Kavli Affiliate: Shunsaku Horiuchi
| First 5 Authors: Brandon M. Roach, Steven Rossland, Kenny C. Y. Ng, Kerstin Perez, John F. Beacom
| Summary:
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
$sin^2(2theta)$ for sterile-neutrino masses 6–40 keV. In particular, we
strongly disfavor a ${sim}$7-keV sterile neutrino decaying into a 3.5-keV
photon. In combination with previous results, the parameter space for the
Neutrino Minimal Standard Model ($nu$MSM) is now nearly closed.
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