PM: Search for a Cosmologically-Relevant Boson in Antimuon Decay

University Of Chicago, Chicago IL

Investigators

Abstract

Experiments performed at particle accelerator facilities typically involve the generation of very large signals in the detectors employed. This is the result of the high energies imparted to the accelerated projectiles, which in turn produce very energetic fragments in their collisions. Unexpected products of particle interactions in such experiments are the harbingers of new physics theories. When looking for these signatures of new physics, the traditional concentration has been to push towards higher and higher energies in the sought signals. This award will support an innovative departure from this trend, by concentrating instead on the observation of the smallest possible energies following the impact and decay of a beam of anti-muons striking germanium detectors built specifically for this purpose. The data collected will permit the team to significantly expand the present sensitivity to exotic modes of this decay, specifically one in which an unexpected new particle (a boson) is created. Such bosons have been predicted by many extensions of the Standard Model of particle interactions, a theory now known to be incomplete. By concentrating on low-energies, the search supported by this award will explore, for the first time, the possibility that these bosons are created with slow-enough speeds to be gravitationally bound to galaxies. This would enable them to play interesting roles in cosmological scenarios, including the still unresolved dark matter problem. This award will also support one graduate student towards the completion of their studies. Specialty germanium detectors will be developed with the characteristics required to investigate the mode of decay of an antimuon that generates the new boson and a tell-tale monochromatic low-energy positron. These devices will then be exposed to the M15/M20 surface muon beams at TRIUMF, the Canadian accelerator center. TRIUMF is one of two sources, both outside the US, able to provide the required muon energies for this search. Following short (one week) beam exposures, the pertinent region of boson phase space can be probed down to branching ratios as small as 1E-7, a vast advance with respect to previous searches of this type. Two main factors contribute to this excellent expected sensitivity: an improvement by more than three orders of magnitude in detector energy resolution with respect to previously employed large scintillator calorimeters, and the modest background rate expected in the tiny detectors to be used. 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.