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Baryons

Table 21.2. I Quark properties.

Name Symbol Charge Mass (GeV)

(+ others) p (proton) (muon, neutrino) (+ others) t, vT (tau, neutrino)

are the proton and neutron, and the most familiar meson is the pion.

The leptons appear to be the simplest group. In addition to the electron, the ^ (mu) and t (tau) particles are heavier versions of the electron. Their masses are 207 and 3660 times that of the electron, respectively. Neutrinos are also leptons. We think that there are three types of neutrinos, one to go with each of the other leptons (e, t). That is, in reactions for which we see an electron, we will see an electron neutrino, and so on. All of the evidence to date indicates that the leptons are truly fundamental. They seem to have no internal structure. We still don't know if the six leptons are all that there are. There is some evidence to suggest that there are no others, but we have been surprised before.

The hadrons do appear to have internal structure. This can be seen in experiments that have sufficiently high energy to probe the charge distribution within a proton. We now think that the hadrons are composed of particles called quarks. In the original theory of quarks, there were only three; now six have been found. That makes six quarks and six leptons, a balance which theorists seem to like.

The properties of the quarks are given in Table 21.2. Notice that they have fractional charges, coming in units of (plus and minus) one-third and two-thirds of the fundamental charge, e. However, the quarks can only combine in ways that produce integral net charges. Each quark has its own antiquark. All of the properties of a given antiquark (except mass) are the negative of those for the corresponding quark.

In the quark theory, any baryon is a combination of three quarks. For example, a proton is uud

Table 21.2. I Quark properties.

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u

+ (2/3)e

0 0

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