The rate at which protons capture muons has been accurately measured for the first time by the MuCap collaboration at the Paul Scherrer Institute (PSI) in Switzerland. This process, which can be thought of as beta decay in reverse, results in the formation of a neutron and a neutrino. The team has also determined a dimensionless factor that influences the rate of muon capture, which was found to be in excellent agreement with theoretical predictions that are based on very complex calculations.
Muons are cousins of the electron that are around 200 times heavier. Beta decays demonstrate the weak nuclear force in which a neutron gets converted into a proton by emitting an electron and a neutrino. Now, replace the electron with the heavier muon and run the process backwards: a proton captures a muon and transforms into a neutron while emitting a neutrino. This process – known as ordinary muon capture (OMC) – is crucial to understanding the weak interaction involving protons.