| MadSci Network: Physics |
Hi Aditya,
When particles combine, what comes out is determined by the sum of various numbers associated with the particles. These numbers include familiar quantities such as charge. There are also somewhat less familiar quantities. One example is lepton number (electrons and neutrinos, for example, have lepton numbers of 1, other particles have lepton numbers of 0), and baryon number (protons and neutrons, for example, have baryon numbers of 1, while for electrons and neutrinos it is 0). Antiparticles have numbers which are the negative of the values for the corresponding particles, except for their mass and energy. For example, an antielectron (positron) has lepton number -1 and charge +1, while an antiproton has baryon number -1 and charge -1.
When particles combine, or interact to create new particles, we simply add the numbers, and make sure that they have the same totals on "each side" of the reaction. There would be no physical meaning in squaring the nubmers and then adding them. In fact, the reactions which would result would violate many conservation laws which we observe (and which led to the definition of the numbers which I discussed above).
When a particle and its antiparticle combine, all of the numbers, except mass and energy, add up to zero. The simplest particles which have this property are photons, and so photons are indeed the usual result of particle-antiparticle collisions, and carry away the mass-energy possessed by the particle-antiparticle pair going into the collision. It *is* possible to create additional particles, so long as the sum of the quantum numbers is zero, and the total mass+energy equals that of the initial particle-antiparticle pair. Because it takes a great deal of energy to create particles, we usually only see photons coming from particle- antiparticle collisions, except in colliders, where we give the pairs very high energies before allowing them to interact.
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