introduction-weak-interaction-volume-one

CHAPTER FOUR : WEAK LIPTONIC REACTIONS .
4 .1 Phenomenology of ,:uon Decay .
In 1936,
Anderson and Neddermeyer (1) obtained a number of cosmic ra y
cloud chamber tracks at mountain altitudes which were attributed to a particl e
of approximate mass 100 EeV/ c 2 . In 1940, Williams and Roberts (2) obtaine d
cloud chamber photographs
showing a negative particle of mass 120rleV/ c2 , which
decayed into an electron. By comparing Geiger-counter counting rates a t
different altitudes, the lifetime of the new particle, which was named th e
muon, was established as about 2 .ps . Rasetti made a more direct determination
of the muon lifetime . He placed two Geiger counters above a 10 cm thick iro n
absorber, and two others below . Bhen a muon was stopped in the iron, as indicate d
by the anticoincidence of the second pair of counters, the time before th e
emergence of its charged decay product was measured . However, only about hal f
of all the muons stopped in the iron appeared to decay . This was explained by
assuming that, in cosmic rays, there exist equal numbers of muons, which are
negatively-charged, and positively-charged antimuons . The antimuons are
repelled by the Coulomb fields of the iron nuclei, and are thus free to decay ,
but the muons are captured by atomic nuclei . As captured muons cascad e
towards the nucleus, they emit x-rays as they jump from one electron orbit t o
the next . The energies of these x-rays may be measured to within ten o r
twenty electronvolts, and, using the Bohr formul al , we may calculate the muo n
mass a s
105 .66 0 .015 EeV/c2 . (4 .1 .1 )
The currently acknowledged value for the muon mass is (3 )
105 .65948 ± 0 .00035 NeV/c2 . (4 .1 .2 )
The muon lifetime was initially measured by track length in nuclear emulsions ,
and then by accelerator time-of-flight measurements . The current value is (4 )
(2 .1994 ± 0 .0006) X 10-6 6 . (4 .1 .3 )
Since the electron in muon decay is not monoenergetic, we may deduce tha t
there are
two unobserved neutral particles present in the decay, and thus