UC San Diego

Numerous evidence suggests that the majority of matter in the Universe is made of a rarely interacting, non-luminous component, termed dark matter. The XENON1T experiment, utilizing a two-phase liquid xenon time projection chamber, was primarily designed to search for Weakly Interacting Massive Particles (WIMPs), one of the most promising dark matter candidates. With one tonne-year exposure, XENON1T placed the most stringent upper limits of WIMP interaction strength for a large range of WIMP masses and a variety of interaction types. The unprecedented low background in XENON1T also enabled competitive searches for electronic recoil signals. An excess was observed above the known background at low energies and is most prominent between 2 and 3 keVee. This excess favors solar axions over backgrounds at 3.4 sigma, a hypothetical particle arising from the Peccei-Quinn theory to solve the strong CP problem. The resulting axion couplings, however, are in strong tension with astrophysical constraints. The excess can also be explained by beta decays of tritium at 3.2 sigma with a trace amount, which can neither be confirmed nor be rejected with the current knowledge of its production and mitigation mechanisms. If an unconstrained tritium component is added to both alternate and null hypotheses, the significance of the solar axion hypothesis is reduced to 2.0 sigma. This search also includes other electronic recoil signals, such as an enhanced neutrino magnetic moment, bosonic dark matter, and leptophilic dark matter. The prospect of XENONnT, the next-generation experiment that is expected to take science data in 2021, is also discussed in the context of searching for WIMPs and deciphering the excess observed in XENON1T, respectively.