The particles from early machines were smashed into fixed targets where they would collide with the atomic nuclei in the target causing nuclear reactions. As beam energies increased, particles and their antiparticles were created leading to many new discoveries.

In all events, momentum must be conserved. Before the collision, the only particle with significant momentum is the particle from the accelerator; although the atoms in the target have thermal motion, this is of the order of eV rather than MeV and can be ignored. After the collision this momentum must be carried away by some particles along the initial direction of the beam and so the amount of energy available to create new particles only increases as the square root of the beam energy.

In collider experiments, beams of equal but opposite momentum are brought together. The initial momentum is zero and so all the energy of both beams can, in principle, be converted into new particles.

Experimenters can make use of the fact that particles and their antiparticles are oppositely charged and so will orbit in different directions in the same magnetic field – both can be accelerated in the same tube, saving a lot of effort and money. The particles will annihilate on collision releasing their total energy, (i.e. both kinetic and rest energies) for particle creation.

All major recent accelerators - the SPS, LEP, Fermilab’s Tevatron and the LHC – have been synchrotron colliders but, although there is an energy advantage, the design introduces extra problems.

Firing a particle beam into a solid metal target or large tank of liquid ensures that almost every particle will collide with a nucleus but getting two particle beams to interact is much harder. Beams must be compact and quadrupole magnets are used to focus the particles into tight bunches but, the tighter the beam, the larger the repulsive Coulomb forces between the similarly-charged particles.

Colliders that use antiparticles must find ways of producing them in large numbers and then storing them while the beam is brought up to full energy and they then collide. Unless the beams are very tightly focused there will only be a few collisions per revolution so the antimatter may have to be stored for hours.

Leptons (electrons, positrons, muons, taus and neutrinos) are fundamental particles – we have no evidence that they are made up of anything smaller. Hadrons (protons, pions, neutrons, kaons etc.) are not fundamental – they are made up of quarks and antiquarks and the gluons that hold them together.

In a lepton beam of known energy, each particle has this energy and so precision measurements of interactions in a detector are possible, balancing the energy before the event with the observed energy afterwards.

In a hadron beam, however, each hadron’s energy is shared out between its constituent particles in a constantly changing way. At high energies, interactions are between the quarks and gluons rather than between the hadrons as a whole, and so the initial energy of the two colliding particles cannot be known very accurately.

The two types of machine compliment each other: hadron colliders are useful for discovering new physics or searching for new particles as they explore a wide range of collision energies with one beam energy. Lepton machines can be used for precision measurements of particles after their discovery.

For example, the W and Z particles were discovered in CERN’s SPS synchrotron by colliding protons and antiprotons. The LEP collider was then built to measure the Z mass to very high precision by colliding electrons and positrons at precisely the rest energy of the Z.

Because the electron is so light, cyclic machines suffer such large synchrotron losses that they cannot be realistically constructed much larger than the LEP, as the power loss increases with g much faster than it decreases with increased orbit radius.

Future lepton machines will probably be linear colliders such as CERN’s proposed CLIC accelerator, although the possibility of colliding muons with antimuons in a large circular collider is an interesting possibility. Muons have a mass about 200 times larger than the electron but their lifetime in a rest frame is only 2 ms. When accelerated, this laboratory lifetime increases with g, but it will be a very different experiment from current machines as long-term beam storage will not be possible.

In less than 100 years, accelerator designers have raised the energy available to experimenters from Rutherford’s 10 MeV alpha particles to the LHC’s 7 TeV proton beams, a factor of nearly a million. The known particles have increased from Rutherford’s proton, electron, photon and suspected neutron to the huge number possible in the standard model.

The LHC should complete the standard model by discovering the Higgs particle and possibly show the way forward, detecting some of the lighter particles predicted by supersymmetric theories but this is not the end – plans are already being made for the Very Large Hadron Collider (VLHC) which will have an orbit of 240 km and a beam energy of at least 50 TeV.

Introduction        Direct voltage and cascade machines           Cyclotrons

Betatrons                    Linear accelerators and the synchrocyclotron                Synchrotrons       

    Fixed target verses collider machines        Lepton verses Hadron machines        The Future?