The Muon Lifetime Experiment

μ­

TaskThe main goal of this lab course is to measure themean liefetime of secondary cosmic ray muons.[1]

Muons are charged elementary particles. They areunstable and decay via the weak interaction. Becausecharge must be conserved, one of the products ofmuon decay is always an electron of the same chargeas the muon. This fact is used in the measurement ofthe muon lifetime.

ApparatusAll measurements are conducted using a "muontelescope", which consists of three plastic scintillationdetectors stacked one on top of the other. Adedicated integrated circuit allows a rule basedselection of relevant events from the detectors.Muons coming to full rest inside one of thescintillators will trigger a signal.

After spending some time (~μs) inside the scintillator,the muon decays. The rest mass is distributed amongits decay products. On average, the resulting electroncarries enough energy to also cause ionization insidethe scintillator. Hence the detector will register twoevents within a small period of time. The differencebetween the rising edges of the pulses indicates thedecay time.

Measurements and AnalysisAbout 10,000 muons reach every square meter of theearth's surface a minute. But due to the low detectionrate of ~4 events/minute measurements need to betaken over an extended period of time in order to getsignificant results.

The collected decay times can be plotted in ahistogram. A least square fit with an exponentialdecay applied to the top-right edges of the bins thengives an estimate of the mean lifetime. To assess thequality of the fit and to find the optimal number ofbins for the data a Chi² test can be conducted. Thedegree of freedom for this test mainly depends on thenumber of bins.

The mean lifetime determined in this setup deviatesfrom the literature value of 2.1969811(22)μs formuons in vacuum.[2] This is due to muon-scintillatorinteraction.

Days42

Decay events250,000

Telescope Signal

Significance level> 0.1

µs mean lifetime2.073(13)

MeV/c2105.7

Electric charge-1

Literature[1] S. Federici and N. Hakansson. The Scintillation Experiment Description.Universität Potsdam - Insititut für Physik und Astronomie, May 2014.[2] K.A. Olive et al. (Particle Data Group) and Chin. Phys. C. 2014 review ofparticle physics. , 38, 090001, 2014.

Primary decay channelμ−→ e− + νe + νμ

Julien Gout