RUI: Hidden Sectors in the Early Universe and Today

This award funds the research activities of Professor Brian Shuve at Harvey Mudd College. Observations of the cosmos provide strong evidence that our understanding of the natural world is incomplete. The universe is full of dark matter, which we can neither see nor directly touch, but which dramatically influences the motions of stars and gas through its gravitational pull, and whose identity and origin remain a mystery. At the same time, the ordinary, visible matter making up atoms and molecules is the result of a primordial excess of matter over antimatter produced in the instants after the Big Bang, but none of the known forces can account for the discrepancy between the amounts of matter and antimatter. Professor Shuve will investigate the possibility that "hidden sectors," which are collections of particles and forces that interact so feebly with visible matter that they have so far escaped detection, can account for the phenomena of dark matter and the matter-antimatter asymmetry. Under this grant award, Professor Shuve will develop novel methods to test for the existence of hidden sectors using data from high-energy particle collider experiments, improving the prospects for discovering new elementary particles. Professor Shuve will also study how hidden sectors alter the evolution of the universe after the Big Bang, providing new ways for particle physics experiments to shed light on the forces that shaped the early universe. Research in this area advances the national interest by promoting the progress of science in one of its most fundamental directions: the discovery and understanding of new physical laws of nature. This project has significant broader impacts. The research will involve undergraduate students at all stages, providing advanced training experiences that will be transformative for the students' career trajectories and contribute to the development of the STEM workforce. Professor Shuve will give public lectures on the results of this research. He will also develop curricular materials that incorporate these findings and prepare current and future students to engage in research addressing some of the most challenging open questions in science. More technically, Professor Shuve will pursue two complementary research directions into the physics of hidden sectors. In the first direction, he will explore both bottom-up and top-down motivations for hidden-sector phenomenology, developing new models of hidden-sector dynamics that inspire novel experimental methods, while studying known gaps in experimental coverage to propose tests to fill in these gaps. This work will develop new approaches to analyzing high-energy particle collision data, such as those collected by the Belle II experiment and by experiments at the Large Hadron Collider. These approaches will lead to new tests of baryon and lepton number symmetries and their possible violation, search strategies for tagging new physics in low-mass semi-visible particle decays, and searches for new charged states predicted in ultraviolet completions of mediator effective field theories such as axion-like particle and leptophilic scalar models. In the second direction, Professor Shuve will identify new hidden-sector mechanisms for resolving outstanding problems in cosmology, which could point towards entirely new tests that would cover uncharted yet motivated parts of parameter space. He will comprehensively investigate scenarios in which baryogenesis and dark matter abundances are simultaneously established through dark matter freeze-out, and study new regimes of baryogenesis through dark matter freeze-in. 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.