In their namesake “puzzle,” these particles have a higher-than-expected detection rate in the LHC Forward experiment. The Higgs boson was discovered by both the ATLAS collaboration and the Compact Solenoid detector named for these particles. An inconsistency in the Standard Model is the 3.5-standard-deviation discrepancy between this particle’s measured and predicted g-factor. By increasing the reduced mass of hydrogen, these particles are hypothesized to catalyze a type of cold fusion. The Rossi–Hall experiment showed time dilation (10[1])by observing discrepancies in the 2.2-microsecond decay time of these particles after they were produced in the atmosphere by cosmic rays. For 10 points, name this unstable lepton with a mass between that of the tauon and electron. ■END■