13 Apr 2012 23:10:34 IST
Pair production refers to the creation of an elementary particle and its antiparticle, usually when a photon (or another neutral boson) interacts with a nucleus. For example an electron and its antiparticle, the positron, may be created. This is allowed, provided there is enough energy available to create the pair ? at least the total rest mass energy of the two particles ? and that the situation allows both energy and momentum to be conserved. Other pairs produced could be a muon and anti-muon or a tau and anti-tau. However all other conserved quantum numbers (angular momentum, electric charge, lepton number) of the produced particles must sum to zero ? thus the created particles shall have opposite values of each other. For instance, if one particle has electric charge of +1 the other must have electric charge of ?1, or if one particle has strangeness of +1 then another one must have strangeness of ?1. The probability of pair production in photon-matter interactions increases with increasing photon energy and also increases with atomic number approximately as Z2.