# Identification of particles and anti-particles

The identification of an electron as a particle and the positron as an antiparticle is a matter of convention. We see lots of electrons around us so they become the normal particle and the rare and unusual positrons become the antiparticle.

My question is, when you have made the choice of the electron and positron as particle and anti-particle does this automatically identify every other particle (every other fermion?) as normal or anti?

For example the proton is a particle, or rather the quarks inside are. By considering the interactions of an electron with a quark inside a proton can we find something, e.g. a conserved quantity, that naturally identifies that quark as a particle rather than an antiparticle? Or do we also just have to extend our convention so say that a proton is a particle rather than an antiparticle? To complete the family I guess the same question would apply to the neutrinos.

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Proposal : if a left-handed matter spin $\frac{1}{2}$ "entity" is part of the weak interaction, it is a particle, otherwise it is an anti-particle –  Trimok Oct 22 '13 at 17:02
I think we've seen a version of this question on the site before. Looking... –  dmckee Oct 22 '13 at 17:11
@dmckee: I've seen questions on this theme, but never a really clear answer. Trimok's comment is the closest I've seen to a definitive statement of the difference. –  John Rennie Oct 22 '13 at 17:13
@JohnRennie: re neutrinos, these would end up as particles via the lepton number; connecting to the quark sector is not as obvious, though there's (conjectired) stuff like B-L and leptoquarks –  Christoph Oct 22 '13 at 17:27
@Christoph: ah, yes, good point. –  John Rennie Oct 22 '13 at 18:22

In terms of QFT, the relevant (almost-)conserved quantity is the "charge parity," the eigenvalue of the combination of operators $\mathcal{CP}$.
I think so, since GUTs group the quarks and leptons together into unified multiplets, e.g. $(e,\nu_e,u,d)$. But I'm not personally familiar with those theories so I couldn't tell you the details. That probably would be a good topic for another question. –  David Z Oct 22 '13 at 20:00
@JohnRennie : In a $SU(5)$ model, it seems that particles and anti-particles may be mixed (if we look at the decomposition in $SU(3)*SU(2)*U(1)$), for instance the anti-symmetric representation $10$ decomposes as $10 \to (3,2,\frac{1}{6}) \oplus (3^*,1, -\frac{2}{3}) \oplus (1,1,1)$ (here all the particles/anti-particles have the same handedness). –  Trimok Oct 23 '13 at 9:35