Atomically challenged

It's big news for small matter. Electrons behaving flirtatiously in Germany may prompt scientists to re-write the laws of physics.

Crowds gather in the suburbs of northern Hamburg to watch one of the most famous German football teams, Hamburg SV. The ball moves back and forth and the crowd yells. Meanwhile, electrons whistle silently beneath their feet at close to the speed of light in experiments to observe the smallest structure of matter.

The electrons are created at the Deutsches Elektronen Synchrotron (Desy), Germany's national particle physics laboratory. Founded in 1959, today its principal machine is Hera (Hadron Elektron Ring Anlage) - a proton positron collider - and it is Europe's second international particle physics lab.

In recent weeks, the physicists who work on these experiments have become as excited as the football fans. They have uncovered data that could revolutionise their understanding of the laws that govern the universe.

Physicists understand the laws of physics in terms of the so-called Standard Model, which describes matter in terms of four fundamental forces (strong nuclear, weak nuclear, electromagnetic and gravity) acting on simple point- like particles known as quarks and leptons. This model, formulated in the 1970s, has withstood every subsequent challenge - although physicists know it is incomplete in many ways. It contains many arbitrary constants, and it does not incorporate the interaction of particles and the force of gravity.

The particles of matter in the universe, and their anti-particles, can be divided into quarks and leptons. The difference between them is their behaviour with respect to the most powerful of the four forces, the strong force, which binds the protons and neutrons inside the atomic nucleus, and confines the quarks themselves inside these protons and neutrons. While quarks are dominated by the strong force, leptons are totally immune to it. Their behaviour is controlled by the electromagnetic force, the most familiar everyday force which is responsible for operating electrical devices, and the weak force, responsible for the burning of stars and the decay of radioactive elements.

This very different behaviour of quarks and leptons is crucial to the world as we know it. For example, the electron is the lightest electrically charged lepton. Because they are unaffected by the strong nuclear force, electrons remain outside the atomic nucleus rather than being absorbed into it. Electrons are the active participants in all atomic behaviour. If they felt the nuclear force, the entire universe would be a soup of nuclear material.

In the 100 years since its discovery, the electron has seemed simple and well-behaved. It does not appear to have any size, it is completely stable, and it shows no interest in forming any more complicated or excited states with quarks or other leptons. But now, in the centenary of its discovery, the first evidence is accumulating that the electron may, after all, be involved in intimate relations with quarks.

This surprising suggestion comes from the two major experiments at Hera known as H1 and Zeus. These experiments involve large international collaborations of physicists and engineers. The main purpose is to fire electrons at protons at very high energies. By observing the energy and angle of the scattered electron, as well as the remnants left when the proton breaks up under the impact, the properties of the constituents - such as the quarks inside the proton - can be reconstructed. Hera can be thought of as the world's largest electron microscope, looking at the sub-nuclear rather than the molecular level of the best conventional electron microscopes.

A feature of both types of microscopes is that the higher the collision energy (that is, the more direct the clash) between electron and target, the smaller the detail which can be resolved. However, such energetic collisions are very infrequent. Far more commonly, an electron strikes a glancing blow off the quarks. But data from the rarest events at the very highest resolutions from Zeus and H1 indicate anomalies that have caused the excitement.

Until now, all data from the two experiments could be explained within the framework of the Standard Model. But on 19 February, both teams announced the first evidence that the electron is more likely to scatter from the quarks at high energies than predicted.

There are many possible explanations for this unexpected behaviour. The most likely is still that most ubiquitous of laws in experimental science, which can be formulated as "If Nature can fool you, she will".

Because the events in question are very rare, the two experiments each gathered only a handful. The statistics of small numbers means that the events observed might simply be a chance upward fluctuation which, as more events are gathered, will average out to return to the Standard Model prediction. But the larger the size of the excess observed, the smaller the chance that it can be a fluctuation.

It's like the National Lottery - there is a small chance that the number 15 will come up in two successive draws, and a smaller one of appearing in three successive draws, and so on. If 15 kept on coming up in draw after draw, a normal scientist would either formulate a new law of probability, or offer the more likely hypothesis that someone was introducing extra balls marked with a number 15. At Hera, the scientists are increasingly suspicious that something strange is going on, but can't yet point conclusively to the culprit.

The fact that both experiments see a similar effect makes a statistical fluke less likely, in which case a variety of explanations suggest themselves. The quantity of evidence is insufficient to favour a particular possibility; however, most involve new physics which would open up a new world of phenomena.

One interpretation is that a new type of particle, known as a "leptoquark", is being formed. Such particles would be a hybrid of a lepton - in this case an electron - with a quark. More electrons are scattering with quarks at high energies than predicted because at these very small distances, the electron and quark find they like to embrace each other and for a tiny instant form this new type of matter, the leptoquark, before falling apart again into leptons and quarks.

Conventional wisdom suggested that leptoquarks would not be the first new particle to be discovered. Therefore, if the new data stands the test of time, physics will need a major rethink: the effects would stretch back to our understanding of the Big Bang and the first fractions of a second of the life of the universe.

The writer is professor of experimental physics at the University of Bristol

For details on the Hera experiments, see http://www-h1.desy.de/h1/www/html/hiq2.html

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