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If an electron and a positron meet, they annihilate. Assuming that they are not moving, all their energy just comes from their mass E=mc^2. The mass of the electron (and the positron) can be written as 0.5 MeV/c^2. The units are such that when multiplied by speed of light squared we get the mass energy of the electron to be E=0.5 MeV.

The energy released in one annihilation of a (non-moving) electron-positron pair will then be 1 MeV. I remind you that eV is just a unit of energy.

1 MeV is 1 Mega electron volt which is 10^6 eV or 1.6 \times 10^{-13} J

which may not be a lot in day-to-day life, but it is a fair amount of energy to give to a single particle.

The goal of an accelerator of particles is to move particles to higher and higher energy (by increasing their momentum). As we saw, the relativistic energy of a particle is given by the relation.

E^2 = m^2c^4 + p^2c^2

By increasing the momentum p, the energy increases. When accelerated electrons and positrons annihilate at high energy, we can explore new regimes of energy and produce new particles beyond the photon!

Large Hadron Collider

Over the years, we have built many different particle accelerators. The latest, biggest, baddest and strongest machine is the Large Hadron Collider (the LHC) where they collide protons with protons.

This is a 27 km long underground ring in which we accelerate protons (in both directions) until they go at nearly the speed of light (not quite, but some 99.999999% c). Their momentum grows to huge values and in fact the energy due to their motion is much more important than their mass energy (the p^2 term in the formula for energy dominates). (remember how we said before that when a massive particle approaches c, its energy grows towards infinity) The LHC accelerates protons until they have roughly the energy of the order of

1 TeV or 10^{12} eV. This is a million times the rest mass energy of the electron.

Once in a while, the protons are allowed to collide with the protons going the opposite way in the beam. The collisions occur at four different sites where we have big (really big) experiments and detectors such as CMS and Atlas (see Fig "Particle Colliders").

At these locations where the protons are allowed to collide, the result is a shower of particles shooting in all directions (see Fig "Particle Production at LHC"). Among the mess, physicists are trying to find new clues about the underlying structure of the Universe.