CERN's accelerator complex...
Each bunch contained some 250 billion particles. A bunch was just over a centimetre long and fractions of a millimetre high and wide. That gives you an idea of how incredibly tiny electrons are.
The bunches rushed around LEP at a shade under the speed of light. Their speed was so close to that of light that if a bunch of LEP electrons could be made to compete with a beam of light in a race to the moon, 384,500 kilometres away, the light would win by a margin of just five millimetres! At this speed, the particles in LEP made 11,200 laps per second, meaning that the bunches passed through each other 44,800 times per second in the four detectors. The fact that collisions between individual electrons and positrons only happened a few times per second is another indication of how small electrons are.
CERN's accelerator complex...
...is largely underground. In this picture, the white rings show where they are.
The particles used for collisions in LEP began their lives in the Linear Injector for LEP, LIL. They were then accumulated in a device called the Electron-Positron-Accumulator, EPA, until there were enough of them to form a bunch. Bunches were successively accelerated through smaller accelerators called the PS and SPS, both research machines in their own right, before being injected into LEP. There they received their final boost of energy before being brought into collision inside the four experiments. A circular particle accelerator like LEP is made mainly of magnets that steer the particles around the ring, it works like this:
Charged particles follow curved paths in a magnetic field...
...so powerful dipole magnets are used to keep particle bunches in their orbits.
Because particle energies are depend directly on accelerating voltages, physicists use a unit of energy based on voltage. An electron accelerated from standstill through a field of 1 Volt will have an energy of 1 electron Volt, eV. This corresponds to about 10-19 Joules, which would be a rather awkward unit to use in particle physics.
If an electron and a positron are accelerated in a field of 470 Megavolts, each will have an energy of 470 Mega electron Volts or 470 MeV. If they then collide head-on, the total energy of the collision will be 940 MeV.
This energy would be sufficient to produce a neutron according to Einstein's famous equation E = mc2, which tells us that matter and energy are convertible. For this reason, particle physicists also speak of particle masses in terms of electron Volts:
For simplicity, physicists often just quote masses in electron Volts, remembering that the c2 is always there. That's why you'll often find physicists using electron Volts as units of both energy and mass.
To generate the data on this CD-ROM, the electrons and positrons in LEP were accelerated to about 45 billion electron Volts, 45.625 GeV (Giga electron Volts) to be precise, resulting in collisions with an energy of 91.25 GeV. This energy corresponds exactly to what is needed to produce a Z particle.
It sounds like a lot of energy but in fact it is only about a tenth the kinetic energy of a flying mosquito. The difference is that it is concentrated into a tiny space, about ten thousand million million times smaller than a mosquito, and that's what gives rise to the phenomena physicists are interested in.
