Blasting into inner space

Smash site The path of the underground accelerator and smaller booster ring at CERN, where violent collisions occur

A collision machine It takes machines the size of a small city to create the huge energies needed to study subatomic particles. The ring of LEP, which has been operating at CERN since 1989, sits in a tunnel 100 metres deep. Along it are thousands of magnets which bend and focus opposing beams of electrons and positrons. The beams - siphoned in from a series of smaller accelerators - are given a final boost by a radio-frequency electric field. At four places the beams are brought together into a fine point by superconducting magnets. The debris from the resulting collisions is analysed on a computer.

In this dossier Particle quest - from atoms to quarks The elementary building blocks of nature Giant particle smashers More than just another 'theory of everything'

For the first time,new super-machines are opening up the incredible worlds deep inside matter

How do scientists go about testing theories and discovering new particles? In the early years of particle physics scientists relied on natural processes that create subatomic particles - for example, radioactivity in which nuclei with unstable arrangements of protons and neutrons break down to release electrons, helium nuclei (alpha particles) and gamma radiation. Cosmic rays provided the other source of particles.
What scientists wanted were machines that could smash up the nuclei of ordinary matter to reveal what's inside. But it requires high energies to overcome the powerful nuclear "strong force" The obvious way to do this is by bombarding the target nucleus with fast-moving electrons and protons in the hope of knocking it open.
It's a bit like crashing a car into a wall and hoping to find out how the engine works. But physicists have made some remarkably accurate inferences by detecting particles that are kicked out by the collision and measuring which way they go.

UNITS OF ATOMIC ENERGIES Electron Volt (eV) - the energy one electron acquires when accelerating through one volt of potential difference MeV - mega (106) eV GeV - giga (109) eV TeV - tera (1012) eV Particle masses can be expressed as energy according to E=mc2, because m=E/c2 (m = mass; E= energy)

Charged particles are accelerated by passing them through an electric field. The first machines, in the 1930s, generated 700,000 volts to accelerate protons (hydrogen atoms without their electrons). When they hit the target material they caused some of the nuclei to break up.
A real breakthrough came with the cyclotron, invented by Ernest Lawrence in 1931.

Charged particles introduced into the machine were sent whirling round in a circle by the magnetic fields applied from above and below. An alternating electric field between a pair of D-shaped electrodes caused the particles to accelerate every time they crossed the gap. This first cyclotron reached the then-astonishing energy of 80,000 electron volts (80 keV) (see units box).

Today, the most powerful accelerators are synchrotrons, which consist of a circular vacuum-filled tube. Electromagnets situated along the ring at regular intervals bend the paths of the particles as they whizz down the tube, and keep them going in a constant circular orbit.

Electrons amd positrons annihilate each other in bursts of pure energy

As well as bending magnets (which are dipole), there are special magnets with four or six poles which focus the particles into a pencil-like beam. Also along the ring are devices that produce a radio frequency field, which accelerates the particles close to the speed of light.

Giant detectors in the accelerator To detect collisions in the giant LEP accelerator there are four giant instruments - Aleph, Delphi, L3 and Opal - placed at intervals along the ring. These record every nuance of every particle "event" that happens in their vicinity. Each detector has a slightly different configuration and measures slightly different things. This way they can verify each other's results. Aleph is typical: it is an onion-shaped structure through which the accelerator tube passes. Six layers measure the type, direction and energy of the particles produced in the collision. The inner three layers are composed of solid- state and MWPC detectors (see box, far right), which measure the tracks of charged particles. A superconducting magnet surrounding the layers bends the particle tracks according to their mass and electric charge. This allows their momentum to be deduced. Direct hit! A computer reconstruction (right) of particles emerging from a collision of an electron and a positron at the centre of the Aleph detector (above) Two outside layers stop particles to measure their energies; only neutrinos and muons escape. Muons are detected in wire chambers in the outer shell. But neutrinos are traced by the momentum left when the momentum of all the other particles has been added up. All the data is collected via electronic channels, then processed and analysed.

The reason synchrotrons are used is because they keep particles circulating and accelerating. There is one disadvantage: particles travelling in a magnetic field at near-light speeds lose energy by emitting it as radiation. The more sharply they bend, the faster they go, and the more energy they lose. So the more energy you want to achieve, the larger the accelerator ring must be.
The largest accelerator, called LEP (Large Electron Positron Collider), is at CERN in Geneva and is a whopping 27 kilometres in circumference. As one of a new generation of super-machines, it uses a lot of energy - a tenth of the electricity that Geneva uses. These accelerators collide beams of particles circling in opposite directions, so that they meet at twice the energy of each beam.
LEP smashes electrons and their antimatter counterparts, positrons, at energies of 100 GeV, which annihilate each other in a burst of pure energy. This crystallises into the Z, one of the recently discovered particles that carries the weak force. The Z decays rapidly into a shower of other particles. The particles, which are in bunches, then meet at four crossing points along the ring, where there are detectors to record the collisions.
LEP's job is to test the Standard Model to a very high precision. So far results have proved that there are only three generations of particles (some scientists thought there could be more), and confirmed many other predictions.

How to see 'invisible' particles You might wonder how you I can see something as small as a subatomic particle. Fortunately almost all particles interact with something to give a tiny signal, which can be amplified into something visible. For instance, a charged particle produces a tiny flash of light in some crystals. A device called a photomultiplier can convert the flash into an electrical signal, which can then be visualised electronically. Cloud chambers were the first detectors. They worked by producing a tiny vapour trail, similar to that caused when water vapour condenses on the exhaust of an aeroplane. Later, photographic emulsions were used to take a snap- shot of charged particles, which leave dark tracks on the plate. Emulsions were very useful for detecting cosmic rays and are still used for small experiments. The bubble chamber that CERN used in the 1970s and 1980s Bubble chamber These are the most famous kind of detectors. Inside the bubble chambers, particles produce beautiful swirling shapes. Liquid hydrogen in the chamber is allowed to expand suddenly when a particle passes through it, and a trail of bubbles is left in the particle's wake. Until recently the bubble chamber was the mainstay of particle physics experiments. But it is not suitable for accelerators that used colliding beams, as in these particle collisions occur in the accelerator, rather than an external chamber. In a drift chamber fine wires detect the tracks of charged electrons The multiwire chamber Colliders like LEP use a whole range of new detectors such as the drift chamber and the Multi- wire Proportional Chamber (MWPC). This clever device won George Charpak, who works at CERN, the Nobel physics prize last year. The MWPC consists of a gas-filled box with a plane of "sense" wires stretched inside it. When a particle passes through the gas it knocks electrons out of the gas molecules, which drift towards the positively charged wires. Each electron knocks out further electrons, producing an avalanche of electrons around the wires. This multiplies the electrical signal 100.000 times. The CCD chip used to detect particles is also found in camcorders Solid-state detectors The silicon chip has found its way into particle detectors. One of the most important devices is the charge-coupled device, or CCD. This is a chip covered in tiny pixels, squares that can store the charge produced when charged particles strike them. The resulting two-dimensional electronic "image" is then converted into a series of pulses of varying voltages, which in turn become a computer image.

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