Strange case of the unbreakable proton

Physicists in Germany are puzzling over particles that can survive very violent collisions. Brian Foster reports
The leafy outer suburbs of Hamburg, Germany, are home to one of the world's largest electron "microscopes". The device, known as Hera, is actually a large subatomic racetrack around which researchers fire elementary particles in order to probe the fundamental constituents of the material universe.

Over the past 50 years, researchers have collected an increasingly exotic menagerie of subnuclear particles, whose sometimes whimsical names reflect their esoteric nature: pions, muons, quarks, and the gluons that bind (or "glue") the quarks together. But there is now a lot of excitement at Hera, where they believe they have detected a particle so exotic that even the physicists d not think it can really exist. Its function is to do nothing - well, almost.

Hera collides electrons, the negatively charged particles that carry electricity along cables to our homes, with highly energetic protons, the positively charged particle that is the nucleus of the hydrogen atom. By observing the debris left after very energetic collisions between the electrons and protons, experimenters can deduce the properties of the constituents of the proton itself. The belief is that these constituents are yet smaller particles, known as quarks, which are never seen by themselves.

This is the essence of the physicists' dilemma. They expect that, when the electron slams into a proton at Hera, the violence of the collision should break the proton up into its constituent parts - as you might expect, for example, if you were to fire a high-velocity bullet into an alarm clock.

Most collisions at Hera do indeed result in the break-up of the proton - but not always, it seems. Last year at Hera, one of the two major experiments, known as Zeus, discovered that sometimes the proton emerged intact from the debris of a very violent collision. This was particularly surprising since the debris identified by the Zeus detector seemed broadly similar to that seen in the normal interactions. It seemed as if, although having demonstrably been involved in a violent collision, the proton had shrugged it off and carried on its way with the very minimum of deviation. This seeming paradox has reopened one of the oldest puzzles in particle physics.

When high-energy particles interact with each other, they do so by "exchanging" other particles. That, at least, is the basis of our current understanding, expressed in the so-called Standard Model. This has been phenomenally successful in explaining the development of the universe since the Big Bang on the basis of a small number of point-like "elementary" particles, the quarks and leptons (such as the electron and its near relations). Four forces - gravity, electromagnetism, the strong and the weak nuclear forces - govern the behaviour and interactions of these particles. When two particles approach sufficiently closely they interact by exchanging the appropriate carrier. The emission of this carrier by one particle and its reception by the other changes the properties of both.

Most of these exchanged particles can be produced in the right conditions as real particles. Some, such as the photon, an example familiar in everyday life as light, will exist until they are reabsorbed by a charged particle. Others, such as the W bosons, which are the exchanged particles mediating the weak interaction, decay shortly after being formed. Nevertheless, they can be detected by looking at the properties of the particles into which they decay.

There is one exception to this pattern and one exchanged "particle" that seems to have an existence no longer than the brief moment in which it mediates an interaction. This particle may hold the key to the strange invulnerability of the proton in the Hera experiments. Known as the Pomeron, after the Russian physicist Lev Pomeranchuk, who first devised it as a theoretical possibility, its only effect is to transfer some energy and momentum between the interacting particles, leaving all the other properties, and the two original particles, unchanged. This is in marked contrast to all other strong interactions, which always involve changing some aspect of the nature, or "quantum numbers", of the interacting particles.

The Pomeron is an anachronism, a survivor of an earlier age of particle physics, predating the concept of quarks and leptons. But the picture that is beginning to emerge from the experiments at Hera is one in which the proton emits a Pomeron and is only slightly deviated, while it is the Pomeron that takes the full force of the violent collision with the electron.

Since the Hera machine was designed to explore the interior of the proton, it is also capable of exploring the Pomeron, which the proton has so surprisingly provided as a substitute target. Physicists are thus in a position to explore for the first time the characteristics of this hitherto elusive particle. The first indications from Zeus and its sister experiment, H1, are that like the proton, the Pomeron is composed of point-like particles that seem to be quarks and gluons. However, the Pomeron has a significantly greater gluon content than the proton. Indeed, it seems that most of its energy is carried in the form of internal gluons.

This is a remarkable discovery. Most particle physicists had assumed that the Pomeron was merely a mathematical abstraction, a convenient way of visualising a process resulting in two particles giving each other a gentle nudge. Instead, not only does it seem to participate in violent collisions, it also shows alarming signs of having all the internal structure of a bona-fide particle such as the proton.

As such, it opens up a whole new field of experimentation and speculation. It dominated discussion at the international gathering of particle physics held in Paris at the end of April. Theories and papers that have been gathering dust for 30 years are being brought out, dusted off and re-examined. A large body of physicists is still strongly of the opinion that the Pomeron has no real existence and that the Hera experiments are showing indications of other, perhaps even more exotic, effects.

Pomeranchuk would surely have been surprised by the controversy over his particle and astonished by the idea that the experiments at Hera were revealing its internal structure. At the moment, particle physicists are in the interesting position of exploring a particle that may well not exist.

The writer is a reader in physics at Bristol University.