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Muon Lifetime

With only one scintillation detector and the QuarkNet DAQ board, we can measure the lifetime of the fundamental particle, the muon. The muon only lives approximately 2 microseconds. Our detector was specifically designed to trigger on a muon and then wait 25 microseconds to record the next pulse coming from the detector. Most muons will have decayed by that time. We use the same apparatus to see the muons decay product. The muon decays into an electron and two neutrinos. The two neutrinos escape our apparatus and interact so little that they will go directly through the Earth without stopping. The electron is charged and will give off light going through the scintillator.

We must have patience. Ten muons per second go through our detector, but in order to see them decay, they must stop inside our detector. Most muons have so much energy that they go through. However only 1 muon out of every thousand lose enough energy to stop and decay inside our scintillator. We made our scintillator extremely thick (3 inches) to maximize the number of stopping muons. We can improve the rate of stopping by adding dense shielding above our detectors so the muon first loses energy and slows down in the steel or lead. In fact, the UIC detector is located on the second floor of a 4-story building so there is quite a bit of concrete and steel above the detector that already slows down the muons.

How can we tell the difference between a muon pulse and an electron pulse in the scintillator? All charged particles create light in the scintillators. The pulses from muons and electrons are slightly different in size, but we ignore that effect and use the timing information. Obviously the electron appears after the muon pulse. As long as there are very few other pulses, the next pulse after the trigger belongs to the electron. Thatís it! We plot the time between trigger and second pulse and find an exponential distribution with characteristic time of approximately 2 microseconds.

The rate of muon decays vesus time is shown for two definitions of incoming muons. The slope of the rate on a log plot gives the lifetime of the muon. The plot on the right requires a cleaner muon definition and the flat horizontal tail is suppressed.
QuarkNet Muon Lifetime

In an newer effort at UIC (CHAMP search) presented at the 2010 APS meeting, the muon time is shown for muons that stop.

  • Muon Lifetime result from APS 2010 presentation
  • Full APS 2010 presentation
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