A MEASUREMENT OF THE POSITIVE pi- Mu DECAY LIFETIME

The lifetime for the decay of a {pi} meson into {mu} meson and neutral particle was first measured by Richardson and later by Martinelli and Panofsky. The method was the same in both cases: The fraction of {pi} mesons surviving various times of flight is measured by placing photographic detectors at various path lengths from the target. In the experiment reported here we observe the time lag between the two bursts of fluorescence due to mesons decaying in a scintillation crystal. The first burst is due to the stopping of the entering {pi} meson, the second to the {mu}-meson. As is shown in Fig. 1, a particle penetrating the first and into the second crystal starts the sweep (10{sup -8} sec/mm) of an oscilloscope. The pulses in the second crystal are delayed 0.5 x 10{sup -6} sec to allow the sweep to start and brighten and are then photographed. If the responsible particle is a {pi}{sup +} meson which stops in the crystal, it undergoes {pi}-{mu} decay and two pulses appear on the trace. The {mu}{sup +} meson has a range of only 2 mm in the crystal. If its decay electron is detected some time (.5-2.5 x 10{sup -6} sec) later; a neon light flashes and is photographed together with the scope trace. Only such marked traces are measured. Of these marked traces, 650 or roughly one-half, show the two pulses of the {pi}-{mu} event. Five percent are calculated to be due to random delayed coincidences, and another 3 percent due to {pi} mesons which have decayed in flight and come to rest in the second crystal as {mu} mesons. The remaining traces are due to {pi}{mu} decays which are too fast to be resolved. The sweep speed of the oscilloscope is calibrated periodically with an oscillator of known frequency