bei48482_FM

Neutrino

Antineutrino

Figure 13.3Neutrinos and anti-

neutrinos have opposite direc-

tions of spin.

motion; viewed from behind, as in Fig. 13.3, the neutrino spins counterclockwise. The

spin of the antineutrino, on the other hand, is in the same direction as its direction of

motion; viewed from behind, it spins clockwise. Thus the neutrino moves through

space in the manner of a left-handed screw, while the antineutrino does so in the

manner of a right-handed screw.

Prior to 1956 it had been universally assumed that neutrinos could be either left-

handed or right-handed. This implied that, since no difference was possible between

them except one of spin direction, the neutrino and antineutrino are identical. The as-

sumption had roots going all the way back to Leibniz, Newton’s contemporary and an

independent inventor of calculus. The argument is as follows. If we observe an object

or a physical process of some kind both directly and in a mirror, we cannot ideally

distinguish which object or process is being viewed directly and which by reflection.

By definition, distinctions in physical reality must be capable of discernment or they

are meaningless. But the only difference between something seen directly and the same

thing seen in a mirror is the interchange of right-handedness and left-handedness, and

so allobjects and processes must occur with equal probability with right and left

interchanged.

This plausible doctrine is indeed experimentally valid for the strong and electro-

magnetic interactions. However, until 1956 its applicability to neutrinos, which are

subject only to the weak interaction, had never been actually tested. In that year Tsung

Dao Lee and Chen Ning Yang suggested that several serious theoretical discrepancies

would be removed if neutrinos and antineutrinos have different handedness, even

though it meant that neither particle could therefore be reflected in a mirror.

Experiments performed soon after their proposal showed unequivocally that neutrinos

and antineutrinos are distinguishable, having left-handed and right-handed spins

respectively.

Other Leptons

The muon,, and its associated neutrino were first discovered in the decays of

charged pions:

Charged pion decay → → (13.1)

The pion was discussed in Sec. 11.7 in connection with the strong force between

nucleons, which it mediates. The pion’s mass is intermediate between those of

the electron and the proton, and it is unstable with a mean life of 2.6 10 ^8 s for

. The neutral pion has a mean life of 8.7 10 ^17 s and decays into two gamma

rays:

Neutral pion decay ^0 → (13.2)

The neutrinos involved in pion decays are not the same as those involved in beta

decay. The existence of another class of neutrino was established in 1962. A metal

target was bombarded with high-energy protons, and pions were created in profusion.

Inverse reactions traceable to the neutrinos from the decay of these pions produced

muons only, and no electrons. Hence these neutrinos must be different in some way

from those associated with beta decay.

Positive and negative muons have the same rest mass of 106 MeV/c^2 (207 me) and

the same spin of ^12 . Both decay with a relatively long mean life of 2.2 10 ^6 s into

electrons and neutrino-antineutrino pairs:

Elementary Particles 479

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