Identify the mediators for the two interactions described in the Feynman diagrams shown in Figure.

(a) The Feynman diagram in ANS. shows a neutrino scattering off an electron, and the neutrino and Electrons do not exchange electric charge. The neutrino has no electric charge and interacts through the weak interaction (ignoring gravity). The mediator is a $$Z^0$$ boson.
(b) The Feynman diagram shows a down quark and its antiparticle annihilating each other. They can produce a particle carrying energy, momentum, and angular momentum, but zero charges, zero baryon number, and, if the quarks have opposite color charges, no color charge. In this case, the mediating particle could be a photon or $$Z^0$$ boson. Depending on the color charges of the d and d quarks, the ephemeral particle could also be a gluon, as suggested in the discussion of Figure.
For conservation of both energy and momentum in the collision, we would expect two mediating particles; but momentum need not be strictly conserved, according to the uncertainty principle, if the particle travels a sufficiently short distance before producing another matter-antimatter pair of particles, as shown in ANS.