Exchange particles
4.5
(a) Exchanging a heavy object
KEY IDEAS
›› Hadrons are composed of particles called quarks. ›› Quarks have antiparticles, known as antiquarks. Antiquarks have the same mass but opposite values of charge, baryon number and strangeness.
›› Baryons are composed of three quarks or
(b) Exchanging a boomerang
three antiquarks.
›› Mesons are composed of a quark and an antiquark.
›› Three types (or flavours) of quark, up, down
and strange, were sufficient to describe all the hadrons that were known up to 1970.
Figure 11 Analogy of particle interactions
4.5 EXCHANGE PARTICLES The theory of interactions between particles proposed a third group of fundamental particles. On the subatomic scale exchange particles are responsible for transmitting the fundamental forces that hold nucleons, nuclei and atoms together. On a larger scale they are responsible for the everyday forces that affect our lives.
Exchange particles and Feynman diagrams When two bodies exert a force on each other, perhaps the floor exerting a force on your shoes, what is it that happens between the two bodies? What happens to make the two objects repel each other? The Japanese physicist Hideki Yukawa suggested that when two particles, A and B, exert a force on each other, a virtual particle is created. This virtual particle can travel between particles A and B and affect their motion. The virtual particle, which may exist for only a very short time, is referred to as an exchange particle and it is the mediator of the force. The idea of a force being carried by an exchange particle can be pictured by considering two people on ice skates (Figure 11). If one person throws a ball to the other, both skaters’ motion will be affected. In fact, they will be pushed away from each other. We have to stretch the analogy a bit to include an attractive force, but if you imagine a boomerang being thrown, rather than a ball, then the two skaters will be drawn together. The analogy of a ball, or boomerang, being exchanged is not to be taken too literally. In some
interactions, it is not, or not just, momentum that is being exchanged, but charge or other properties. Indeed, in some interactions, involving the weak force, this property is being changed. The exchange particles that are transferred between fundamental particles are known as gauge bosons and each fundamental force has its own boson or set of bosons.
The electromagnetic force The electromagnetic force is carried between charged particles by the photon, γ . When two charged particles, say two electrons, exert a force on each other, a virtual photon is exchanged between them. We can use simple diagrams to represent what happens (Figure 12). These are called ‘Feynman diagrams’ after Richard Feynman (Chapter 2), who devised them. The photon is a massless, chargeless particle. In fact it is its own antiparticle: a photon is identical to an antiphoton.
The strong interaction Hideki Yukawa was working on the strong nuclear interaction when he first proposed the idea of exchange particles in 1935. He suggested that these exchange particles could be travelling at close to the speed of light across the nucleus. An exchange particle moving at close to the speed of light has to exist for about 10–23 s if it is to have time to travel across the nucleus. This enabled Yukawa to predict a maximum
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