4 THE STANDARD MODEL
−
p
e−
e
n π0
−
e = an electron γ = a virtual photon
γ Time
p e
−
e
This Feynman diagram shows a pion being exchanged between a proton and a neutron. This exchange is the source of the strong nuclear force. p = a proton n = a neutron π0 = a neutral pion
n
−
Figure 14 Diagram representing pion exchange Feynman diagrams represent the events in an interaction. In the diagrams in this book the sequence of time goes up the page. Note that Feynman diagrams tell us nothing about the actual paths of the two particles. The angles in the diagrams have no significance.
Figure 12 Diagram representing two electrons feeling the electric force as a result of a photon being exchanged between them
mass for the particle, which became known as the pi meson or pion (Figure 13). The pion acts between nucleons to hold the nucleus together (Figure 14). We now know that, at a deeper level, the strong interaction is mediated by gauge bosons called gluons that pass between quarks.
The weak interaction The weak interaction has a very short range. Its gauge bosons are relatively massive, and since a large mass (and therefore energy) means a short lifetime, the exchange particles can only travel a short distance. The weak interaction has three gauge bosons, also known as intermediate vector bosons, W+, W– and Z0. These bosons were eventually discovered in 1983 at CERN. The weak interaction acts on both leptons and hadrons. It is the only force, other than gravity, that acts on neutrinos. This explains the fact that neutrinos so rarely interact with anything. The weak interaction can alter the flavour of a quark. Since W bosons carry charge, the weak interaction may lead, for example, to an up quark being changed into a down quark. Look back at Figure 9, which shows the decay of a neutron into a proton (this is what happens in beta-minus decay). Figure 15 shows the Feynman diagram for this interaction, mediated by a W– boson. Figure 16 shows the detail of the changes at a fundamental level.
−
e p −
W
n
νe
The W− is a type of exchange particle known as a boson.
In beta-minus decay, a neutron changes into a proton, and an electron and an antineutrino are emitted. Figure 13 Yukawa’s predicted pion was discovered in 1947 in cosmic ray experiments. This cosmic ray collision produces a spray of particles, including 16 pions, shown in yellow.
Figure 15 Diagram representing the exchange involved in beta-minus decay
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