We’ve presented beta decay as if it were perfectly normal for one kind of matter to change spontaneously into a completely different kind of matter. For example, it would be energetically favorable for a large truck to spontaneously turn into a Cadillac and a VW Beetle, ejecting the Beetle at high speed. But it doesn’t happen.

Once you stop to think of it, the process $\text{n}\to {\text{p}}^{+}+{\text{e}}^{-}$ seems ludicrous, not because it violates mass-energy conservation but because we have no idea how a neutron could turn into a proton. Alpha decay may be a strange process because tunneling in general goes against our commonsense notions, but it is a perfectly ordinary quantum-mechanical process. Now we’re suggesting that one of the basic building blocks of matter can somehow morph into a different basic building block.

To make matters more confusing, measurements in the 1930s found that beta decay didn’t seem to conserve either energy or momentum. Faced with these difficulties, the Italian physicist Enrico Fermi made two bold suggestions:

1. A previously unknown fundamental force of nature is responsible for beta decay. This force, which has come to be known as the weak interaction, has the ability to turn a neutron into a proton, and vice versa.

2. The beta-decay process emits a particle that, at that time, had not been detected. This new particle has to be electrically neutral, in order to conserve charge, and it has to be much less massive than an electron. Fermi called it the neutrino, meaning “little neutral one.” Energy and momentum really are conserved, but the neutrino carries away some of the energy and momentum of the decaying nucleus. Thus experiments that detect only the electron seem to violate conservation laws.

The neutrino is represented by the symbol $\nu$, a lowercase Greek nu. The beta-decay processes that Fermi proposed are

The symbol $\overline{\nu }$ is an antineutrino, although the reason one is a neutrino and the other an antineutrino need not concern us here. FIGURE 42.21 shows that the electron and antineutrino (or positron and neutrino) share the energy released in the decay.

FIGURE 42.21 A more accurate picture of beta decay includes neutrinos.

The neutrino interacts with matter so weakly that a neutrino can pass straight through the earth with only a very slight chance of a collision. Trillions of neutrinos created by nuclear fusion reactions in the core of the sun are passing through your body every second. Neutrino interactions are so rare that the first laboratory detection did not occur until 1956, over 20 years after Fermi’s proposal.

It was initially thought that the neutrino had not only zero charge but also zero mass. However, experiments in the 1990s showed that the neutrino mass, although very tiny, is not zero. The best current evidence suggests a mass about one-millionth the mass of an electron. Experiments now under way will attempt to determine a more accurate value.