Tom W. Bonner Prize in Nuclear Physics: Precision Muon Physics: Capturing a Moment in a Lifetime

Low-energy precision measurements can both establish key parameters of the Standard Model (SM) and test predictions in the quest for new physics. I will describe the suite of experiments our group has been involved in, all having impactful results, and all involving muons.  In the early 2000s, the Brookhaven muon g-2 experiment measured a value for the muon magnetic anomaly that has been in persistent tension since then with an ever improving SM prediction.  A group of us from that era began studying how to design a "discovery’" experiment capable of reaching a much higher precision.  We also had figure out where it might be located.  The effort resulted in the Fermilab Muon g-2 experiment that is now in its 5^th year of data taking. The recently published results from the year 1 data have already confirmed the BNL finding and, when averged,  exceeds the SM theory prediction with a significance of 4.2 standard deviations.  Of course, this is but the tip of the iceberg as greater than 10 times more data are already acquired and running continues.  In parallel, a world-wide theory initiative is ongoing with breakthrough improvements. Both experimental and theoretical uncertainties are thus expected to improve significantly.  Sandwiched between the the completion of the BNL and start of the Fermilab campaigns, our group developed two precision experiments at the Paul Scherrer Institute (PSI) using low-energy beams of stopped muons.  MuLan measured the positive muon lifetime and determined the Fermi Constant to 0.5 ppm.  MuCap measured the negative muon lifetime in a hydrogen TPC. The difference is the muon capture rate, which is used to determine the nucleon weak pseudoscalar coupling constant g_P in an unambiguous manner.