few billionths

The following article first appeared in Catalyst, a UK Science journal published for GCSE (up to 16 years old). We are grateful that this material is available to us.

A lot can happen in...

...a few billionths of a second.

Goronwy Tudor Jones (University of Birmingham). You will know that atoms consist of a cloud of electrons, surrounding a tiny nucleus made of nucleons (protons and neutrons). Here Goronwy Tudor Jones, a particle physicist, gives an insight into how the substructure of nucleons is studied.

Particle physicists study the basic building blocks of nature and the forces they exert upon each other. The key idea behind many of their experiments is very simple. If you want to know what is inside an object, shoot into it something you know and measure what comes out.

In particle physics we are interested in finding out what protons and neutrons (nucleons) are made of. We investigate this by shooting beams of known particles at nucleons from a machine called an accelerator. What comes out of this collision is measured in a detector.

An accelerator at the European Laboratory for Particle Physics (CERN) straddles the French/Swiss border near Geneva. The way it works is described on another page.

Recently there was a conference at CERN to celebrate the results obtained from a spectacular detector called the bubble chamber. This makes use of the fact that, when a charged particle travels through a specially prepared liquid, such as superheated liquid hydrogen, a trail of bubbles is formed along its path. The bubble trail can be photographed to give a permanent record of the particle's path - a bit like the vapour trail of an aeroplane.

Particle physicists measure and analyse photographs of what comes out of individual collisions between beam particles and nucleons.

Until very recently, the School of Physics and Astronomy at the University of Birmingham was involved, with several other institutions around the world, in analysing millions of bubble chamber photographs, taken at international accelerator laboratories like Fermilab in Chicago and CERN. In a barn-like darkened room containing a dozen 'scanning tables', pictures were inspected carefully by 'scanners' (trained non-physicists) for features considered especially important to particle physicists.

A scanning table at Birmingham University. A projector beside the table projects photographs from the bubble chamber up to a huge metal mirror above the table, which then reflects them down on to the table. The tracks are inspected and interesting tracks measured in detail on other tables.

Here we are looking at a spray of tracks coming from a collision between a neutrino (which itself leaves no tracks because it is neutral) and a nucleon. To get a feel for how we interpret such pictures, you should turn a printout upside down and look at it from a very low angle -- we often have our chins on the table! We would also have two or three other views of the same 'event' to help in the interpretation. The actual tracks in the bubble chamber would be two or three times longer than the projection on the table.

The charged particle trails curve because the bubble chamber is inside a strong magnet. Negative particles such as electrons curve in one direction as they travel, positive particles curve the other way. The more curved the track is, the slower the particle is moving (strictly speaking, the lower its momentum). A little later an exceptional example of a bubble chamber photograph is discussed in detail. It includes a very rare side-show - a head-on collision between an anti-electron (a positron, e+) and a 'stationary' electron (e-).

Nowadays, bubble chambers are being replaced by electronic detectors, such as those in the LEP detectors. Beams of anti-electrons and electrons are fired towards each other and the screens, and the information about collisions is recorded on computer rather than on film.

A REMARKABLE CHANCE

University of Birmingham. E632 Fermilab

This bubble chamber photograph shows the particle physics equivalent of a head-on collision between, say, a moving white snooker ball and a stationary red one. Ignoring complications (due to spin, for example), the white ball stops and transfers its momentum to the red ball, which moves off with the same speed. This can only happen if the balls have the same mass (imagine instead a white ball made of lead and a polystyrene red ball, and vice versa!)

The track beginning near A curves round through almost 180 degrees. At point B it begins to curve the other way, with the same curvature.

The bubble chamber is within a magnetic field. Knowing the direction of the magnetic field through the chamber, we can tell that the particle travelling from A to B is positively charged. The particle causing the track after point B must be negatively charged, because it curves the other way.

The original positive track is produced by a particle called a positron or antielectron, e+. At B the positron runs head-on into an electron, e-.

We can tell that the electron moves off with the same momentum as the positron, because the tracks just before and just after B have the same curvature.

This therefore tells us that the positron (the antiparticle) has the same mass as its corresponding ordinary particle, the electron.

We have used a picture of something that happened in a few billionths of a second to determine the mass of a particle of antimatter!

Further details of this event, and of another unusual event - the annihilation of a positron in flight - can be found in the following articles.

GTJ, 'A simple estimate of the mass of the positron', The Physics Teacher, 31, 95-101 (1993).


© CERN and High School Teachers Programme at CERN	Last modified: 28 June 2002