text: A 2004 experiment used the decay of these particles to confirm the Standard Model's prediction that the Michel rho param- eter equals 0.75; that experiment, which confirms the V-A structure of the weak force, was done by the TWIST collaboration.
full_question: A 2004 experiment used the decay of these particles to confirm the Standard Model's prediction that the Michel rho param- eter equals 0.75; that experiment, which confirms the V-A structure of the weak force, was done by the TWIST collaboration. This particle's decay is also used to measure the Fermi constant characterizing the strength of the weak force. Because these particles are not stopped by electromagnetic or hadronic calorimeters, the ATLAS and CMS experiments have a separate spec- trometer solely for detection of these particles. Charged pions decay into one of these particles and a neutrino. These particles are very useful for performing time dilation experiments since their half-life is around 2.2 microseconds. It's not the positron, but Carl Anderson discovered this particle in his observations of cosmic rays. For 10 points, name this lepton which is lighter than the tauon but heavier than the electron.
first_sentence: A 2004 experiment used the decay of these particles to confirm the Standard Model's prediction that the Michel rho param- eter equals 0.75; that experiment, which confirms the V-A structure of the weak force, was done by the TWIST collaboration.
raw_answer: muons