Particles, Antiparticles, and Electron-Positron Annihilation

Every known subatomic particle has a counterpart called the antiparticle. A charged particle and an antiparticle have the same mass, and opposite charge. If a particle is neutral—for example, the neutron—its antiparticle is still neutral. Then their difference is due to some other property, such as magnetic moment. Some particles, like the photon, are identical with their own antiparticles. An antiparticle cannot exist together with the corresponding particle when an antiparticle meets a particle, the two react and new particles appear. 

Consider the example of the electron and the antielectron, which is the positron. The electron and the positron are identical particles except for their charge, which is equal to e but negative and positive, respectively. The rest mass of either particle is equal to 0.511 MeV. A positron moving in a medium loses 

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