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Subatomic Physics

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Chapter 8<br />

Angular Momentum and Isospin<br />

In this chapter we shall show that invariance under rotation in space leads to conservation<br />

of angular momentum. We shall then introduce isospin, a quantity that<br />

has many properties similar to ordinary spin, and discuss the “breaking” of isospin<br />

invariance.<br />

8.1 Invariance Under Spatial Rotation<br />

Invariance under spatial rotation provides an important application of the general<br />

considerations presented in Section 7.1.<br />

Consider an idealized experimental<br />

arrangement, shown in<br />

Fig. 8.1. We assume for simplicity<br />

that the equipment is<br />

in the xy plane; its orientation<br />

is described by the angle<br />

ϕ. We further assume that<br />

the result of the experiment is<br />

described by a wave function<br />

ψ(x). Next, the equipment is<br />

rotated by an angle α about<br />

the z axis. This rotation is denoted<br />

by Rz(α), and it carries<br />

apointx into a point x R :<br />

x R = Rz(α)x. (8.1)<br />

Figure 8.1: Rotation around the z axis. The angle ϕ fixes<br />

the position of the original equipment axis; it does not denote<br />

a rotation. The equipment is rotated about the z axis<br />

by an angle α. Invariance under rotation means that the<br />

outcome of the experiment is not affected by the rotation.<br />

The rotation changes the wave function; the relation between the rotated and unrotated<br />

wave function at point x is given by Eq. (7.7) as<br />

ψ R (x) =Uz(α)ψ(x). (8.2)<br />

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