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HOW DO YOU SEE THE INVISIBLE?

Sometimes on a clear day you can see the sky criss-crossed with thin white bands. These are vapour trails made by aeroplanes high in the sky. Indeed, the aeroplanes may be so high that they are too small for you to see. Then the aeroplanes themselves seem invisible, but you can tell where they have been by following the vapour trails.

The basic building blocks of which we and everything in the world about us are made are extremely tiny. Even if you enlarged one of these tiny particles a million million times, it would still be smaller than a full stop. Like the high flying aeroplanes they are invisible, but just like the aeroplanes, in the right conditions we can see the trails they make. First we need to knock the particles out of the atoms where they normally hide. Once we have done this we can follow their trails, and the excitement begins!

Particle Detective Work

To study these basic particles you have to be like a detective. First you have to follow the trails left by the invisible particles, then you have to identify them and finally you have to put them all together to decide what happened "at the scene of the crime".

Tracking the particles ...

To make the particle trails visible, we need to send the particles through a DETECTOR containing a suitable substance. This can be a solid, a liquid or a gas. In one kind of detector, called a CLOUD CHAMBER, vapour trails form along the tracks left by particles, so that we see where they have been just as we see the aeroplanes high in the sky.

... and recognising the footprints

To learn about particles we need to be able to recognise the trails left by different types of particle. One trick we can play to help us here is to make the particles pass through a magnetic field. Particles with positive electric charge will bend one way, those with negative charge will bend the opposite way. But the particles help us as well, because some of them leave very distinctive trails that we can recognise them.

In a detector called a BUBBLE CHAMBER, particles leave trails of bubbles in a liquid. ELECTRONS, which are found in all atoms, produce delicate curling, branched tracks, unlike those of any other particle. PROTONS, also found in all atoms, leave thicker, unbranched tracks.

Did YOU know?

o The first scientists to track particles would sit in a pitch black room for an hour or so until they could actually SEE the tiny flashes of light made as individual particles struck a special material, called SCINTILLATOR.

NOW YOU SEE IT ...

Most modern particle detectors do not make the tracks of particles directly visible. Instead, they produce tiny electrical signals that can be recorded as computer data. A computer program then reconstructs the patterns of tracks recorded by the detector, and displays them on a screen together with information from other kinds of particle detector.

At the scene of the crime ...

Some of the most interesting particles exist for only a very short time before they transform, or "decay", into more common, longer-lived particles. The original short-lived particle often decays before it can leave any direct trace in a detector. The evidence for its existence then comes from a careful study of the longer-lived products. To study such elusive particles in the laboratory we must first make them, usually by colliding together more familiar particles (such as protons or electrons) at high energies.

This is a computer reconstruction of particles recorded in the detector called DELPHI at the European particle physics laboratory, near Geneva. Here we see two sprays, or "jets", of particles shooting out in opposite directions from the centre of the detector, where an energetic collision has occurred. The two jets of particles emerge from the decay of a particle, called simply by the letter Z. The Z is 90 times heavier than a proton; indeed, it is nearly as heavy as an atom of silver! This is why it can decay into so many lighter particles.

The Z lives for such a short time that we can never see it directly. Instead, we calculate its mass from the total mass - and energy - of all the products of its decay. This is how we know that it exists. Other interesting particles are not as short-lived as the elusive Z particle, but they still do not live long enough to enter the tracking detectors and produce trails for us to see. The detective work required to "see" these particles involves special detectors that help to pin-point tracks precisely, as close to the "scene of the crime" as possible.

The Oxford connection

DELPHI, built by a team from many countries, consists of several concentric layers of different types of detector surrounding a pipe in which high-energy particle beams collide. The innermost layer consists of three rings of detectors made from SILICON which record where particles pass through to within 5/1000 of a millimetre.

This information is vital in pinning down where tracks revealed further out in DELPHI originated. In some cases sprays of tracks clearly begin several millimetres from the collision point, but are hidden from direct view within the beam pipe. These sprays originate from particles such as the one known as the B-meson, which lives for only 1.6 picoseconds (a picosecond is a millionth of a millionth of a second).

For a first guide to DELPHI see Events in DELPHI

For further information contact: Dr Gerald Myatt, Denys Wilkinson Building, Keble Road, Oxford OX1 3RH.

Page design by Sam Vaughan, St Birinus School, Didcot.
Text copyright Oxford University, Department of Physics