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The conservation of charge requires only that the number of electrons minus the number of positrons stays fixed. So an electron can be freely created or destroyed, but only if a positron is created at the same time. And electrons and positrons are not alone in this respect. If we turn up the temperature of our box even more, other kinds of particles will begin to be created—starting with pairs of muons and antimuons and working up to Higgs bosons and top quarks.
