ABSTRACT
Any interaction that involves the disappearance of an electron could instead involve the creation of a positron. As mentioned in Chapter 3, in Dirac’s original theory the disappearance of an electron from the negativeenergy sea could be interpreted as the creation of a positron. In QED the negative-energy sea has been eliminated, but there remains the possibility of positron creation. Figure 4.2a can be obtained from Figure 4.1a by replacing the incoming electron by an outgoing positron. e resulting diagram is referred to as pair creation. Similarly, instead of the emergence of an electron and positron pair in the nal state, there can be the disappearance of one. is gives Figure 4.2b, referred to as pair annihilation. e processes in Figure 4.2 are also impossible when all the particles are free.
