Re: Which particles are their own anti-particles?

There are lots of bosons which are their own antiparticles. In addition to the photon, which is a carrier of the electromagnetic force, and the graviton, which is a carrier of the gravitational force, there is a Z^0 particle which is a carrier of the weak force. Unlike the photon and the graviton, the Z^0 has a rest mass. Mesons which consist of quark-antiquark pairs, such as the pi^0 and the Lambda^0, are also their own antiparticles.

We are not sure if there are any fermions which are their own antiparticles. Since any particle which is its own antiparticle must be electrically neutral, the only candidates for such fermions are the three types of neutrinos. If neutrinos were their own antiparticles, then lepton number would not be conserved. There are some theories in which lepton number is not conserved. However, although many experiments have been done to look for this effect, no one has ever found good evidence for violation of lepton-number conservation. Some people believe that the numbers of leptons and antileptons were equal in the very early universe. In that case, processes which violate lepton number conservation would be needed to explain the current state of the universe, where the number of leptons appears to be greater than the number of antileptons.