F5 Creation and Annihilation of Particles
The picture below shows the emergence of an electron-positron pair from a photon of high energy, a so-called γ-quant. The photon does not leave a trail in the bubble chamber, because it has no charge. The electron and the positron, through use of a magnetic field perpendicular to the plane of the image, are diverted by the Lorentz force in opposite directions because of their different electric charge. The photon must have entered the picture from the left. We see all three conservation laws in action at once:
The momentum (and hence also the total energy, kinetic energy and velocity) of the electron and the positron can be determined from the radii at the beginning of the spiral motion, since the strength of the applied magnetic field is known. It is
mv • v2 / r = e • v • B thus p = e • r • B and Etot2 = E02 + p2 • c2
Problem 7 in Section F7 refers to this situation.
Pions, muons and other particles are continuously generated by the millions through impact of high energy cosmic radiation with the atoms of the Earth's atmosphere. Anti-protons are now manufactured in large numbers at CERN and also at CERN scientists have generated anti-hydrogen atoms from anti-protons and positrons.
When a positron meets an electron the two particles ‘decay’ into two photons. The conservation of momentum requires that at least two photons be produced: the total momentum of a single photon cannot be zero in any coordinate system, while the total momentum of two particles in their center-of-mass (barycentric) system is always zero! For the same reason, a photon cannot generate an electron-positron pair without another particle being involved. The event always takes place in the immediate vicinity of an atomic nucleus.