introduction-weak-interaction-volume-one

4 .5 The Conservation of Leptons .
In 1955, Davis (20) suggested the assignment of a quantum number know n
as leptonic charge or lepton number to all particles . The electron, muon, and
neutrinos would have a lepton number of +1, while their antiparticles woul d
have L = -1 . All other particles would have L =0 . Thus, for any particl e
L = e 4- y , (4 .5 .1 )
where e and)). are its electron and muon number respectively (see 4 .3) .
The analogue of lepton number in the strong interactio n7 is baryon number .
The latter is carried by all particles subject to the strong interaction .
If baryon number is conserved, then the proton must be stable, since it i s
the lightest baryon . Stuckleberg and Wigner have studied the stability of th e
proton, and have thus deduced that baryon number is conserved to better than
one part in 1 0 43 . From studying leptonic and semileptonic reactions, it i s
easy to see that lepton number is also conserved . Thus the electron must b e
stable, since it is the lightest charged lepton . The neutrinos cannot decay ,
since they are the lightest leptons .
We now consider some of the evidence for lepton conservation . If thi s
conservation law holds good, then the nuclear deca y
(Z, A) >(Z+ 2, A) t 2e , (4 .5 .2 )
known as double beta decay, should never occur, since it violates the law of
the conservation of lepton number . The decay
(Z, A)• :. (Z+ 2, A) + 2e + 2ve (4 .5 .3 )
is, however, permitted . The decay (4 .5 .3) may be distinguished from (4 .5 .2)
in experiments by observing the electron energy distribution . In (4 .5 .2) ,
the electrons share the full decay energy, whereas in (4 .5 .3), some of thi s
is removed by the antineutrinos . If (4 .5 .2) were allowed, then it shoul d
have a rate about 10 5 times greater than that of the permitted decay (4 .5 .3) .
Recently, it has been suggested that double beta decay occurs in nature (21) :
Te 130 @F > Xe 130 ( 4 .5 .4 )
and there is convincing geological evidence to support this view . The measured
half—life for the process (4 .5 .4) is 10 21 .34 t0 .12 yr . The theoretical
prediction for a neutrinoless decay is 1 01.3 '' 2 yr and that for a decay with