Experimental comparison of neutrino and muon velocities

The velocities of neutrinos and muons originating from π+ and K+ decays have been compared using the California Institute of Technology-Fermilab detector in a narrow-band neutrino beam. A time-of-flight comparison between positive muons and neutrinos, as indicated by the final-state negative muons in neutrino interactions, yields |βν-βμ|<~0.4×10-3 (99% confidence level). We assume that |1-βμ|<10-5 for these muons.