644 Particle Physics and CosmologyP46.65(a)The mediator of this weak interaction is aZ boson0.(b)The Feynman diagram shows a down quark andits antiparticle annihilating each other. They canproduce a particle carrying energy, momentum,and angular momentum, but zero charge, zerobaryon number, and, it may be, no color charge. Inthis case the product particle is a photon .FIG. P46.65For conservation of both energy and momentum in the collision we would expect twophotons; but momentum need not be strictly conserved, according to the uncertaintyprinciple, if the photon travels a sufficiently short distance before producing another matter-antimatter pair of particles, as shown in Figure P46.65. Depending on the color charges ofthe d and d quarks, the ephemeral particle could also be a
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