Thesis (Ph. D.)--University of Rochester. Department of Physics and Astronomy, 2019.
Astrophysical observations are central to the quest for new physics including the
search for dark matter. The search is based on identifying potential deviations from
the Standard Model in the cosmic-ray and the electromagnetic spectrum of astrophysical
sources. The deviations could either be signatures of dark matter or have
consequences for our understanding of known sources. The last decade of precision
measurements from detectors in space, such as the Fermi Gamma-ray Space Telescope,
and the Alpha Magnetic Spectrometer for detecting cosmic rays aboard the
International Space Station, have identified certain “anomalies” or unexpected spectral
features, that challenge the standard models of how cosmic rays are produced
and propagate through the Galaxy. Examples include an unexpectedly hard spectrum
of cosmic-ray antiprotons at energies above a few hundred GeV, and an unexplained
excess of very-high-energy gamma rays from the Sun. An excess of cosmic-ray antiprotons
and a hard spectrum of gamma rays from the Sun also feature in the predictions
of various models of dark matter annihilation. However, without a complete understanding
of the antiproton spectrum, and the production mechanisms of solar gamma
rays, it is impossible to differentiate new physics from the standard astrophysical
foreground flux of these particles. Measuring these fluxes at energies that extend into
the TeV range is an observational challenge that we explore in this thesis. The High AltitudeWater Cherenkov (HAWC) Observatory is a wide field-of-view
array that is currently the only detector capable of making high-statistics measurements
of cosmic rays and gamma rays at multi-TeV energies. This work uses data from HAWC collected between 2014–2017 to constrain two unique fluxes at the TeV
scale: antiprotons in Galactic cosmic rays, and gamma rays from the quiescent Sun
— both relevant foregrounds for astrophysical searches for physics beyond the Standard
Model. Cosmic rays in the inner solar system are subject to deflection by the
magnetic fields of the Earth and the Sun, affecting the observed deficit or “shadow”
of the Moon/Sun. Cosmic rays also interact with the Sun’s atmosphere to produce a
steady emission of gamma rays up to at least 200 GeV, though the exact underlying
mechanism remains a puzzle. We present the strongest upper limits on the antiproton
to proton ratio in TeV cosmic rays at 1% using the Moon shadow as a momentum/
charge discriminant. We also discuss our search for excess gamma rays from the
Sun above 1 TeV, and present the resulting implications for models of dark matter
capture and annihilation in the Sun. Our results constrain the steady gamma-ray
emission from the Sun up to a few times 10−12 TeV cm−2 s−1 at 1 TeV. For dark
matter annihilation with long-lived mediators in the Sun, we present the strongest
upper limits on dark matter-proton scattering cross section up to ~10−45 cm2, which
is a potential improvement of four orders of magnitude compared to direct-detection
experiments for dark matter mass of 1 TeV.
