Science: Inside the mysteries of the light fantastic: Scientists in Germany are using a giant particle accelerator to glimpse the secret world of the photon. Brian Foster reports on their first discoveries
Monday 07 June 1993
At the Desy laboratory, the scientists' pride and joy is a machine called Hera, which accelerates subatomic particles to very high energies. The first data garnered from Hera has shown that light, when looked at under this most powerful of 'microscopes', is not as simple as it appears.
Hera accelerates two kinds of subatomic particles: protons, the positively charged particles that form the nucleus of the hydrogen atom, the simplest of all the chemical elements; and electrons, the negatively charged particles that carry electricity along the cables into our homes. The electrons circulate inside a huge vacuum chamber 6.3km (3.9 miles) in circumference and the protons circulate in the opposite direction, before they are brought together to collide head-on.
In this giant electron microscope, the violence of the collisions between the electrons and protons means that physicists can probe the interior of the proton with 10 times the precision previously attainable. Although the main purpose of the experiments is to examine the proton's innermost structure, Hera is also starting to uncover the mysterious properties of light.
Light is familiar to everyone. Yet, to physicists brought up on quantum mechanics, any beam of light is actually a stream of elementary particles, called photons. Via the wonders of quantum mechanics, the myriads of individual particles - photons - behave in many situations as if they were waves of light. The photon is a sort of intermediary particle that 'carries' the electromagnetic force, familiar from the electric current in a light bulb or the force between a piece of iron and a magnet.
The Hera accelerator produces a very intense source of photons. Physicists believe that, in the violent collisions between the electrons and protons, the electron emits a photon which a 'quark', one of the proton's inner constituents, then absorbs. The photons in the most violent collisions appear to be very simple, point-like particles, conveying the electromagnetic force between the electron and quark. These very violent collisions happen relatively infrequently, and therefore only a handful have been seen in the experiments. It is the properties of photons involved in less violent but more probable collisions that have proved so interesting.
In the less violent collisions, photons no longer behave as if they were simple particles with zero size. Instead, they seem to behave as if they were a very complex, spatially extended, jumble of quarks, antiquarks, electrons and other particles.
For a photon to turn itself into a quark-antiquark pair violates a fundamental principle of physics: that energy is always conserved. But in quantum mechanics, Heisenberg's Uncertainty Principle allows the photon to do just that, provided it does so for an extremely short time. Heisenberg's principle puts a limit on how precisely we can measure energy and time, so there can be a large 'uncertainty' in the photon's energy (as it transforms itself into a quark-antiquark pair) but only if its duration keeps within the limit set by Heisenberg's principle.
Thus, if we can examine a photon over such short times, it may appear not as a simple intermediary of the electromagnetic force, but as subject to other forces including the 'strong' and 'weak' nuclear forces - responsible respectively for holding the nuclei of atoms together and for radioactive decay. The once simple photon now appears as a complicated object. This complexity implies a richness of possible phenomena and a unique chance to examine the interplay of fundamental forces.
In previous experiments carried out at lower energy on other machines, the photon often interacted with protons as if it felt the strong force. But the evidence for this could be inferred only indirectly. The maximum energy of photons in previous experiments was about 20 times lower than can be achieved at Hera. Now the two major experiments at Hera, known as H1 and Zeus (each involving complex arrays of electronic detectors weighing several thousand tons and operated by several hundred physicists), have been able to utilise the much higher photon energy to observe unambiguously the transitory quark constituents of the photon.
Quarks have never been isolated in experiments. The strong force acts rather like an elastic band tying two quarks together. If a photon strikes a quark in a weak collision, the elastic band merely stretches and restores, returning the quark to its neighbours and causing the parent particle to deviate slightly. More energetic collisions cause the elastic band to break. There the analogy with elastic bands fails, because a new quark-antiquark pair appears at the break, and then the shorter elastic band is stretched further as the original quark continues to recede.
Further breaks occur, until the final result of the original collision is not an isolated quark but a number of particles of similar type to the proton, each containing quarks and antiquarks. At low energies, the pieces of the broken elastic band originating from the photon cannot travel very far and get inextricably jumbled up with similar particles produced by the quarks within the proton. At the high energies of Hera, however, the struck quark travels a large distance, producing a swarm of particles known as a 'jet'. The jet follows the original direction of the struck quark quite closely. By measuring the energy of the jets observed as the particles travel through electronic detectors, the experimenters can reconstruct the properties of the original quarks inside the photon.
Having firmly established the existence of quarks inside the photon, scientists at Hera have gone on to make detailed measurements of the energy and properties of these fundamental constituents of our material world.
The observations reported here were made with data collected in the first few weeks of Hera's operation, and with intensities of protons and electrons several hundred times smaller than will eventually be achieved. A great increase in the quantity and quality of data is therefore expected over the coming months. This should allow a fascinating glimpse inside the mysterious world of the photon, and how light flashes into matter and then back into light again.
FORCES TO BE RECKONED WITH
The photon is important in particle physics because of its central role in the 'Standard Model'. This seeks to explain the behaviour and evolution of the universe in terms of four forces acting on 12 'elementary' particles. (Each particle has its antimatter equivalent, known as an antiparticle, with equal mass but otherwise opposite properties.)
The four forces are electromagnetism, gravity, the weak force and the strong force. The electromagnetic force and gravity are familiar in everyday life. The strong and weak forces are unfamiliar since they govern the behaviour of matter inside the unimaginably small atomic nucleus.
Six of the 12 elementary particles, the ones known as quarks, feel the effects of all four forces, while the other six, the leptons, do not feel the strong force. The electron, for example, is a lepton.
All the forces act on the elementary particles via intermediary particles. The photon is the intermediary of the electromagnetic force, and thus conveys this force between electrically charged quarks and leptons.
Dr Brian Foster is a reader in physics at Bristol University.
(Photograph and graphic omitted)
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