Measurement of the Lifetime of Cosmic Ray Muons

In this experiment cosmic rays create scintillation light as they pass through a 5 gallon mineral oil/scintillator detector. Some of the low-energy cosmic muons come to rest in the detector and their subsequent decays ($\mu^+ \to e^+ \, \nu_{\mu} \, \bar{\nu}_e$ + \, $C.C.$) are observed as a second burst of light. The decay lifetime of 10,392 stopping muons were measured and the mean muon lifetime ($\tau_{\mu}$) was calculated. The composition of cosmic muons includes both positive and negative muons; however, a small fraction of the $\mu^-$ cosmic rays are captured by hydrogen atoms in the mineral oil ($CH_2$) thus affecting the accepted lifetime of muon decays (2.197 $\mu$s). The muon lifetime measured in this experiment $\tau_{\mu} = (2.092 \pm 0.019) \,\mu s$ does not differentiate between positive or negative muons and is consistent with the occurrence of $\mu^-$ capture on hydrogen.