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, as observed by three photomultiplier tubes. Some of the low-energy cosmic muons come to rest in the detector and their subsequent decays ($\mu^+ \to e^+ \, \bar{\nu}_{\mu} \, \nu_e$ + \, $C.C.$) are observed as a second burst of light. The decay lifetime of 498,987 stopping muons were measured and the mean muon lifetime ($\tau_{\mu}$) was calculated to be $\tau_{\mu} = (2.119 \pm 0.003) \,\mu s$. The composition of cosmic muons includes both positive and negative muons, and a small fraction of the $\mu^-$ cosmic rays are captured by hydrogen and carbon nuclei in the mineral oil ($CH_2$) resulting in a capture lifetime of $\tau_{cap} = (57 \pm 2)\,\mu s$.