Particle Accelerators and how they Work

Particle colliders or accelerators are used to study the fundamental constituents of matter; the quarks, leptons and their interactions. The basic idea of a collider is to accelerate particles into two beams and collide them together with a certain amount of energy. This energy is then used to create new particles from the vacuum. Therefore if you pump more energy into each collision you can create more massive, elusive and exciting particles. The particles produced are generally unstable so they decay quickly and the Scientists at the collider study the spray of particles produced in each decay. Using information about the decay products one can reconstruct the initial collision and any information about the particles produced.

Leading the way in collider technology are Hadron colliders such as the Large Hadron Collider at CERN. The advantage of Hadron colliders is that Hadrons are much more massive than Leptons and as such do not suffer so much from energy loss due to radiation when being accelerated. This radiation, known as Synchrotron radiation, arises when a charged particle is being constantly accelerated causing it to emit Electromagnetic radiation. This is a common problem in circular accelerators such as Cyclotrons and Synchrotrons and is proportional to the inverse of the mass of particle being accelerated. This is not such a problem for straight line accelerators such as Linacs but the collision energies attainable in circular colliders are highly advantageous.

The two beams of protons smashed together at Hadron colliders propagate in opposite directions along what is commonly referred to as the beam-pipe. If a particle has any motion perpendicular to the beam pipe it is said to have transverse momentum. The component of motion along the beam-pipe is known as longitudinal momentum. By these definitions any momentum vector can be decomposed into these two constituents and we can describe collisions in both the transverse plane and longitudinal plane. In Hadron colliders the Hadrons are moving at a known velocity but their constituent Quarks and Gluons are not. We thus do not know the full momentum with which the Quarks and Gluons and colliding. However we do know that there should be negligible transverse momentum and so we can perform a physical analysis in the transverse plane.

Once a collision has occurred we analyse the decay products. Collisions are positioned such that they occur in detectors which are wrapped around the beam-pipe to study emerging particles in every transverse direction. A universal detector will be divided up into various components that are stacked so that any particle will propagate through each layer sequentially. The innermost layer is the tracking chamber which is usually embedded within a magnet system causing charged particle tracks to bend (positive and negative in opposite directions). The curvature of the track is also related to the momentum. Sophisticated tracking devices known as vertex detectors are placed close to the beam pipe to detect tracks of short lived particles like the Bottom or Charm Quarks. The next few layers are the calorimeters which measure the variety and energy of particles progressing through the detector. The Electromagnetic and Hadronic calorimeters will be composed of different materials depending upon the variety of particle they are stopping. The Muon chamber is the outermost layer because Muons are highly penetrative.

Every particle leaves a characteristic signature in the detector from which we can determine its mass, momentum etc all except for weakly interacting particles like the Neutrino. The Neutrino travels through the detector without detection but when we balance the transverse momentum of the other particles we can infer it’s existence through conservation of momentum. Clever ,mathematics and physical analysis can then reconstruct information about the initial resonance and collision.