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4. Fundamental families page 16
The leptons
We have seen that the electron has an anti-particle – the positron. Like the electron, this is a fundamental particle and is a member of the family of leptons. They are both found in normal matter and are known as first generation leptons. However, as we shall see, there are other, almost invisible, particles in the first generation of leptons. They are tiny, only interact weakly with other particles and are therefore extremely difficult to detect. They are neutrinos (meaning little neutral one).
Spectrum of energy for beta particles
Picture 4.2 The energy spectrum of beta particles emitted during beta decay.
Beta trouble
In the 1920s and 1930s, it was known that beta decay was not straightforward. The fast moving electron came from the nucleus, which didn't contain any electrons. And its beta particles were emitted with a range of energies. So why was this a puzzle?

Like any event involving particles, beta decay must obey the laws of conservation of momentum and energy. In beta decay, a neutron decays into a proton sending out an electron. This decay should release a fixed amount of energy which would be shared between the proton and the electron. According to conservation of momentum, the proton's recoil speed is a fraction of the electron's. Its kinetic energy, therefore is insignificant. So the energy released in the decay is far from equally shared between the proton and the beta particle.

So why did most beta particles have less energy than the maximum energy? Why weren't they all given out with the same energy? The answer is that some of the energy must be taken away by another means. There was no evidence of any radiation being given out so scientists considered that it might be another particle. This is what Wolfgang Pauli proposed in the early 1930s, calling the particle the neutrino. He suggested that the missing energy was carried away by the neutral neutrino, which had to be extremely light (possibly massless). For a long time, it was thought that the neutrino had zero mass. However, recent evidence in 2003 suggests that it does have some, though very little, mass.

A new force
The neutrino does not feel the strong nuclear force (because it is a lepton) or the electrostatic force (because it is neutral). Its behaviour is affected by the weak force only (see page 23). This force has so little effect that billions of neutrinos have passed through you in the last second without being deflected by the other particles in your body.

The neutrino wasn't found in an experiment until 1957. This is because the forces between it and other particles are extremely weak and short range. It is thought that a neutrino can pass through the Earth with only a 1 in 200 million chance of interacting with any other matter.

1st generation Leptons
Particles Antiparticles
electron neutrino positron anti-
neutrino
m
/kg
10–30 0? -10–30 0?
q
/C
–1.6x10–19 0 1.6x10–19 0
F EM, weak, gravity weak, gravity EM, weak, gravity weak, gravity
Table 6. The first generation of leptons showing mass charge and the forces they feel.
Getting the evidence

We now know that the third particle in beta decay is actually an anti-neutrino. A normal neutrino is given out in beta plus decay. Both types of beta decay give out one particle and one anti-particle.

The equations for these decays are:

Beta minus decay:  Equation Notice the symbol for an anti-neutrino. It has a bar on top to show that it is an anti-particle.
Beta plus decay: Equation For completeness, there is a bar over the beta plus symbol.

[Remember an isolated proton does not decay; it is only when the proton is in a nucleus that it will undergo beta plus decay.]

We now have two particles and their antiparticle in the lepton family. They are the only leptons found in normal matter and make up the first generation of leptons (table 6).

The generation game
In 1936, two American physicists discovered a new lepton. Carl Anderson and S.H. Neddermeyer were using a cloud chamber to study cosmic radiation. They noticed tracks that they could not explain using any known particles. The new tracks came from something like an electron but with 207 times its mass. They called this particle the muon. It is a second-generation lepton.

The muon has similar properties to an electron. However, it is very unstable. It decays with a half-life of 2 microseconds (2 millionths of a second). When it decays, it produces an electron and two different neutrinos. One of these is the neutrino that we have already met and one is a new, second generation, neutrino. Particle physicists are still trying to explain this decay.

What's in a name?

To distinguish between the two neutrinos, physicists name them after the other particle in their generation. So they become the electron-neutrino and the muon-neutrino respectively.

First order leptons
Table 7. The three generations of lepton. Roll over here to show just the electron's generation.
The muon, the muon-neutrino and their anti-particles form a generation of particles that repeats the pattern of the first generation – the electron's generation. However, they are not the end of the story.
3G
In 1974, Martin Perl discovered a third generation lepton called the tau particle. Like the electron and the muon, the tau particle has an associated tau-neutrino and anti-particles. This third generation of leptons repeats the pattern of the other two generations but at a higher mass – about twice the mass of the muon generation.

We now have a picture of one of the families of fundamental particles – the leptons. The other family is the quarks (see page 17). The second and third generations of leptons are not part of ordinary matter and they have extremely short lives. It is, however, a satisfying symmetry that both the leptons and the quarks have three generations of particles.

Question 15
a) Imagine that a beta particle were the only particle given out in beta decay. What can you say about the energies of beta particles from a given radioactive decay?
b) How many generations of leptons are there?
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