A matter of coincidences Electronic recoils from beta decays to dark matter absorption in XENONnT

Dark matter makes up most of the matter in the Universe, yet it has only been observed through its gravitational effects. XENONnT is a liquid xenon time projection chamber operating deep underground at the Gran Sasso laboratory in Italy. It is among the most sensitive experiments searching for dark matter, and its low background also makes it well suited to precision measurements of rare nuclear processes. This dissertation presents a new measurement of the light and charge yields of liquid xenon. It also presents a measurement of the ²¹⁴Bi beta spectrum precise enough to discriminate between competing nuclear structure models. Finally, it reports a search for fermionic dark matter absorption on ¹³⁶Xe that improves existing limits by up to four orders of magnitude.