Question: How many other elementary particles are out there to be discovered? And how will we ever know if we have found them all?

Steve Marsden answered on 7 Mar 2016:

Really good question Josh. We don’t really know how many there are. There are theories that add just one or two to fix certain problems. Others suggest a whole zoo of new particles. One of these is very popular at the moment, and is called Super Symmetry, or Susy for short. This doubles the number of particles we know about, and leaves the others for us to find in the near future.

The way we discover these new particles is by producing them, and then watching them decay. I work at the Large Hadron Collider (LHC) where we small bits of atoms into each other at close to the speed of light. By putting a lot of kinetic energy in one place, we can turn the energy into new matter. The more energy you put in, the heavier the new matter can be. In the very rare instants this new matter is created, it instantly decays, turning into energy and particles we already know about. By looking at the stable particles that come out, we can piece together what happened at the collision, and if a new particle was created.

Importantly, if we don’t find a new particle, we can measure how much we didn’t see it. By doing this we can try and rule out the particle’s existence.