Collider Physics – The Art of Smashing Particles

Particle colliders are the favourite tools of particle physicists. While there are plenty of other ways to study the fundamental building blocks of the universe, nothing quite beats getting two beams moving at close to the speed of light, then letting them smash together. This is what is done at the Large Hadron Collider in Geneva, the biggest and most recent of a long line of colliders.

Why do we do this? The aim is to search for signs of new scientific phenomena in the resulting particle debris. Why it takes high power precision destruction to do this can be explained in various ways. One view of particle physics is that we are looking back in time, 14 billion years, to the universe just after the Big Bang. At this time, as all the energy in the universe was in a tiny volume, the average speed of the individual particles was very high. Colliders let us reproduce these primordial energies in controlled conditions where we can see what is going on.

Another view is that a particle collider is a microscope probing the smallest scales. We study the structure of an atom by firing particles at it and watching how they are deflected. A slow particle is nudged aside by the electric field of the electrons surrounding the nucleus. A higher energy particle can be fired right into the nucleus of protons and neutrons. By firing high-energy protons at each other, we can probe the quarks and gluons inside them, as well as other particles and the forces through which they interact.

The main gadget in a collider is the accelerator, the device that boosts the beam particles to near light-speed. Having accelerated your beam, you could direct it into a fixed target and watch as the beam particles wham into stationary atoms. However the approach taken in a collider experiment is to accelerate two beams in opposite directions, then steer them to a point where they strike each other head-on.

LHC_tunnel_2

The LHC tunnel. Photo: Brice, Maximilien: CERN.

Many students assume the huge superconducting magnetics seen in photographs of the Large Hadron Collider tunnel are what accelerates the protons, but this is a misconception. It is taught at school that the force a magnetic field exerts on a particle is always perpendicular to its direction of motion. Magnetic fields do not accelerate particles. They bend the path of the beam in a circle. Acceleration requires large electric fields, created in resonant Radiofrequency Cavities.

When you build a particle collider, there are many design choices. First, what shape do you want? You can shoot your particles in straight lines at a linear collider, such as the Stanford Linear Collider in California. Alternatively, in a circular collider, such as the Large Hadron Collider, particles are sent round and round, picking up speed with each revolution.

The first particle colliders were relatively small gadgets housed in a single room. These were sufficient to make and study lightweight particles, but it was realised that exploring further would need higher energies. This meant bigger and bigger machines. CERN’s first accelerator, the synchrocyclotron was built in 1957, the proton synchrotron in 1959, followed by the super proton synchrotron in 1974. This was followed by a longer-term plan leading to the Large Electron Positron (LEP) collider and then the Large Hadron Collider (LHC) built in a circular tunnel with a 27km circumference.

What of the future? The logic that future discoveries will need higher energies, and hence bigger machines, still applies. The necessary scale of a future project, and the civil engineering construction costs, mean any future project will be a global project. There is a proposal for an International Linear Collider and more speculative plans for a Future Circular Collider, although it is not yet clear if either of these will be built. A more exotic idea is a Muons Collider. Muons are a heavier sister of the electron, meaning a muon collider could achieve the clean collisions of electron-positron machine, with the bigger mass of proton machines.

There is no shortage of exciting projects for future particle experimentalists.

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