No, not a modification of the now classic "Say of the week" series. Rather, a quote from a very famous Physical Review article which is of relevance to a couple of questions I offered here and here this week:

In order to insure that the detection efficiency be completely independent of the direction of the positron from the decay of the muon, the observers were instructed to measure only those events that were detected by first seeing the decay. All events so found were used in the analysis except those in which the muon decays occurred within 50 microns of either surface.

When a decay was found, the scanner followed the muon to the end of its range and searched for the positron. A considerable effort was made to detect all positrons because it was felt that the loss of positrons could be a source of bias. In only about 1% of muon decays could a positron not be found. For each positron found, a check was made to insure that it indeed originated at the end of the muon track.

For each decay found, the space angle between the initial direction of motion of the muon and that of the positron was measured. This was done to eliminate the effects of multiple scattering which change the momentum but not the spin orientation of the muon. The angles were measured with an accuracy of about +-2°. [...]

2000 complete decays were analyzed [...] and the space angle defined above was calculated for each. For 60% of all events, chosen at random, this angle was recalculated independently; no appreciable discrepancies were revealed. From these data we find



[... there is] a preference for backward emission with respect to the direction of motion of the muon.

(J. I. Freidman and V. L. Telegdi, The Physical Review 106 (1957) 1290.)

B and F in the formula above are, of course, the fraction of backward and forward emission angles of the positron in the muon decay. This small but significant asymmetry is proof of parity violation in the weak decays of the pion and the muon. Parity violation in weak interactions had by then already been established by the famous experiment by C.S. Wu et al. at the National Bureau of Standards... And it had even been observed no less than 29 years before (!) in an unfortunately inconclusive experiment studying the double scattering of beta rays by R.T. Cox et al.... But that is another story.