The lifetime of hyperons

At the Padua Conference, the Rome group showed how hyperon events found in the course of scanning in photographic emulsions could be used to obtain an estimate of the mean lifetime of hyperons. The method was an extension of the statistical procedure introduced by BARTLETT for the analysis of Wilson chamber data, and was designed to utilise to the full the rather poor number of events then available. Since the detection probability of a hyperon decay event is a function of the hyperon velocity, when it is found by scanning, the method involved corrections for scanning bias. However, in the course of several experiments by different groups at Bristol, 14 hyperons have been found in a completely unbiassed waY. It has been possible to use these to obtain a lifetime estimate free from the disadvantage of using corrections for scanning bias which are rather difficult to estimate accurately. These hyperon decays have been found by tracing tracks out from stars without prior knowledge of their identity. The observation procedure then has clearly no bias in favour of hyperons decaying early or late. Two times, t and T, have bee~! defined in the u sual way: t = time of flight to instant of decay, T= time of flight, during which decay could have occurred without escaping detection. Both t and T were measured from the same origin which did not coincide with the instant of production. The reason for this is that a certain minimum length (dependent on ionization) is reqnired, before a hyperon decay will be unambiguously identified. For example, if decay occurs very quickly there may not be a sufficient length of primary track to distinguish a decay from a scattering event.