter

The discovery of high-energy astrophysical neutrinos by IceCube has opened a new window to the Universe. However, the origin of these neutrinos is still a mystery, and some of them could be a result of dark matter interactions such as decay. Next generation gigaton water-Cherenkov neutrino telescope, KM3NeT, is expected to offer significantly improved energy resolution in the cascade channel, and advantageous viewing condition to the Galactic Center; both important for searches of dark matter decay signals. We study the sensitivity of KM3NeT on dark matter decays by performing a mock likelihood analysis for both cascade and track type events, taking into account both angular and energy information. We find that, combining both channels, KM3NeT is expected to produce world leading limits on dark matter decay lifetime in the PeV mass range, and could test some of the dark matter hints in the current IceCube data.

astro-ph.HE↗

arXiv2020

NuSTAR Tests of Sterile-Neutrino Dark Matter: New Galactic Bulge Observations and Combined Impact

We analyze two dedicated NuSTAR observations with exposure ${\sim}190$ ks located ${\sim}10^\circ$ from the Galactic plane, one above and the other below, to search for x-ray lines from the radiative decay of sterile-neutrino dark matter. These fields were chosen to minimize astrophysical x-ray backgrounds while remaining near the densest region of the dark matter halo. We find no evidence of anomalous x-ray lines in the energy range 5--20 keV, corresponding to sterile neutrino masses 10--40 keV. Interpreted in the context of sterile neutrinos produced via neutrino mixing, these observations provide the leading constraints in the mass range 10--12 keV, improving upon previous constraints in this range by a factor ${\sim}2$. We also compare our results to Monte Carlo simulations, showing that the fluctuations in our derived limit are not dominated by systematic effects. An updated model of the instrumental background, which is currently under development, will improve NuSTAR's sensitivity to anomalous x-ray lines, particularly for energies 3--5 keV.

astro-ph.HE↗

arXiv2019

Reverse Direct Detection: Cosmic Ray Scattering With Light Dark Matter

Sub-GeV dark matter candidates are of increasing interest, because long-favored candidates such as GeV-scale WIMPs have not been detected. For low-mass dark matter, model-independent constraints are weak or nonexistent. We show that for such candidates, because the number density is high, cosmic ray propagation can be affected by elastic scattering with dark matter. We call this type of search `reverse direct detection,' because dark matter is the target and Standard Model particles are the beam. Using a simple propagation model for galactic cosmic rays, we calculate how dark matter affects cosmic ray spectra at Earth, and set new limits on the dark matter-proton and dark matter-electron cross sections. For protons, our limit is competitive with cosmological constraints, but is independent. For electrons, our limit covers masses not yet probed, and improves on cosmological constraints by one to two orders of magnitude. We comment on how future work can significantly improve the sensitivity of cosmic-ray probes of dark matter interactions.

hep-ph↗

arXiv2019

Extreme Dark Matter Tests with Extreme Mass Ratio Inspirals

Future space-based laser interferometry experiments such as LISA are expected to detect $\cal O$(100--1000) stellar-mass compact objects (e.g., black holes, neutron stars) falling into massive black holes in the centers of galaxies, the so-called extreme-mass-ratio inspirals (EMRIs). If dark matter forms a "spike" due to the growth of the massive black hole, it will induce a gravitational drag on the inspiraling object, changing its orbit and gravitational-wave signal. We show that detection of even a single dark matter spike from the EMRIs will severely constrain several popular dark matter candidates, such as ultralight bosons, keV fermions, MeV--TeV self-annihilating dark matter, and sub-solar mass primordial black holes, as these candidates would flatten the spikes through various mechanisms. Future space gravitational wave experiments could thus have a significant impact on the particle identification of dark matter.

astro-ph.CO↗

arXiv2019

New Constraints on Sterile Neutrino Dark Matter from $NuSTAR$ M31 Observations

We use a combined 1.2 Ms of $NuSTAR$ observations of M31 to search for X-ray lines from sterile neutrino dark matter decay. For the first time in a $NuSTAR$ analysis, we consistently take into account the signal contribution from both the focused and unfocused fields of view. We also reduce the modeling systematic uncertainty by performing spectral fits to each observation individually and statistically combining the results, instead of stacking the spectra. We find no evidence of unknown lines, and thus derive limits on the sterile neutrino parameters. Our results place stringent constraints for dark matter masses $\gtrsim 12$ keV, which reduces the available parameter space for sterile neutrino dark matter produced via neutrino mixing ($e.g.$, in the $ν$MSM) by approximately one-third. Additional $NuSTAR$ observations, together with improved low-energy background modeling, could probe the remaining parameter space in the future. Lastly, we also report model-independent limits on generic dark matter decay rates and annihilation cross sections.

astro-ph.HE↗

arXiv2018

Unexpected Dip in the Solar Gamma-Ray Spectrum

The solar disk is a bright source of multi-GeV gamma rays, due to the interactions of hadronic cosmic rays with the solar atmosphere. However, the underlying production mechanism is not understood, except that its efficiency must be greatly enhanced by magnetic fields that redirect some cosmic rays from ingoing to outgoing before they interact. To elucidate the nature of this emission, we perform a new analysis of solar atmospheric gamma rays with 9 years of Fermi-LAT data, which spans nearly the full 11-year solar cycle. We detect significant gamma-ray emission from the solar disk from 1 GeV up to $\gtrsim200$ GeV. The overall gamma-ray spectrum is much harder ($\sim E_γ^{-2.2}$) than the cosmic-ray spectrum ($\sim E_{\rm CR}^{-2.7}$). We find a clear anticorrelation between the solar cycle phase and the gamma-ray flux between 1-10 GeV. Surprisingly, we observe a spectral dip between $\sim$30-50 GeV in an otherwise power-law spectrum. This was not predicted, is not understood, and may provide crucial clues to the gamma-ray emission mechanism. The flux above 100 GeV, which is brightest during the solar minimum, poses exciting opportunities for HAWC, LHAASO, IceCube, and KM3NeT.

astro-ph.HE↗