I generally understand that every particle has an anti-particle that is created along with that type of particle with opposite effects (unless there is no charge), and that if that anti-particle comes in contact with a particle of the same type, they both will annihilate. Is there any instance or case that makes this trend not hold true, or is it probable that because of this trend, there will always be an even number of particles in the universe. Perhaps in the case you do not include photons this may hold true (unless a photon is always created in pairs).
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3 Answers
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The creation of particles has to respect a number of conservation laws. For example charge has to be conserved, so it you create an electron with a charge of -1 you have to create a particle with a charge of +1 to balance it out. Likewise lepton number is conserved (in the Standard Model at least). An electron has a lepton number of +1, so if you create an electron you have to create a particle with a lepton number of -1 to balance it out.
However, where particle creation does not violate any conservation laws you can create that particle in any quantity, whether odd or even. For example photons can be created and destroyed in any numbers so there is no requirement for there to be an even number of photons. Likewise for Higgs bosons and Z bosons.
All these particles have integral spin. Generally speaking fermions, with spin $\tfrac{1}{2}$, are always created in pairs. However it's possible there is a class of fermion called a Majorana fermion that is its own antiparticle and could be created in odd numbers (if it exists).
answered Aug 15, 2015 at 5:15
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$\begingroup$ I appreciate the distribution of links immensely. $\endgroup$ Commented Aug 15, 2015 at 5:37
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If you count a $\pi^0$ as a particle, you can create and destroy them individually. This is similar for any particle that is its own antiparticle (like the photon, the $Z^0$ and the Higgs.) You can have an arbitrary number of these.
answered Aug 15, 2015 at 5:16
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There exist particles which are their own antiparticle, even at the elementary particle level. Photons, and gluons are their own antiparticle and thus no need to come in pairs.
