which gives the opposite sign to (10.77). However, this sign ambiguity is completely unphysical, because the p+sector is completely independent of all other ■ actors. This means that amplitudes with one pair can interfere only with other amplitudes with one pair and cannot interfere with amplitudes with a different number of p+p~ pairs. Hence the overall sign of such amplitudes cannot be measured and may be separately fixed by an arbitrary sign convention.
(iv) Application to e+e~ —> e+e~: Our calculation also applies to e+e~ —> e+e" scattering. However, in this case there is another diagram. The two diagrams are shown in Fig. 10.5, and the .M-matrix corresponding to them is
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