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 |β_ν-β_μ|<~0.4×10^-3 (99% confidence level). We assume that |1-β_μ|<10^-5 for these muons.