Final Science from the EXO-200 Experiment

About This Grant

A yet unanswered question in contemporary science is why the Universe is dominated by matter over anti-matter, a fact supported by astronomical observations, when the equations governing physics processes at the smallest scales and those describing cosmic evolution would suggest that matter and anti-matter should have annihilated almost completely shortly after the Big Bang. This award is to support work at the University of Massachusetts, Amherst, to complete the analysis of the data collected by the EXO-200 experiment during its operation between 2010 and 2018. EXO-200 was the first of a novel generation of experiments seeking a possible microscopic origin of the matter/anti-matter imbalance through the study of whether the mass of the lightest fundamental particle having mass, the neutrino, is matter or anti-matter. Indeed, there exists the tantalizing possibility that neutrinos could simultaneously be both, enabling a nuclear process known as neutrino-less double-beta decay, in which a nucleus transforms into another by emitting two electrons and nothing else. If observed, this decay would determine that neutrinos and anti-neutrinos are the same particle. Particles having this property are known as Majorana particles. The UMass group proposes to continue to analyze the large EXO-200 data set to search for all possible mechanisms of neutrino-less double beta decay in the isotopes of xenon and to perform measurements that will inform the design and the technical solutions of xenon-based experiments for rare searches of the future. The activities covered by this award will train highly skilled people for the STEM workforce and develop technologies that align with strategic sectors for the US, such as data science and artificial intelligence, nuclear medicine and medical imaging, and technologies in support of national security. The UMass PI is committed to promoting the science advances enabled by this proposal to the broader physics community and to bringing the excitement of modern physics to the middle and high school curriculum. The EXO-200 detector had at its core a single-phase liquid xenon Time Projection Chamber (TPC) filled with 200 kilograms of xenon enriched to 80% in the mass 136 isotope. The detector design allowed to identify and suppress with high efficiency most background signals mimicking neutrino-less double-beta decays. The detector measured the position and energy of every ionizing event inside its xenon volume. Built with materials selected for their minimal radioactivity content, EXO-200 searched for the feeble neutrino-less double beta decay signal with high efficiency above environmental noise. EXO-200 contributed one of the strongest constraints on the existence of neutrino-less double beta decay of xenon-136 (for which it held the world’s best measurement in 2012) and has reported on other searches for rare events and new physics, including some forms of dark matter particles. The UMass group proposes to complete the analysis of the data collected with EXO-200 with a team of junior scientists who are among the few remaining active collaborators with the necessary expertise to carry out this task, crowning a more than fifteen-year investment by the NSF and other domestic and international partners. Searches will be carried out for subdominant neutrino-less double beta decay modes in xenon-136 and xenon-134, for novel non-standard physics signatures of neutrinos and dark matter, and for yet unobserved ultra-rare nuclear decays. Data collected during a final calibration campaign of EXO-200 using radioactive sources, one of which contributed by the UMass group, will allow the precise measurement of the beta energy spectrum of unstable nuclei produced in the EXO-200 xenon target, notably of xenon-137, benefitting nuclear modeling and contributing to an improved determination of the neutrino output from nuclear reactors. The final data set also provides a precious opportunity to measure subtle effects in the response of liquid xenon detectors that could help to develop background-mitigation strategies in future experiments. Collectively, the proposed measurements will inform the design of large, next-generation noble liquid detectors for fundamental and applied science. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.