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Asked by anon-254590 to Joel on 14 May 2020.
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Joel Goldstein answered on 14 May 2020:
When a particle collides with another particle, several different things can happen as long as conservation laws allow it (energy, momentum, charge etc). Due to the laws of quantum mechanics, we can never predict which one will happen but only the probabilities of the different outcomes.
The possible outcomes include:
– the two particles “bounce off” each other, like two snooker balls would; no energy is lost but we see the paths of the particles are deflected
– if a particle (e.g. an atomic nucleus) contains smaller particles, it may be broken by the energy of the collision and we will see the fragments flying away (think of a cannon ball hitting a wall, with bits of brick flying everywhere)
– entirely new particles can be created from the energy of the collision.This last one is the weirdest, but Einstein’s relativity tells us that energy conservation includes mass as E=mc2, so kinetic energy of the colliding particles can be converted into mass for new ones. Sometimes the colliding particles are completely annihilated in the collision, sometimes they may survive.
As an example, I did the research for my PhD at the LEP collider at CERN, where a beam of electrons was made to collide with a beam of positrons (the antimatter partner of the electron). Most of the electrons and positrons would pass right by each other and keep going round and round the collider; a small fraction every time would be deflected out of the beam; and every once in a while an electron and positron would completely annihilate and all of their energy would be converted into the mass of a completely new particle, a Z boson.
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Comments
Susan commented on :
As Joel says, a number of outcomes are possible.
1. Collision is a glancing blow: the particles effectively bounce off each other. Result: same particles, but their energies and momenta are affected by the collision.
2. Collision causes one or both particles to break up. If the particle that breaks up is something like a nucleus (made of protons and neutrons, which can exist as free particles in their own right), then the outcome is that you get fragments of the original nucleus (protons, neutrons, smaller nuclei). Example: neutron collides with nucleus of U-235 and causes it to split into two smaller nuclei plus some free neutrons.
If the particle that breaks up consists of particles which cannot exist in their own right, e.g. a proton (which consists of quarks, and it seems to be impossible to have a free quark: we’ve looked for nearly 50 years and not found one), then the fragments will cause the creation of new particles, wth the aid of E = mc^2 (some of the energy of the initial collision will go into the masses of new particles). This is what happens in the LHC (which collides protons with protons).
3. If the particles that collide are a particle-antiparticle pair the initial particles may annihilate each other. At low energies, this typically results in the production of a pair of high-energy photons (gamma rays) each with energy = mc^2 where m is the mass of the particle that annihilated. For example, annihilation of an electron and positron at low energies produces two 511 keV emergy gamma rays (we see this in some kinds of radioactive decay). At higher energies, it can result in the production of one or more heavier particle-antiparticle pairs: for example, at LEP, we collided electrons and positrons to produce pairs of W bosons, even though the W boson’s mass is about 160000 times as much as the electron mass (E = mc^2 again).
4. In some cases, one of the particles can transform into a related particle with a different electric charge. For example, if a neutrino collides with a neutron, the result can be an electron plus a proton: the two incoming particles have exchanged electric charge (indeed, this process is sometimes called “charge exchange scattering”). This is important to neutrino physicists like me because we can easily detect the electron whereas we could not detect the original neutrino.