.Particles and anti-particles
Every particle has a corresponding anti-particle. The anti-particle always has the opposite charge, lepton number and baryon number. For particles (e.g. photons and Z bosons) with zero charge, zero lepton number and zero baryon number, they can be their own anti-particle. For instance, consider the reaction below where a down quark and an anti-up quark annihilate and form an electron and an anti-neutrino,
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Both the net baryon number and the net lepton numbers are zero before and after the reaction, since the anti-particles have opposite quantum numbers as compared to their counterparts.