Question: what is a hadron collider

Clara Nellist answered on 19 Mar 2014:

To expand a bit on what Thomas said about hadrons: We know that protons and neutrons, which are a type of hadron and make up the nucleus of atoms, can be split up further themselves. They are made of smaller particles, called quarks, and are held together by particles that carry the ‘strong’ force, called gluons (these glue the hadrons together). There are six types of quarks, but for these hadrons, we only need to know about up quarks (with an electric charge of +2/3) and down quarks (with charge of -1/3). These type of hadrons, called baryons, are made up of three quarks.

To make a proton, we take 2 up quarks and 1 down quark. To add up the charge, we get: +2/3 +2/3 -1/3 = 1. So the proton has an electric charge of + 3/3, or 1, as we expect! Similarly for the neutron we have 1 up quark and 2 down quarks, giving us: +2/3 – 1/3 – 1/3 = 0 which is a neutral particle (particle without electrical charge).

Great, so we know what the hadrons are made of! The other important thing to know is that protons are a lot heavier than electrons. This is important because the Large Hadron Collider is a circle, 27 km long. Before the LHC was there, we had another particle accelerator in the same tunnel that accelerated and collided electrons and positrons (anti-electrons) together. Because electrons are so light, when you make them change direction to go around the circle they lose energy. So, even though you’re giving them more energy to make them go faster, they lose a lot of this changing direction. This means that there is an upper limit of how fast we can make the electrons go and so the energy we get from their collisions is also limited.

So we want to accelerate heavier particles, in this case protons, because they don’t lose as much energy when they change direction, and we get a lot more energy from their collisions. And this is why it took so long to find the Higgs boson. We were waiting to be able to get higher energies from the particle collisions. Because the more energy you have, the heavier the new particles you can make from this energy. (Think of the energy as a form of money to buy new particles, the more money you have, the more choice of things to buy you have!)

But, going back to the protons being made of quarks and gluons. This is a slight problem, because it means we don’t know which quark took part in the collision. For the electron, we know exactly how fast they were going because they do not have any particles inside (as far as we know). But protons have three quarks and any one of them could have been involved in the collisions. This makes the results of the new particles we get out messier and we have to search through it all and try to work out what happened and at what energy. This is more work for us, but we’re happy to do it for the higher energy we get from colliding protons.