Imagine a red car and a yellow car hitting each other head-on at high speed. Both cars are wrecked and debris is strewn across a wide area.

But what is this? The red car, despite all the evidence that it has just been written off, emerges from the collision intact and continues to go in a direction that is little different from its original path.

As bizarre as it may seem, scientists at Germany's subatomic particle colliding laboratory, called HERA, the Hadron Electron Ring Accelerator, believe they are witnessing something very like the imaginary car crash when electrons and protons are smashed into each other at fantastically high energies.

Brian Foster, reader in physics at Bristol University, is one of 65 British particle physicists taking part in an international experiment at HERA, called Zeus, that is looking into this curious behaviour. The scientists are based at University College London, Imperial College, Glasgow University and Rutherford Appleton Laboratory. Their work is backed by the Particle Physics and Astronomy Research Council.

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Dr Foster first explains that subatomic particles such as protons, neutrons and mesons have as fundamental constituents particles called quarks. In a proton there are three quarks. When an electron is smashed into it, as at HERA, the collision normally results in just one of the quarks being struck, causing it to recoil and become ejected from the proton's interior.

As the ejected quark tries to escape, it has to fight against attractive forces that try to keep it inside the proton. These attractive forces are carried by gluons, particles that bind or "glue" quarks together.

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Dr Foster says: "In effect what happens is that the ejected quark has stretched out behind it a string of gluons. You could think of it as a rubber band being continuously stretched." At some point, the gluon rubber band breaks and the quark escapes. In doing so a shower of less exotic particles is created. The shower can travel in many different directions from the path of travel taken by the original proton.

Because of the loss of a family member, the two quarks remaining in the interior of the proton are now unstable and also break up, creating another shower of particles which travels in the same direction as the original path of the proton. The two showers can be thought of as being analogous to the debris created by the two cars colliding.

By detecting and analysing the showers from these two events, particle physicists are able to glean valuable information about the interior of the proton and the nuclear forces that operate there.

But researchers at HERA have noticed something very odd about their collision experiments. Dr Foster says that while most conform to the general chain of events outlined above, one in ten indicate that the proton is "throwing out something" that becomes the target for the electron instead of the proton.

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In fact, just like the red car, the proton carries on almost as if nothing had happened, seemingly oblivious to the chaos caused by the collision behind. And, as with the collision between cars, there is all the evidence that a collision has really happened. There is a shower of particles with characteristics similar to those associated with normal electron-proton smash-ups. Weird. So what goes on?

The answers may lie in a theory developed by the Russian physicist Isaak Pomeranchuk 30 years ago to explain how energy and momentum could be transferred between sub-atomic particles. Dr Foster says: "We think that this mathematical abstraction may actually exist and that we may be seeing the result of the proton throwing out a particle - the pomeron - which collides with the electron instead of the proton." He points out that the similarity between the showers resulting from normal collision and those from the unexplained smash-ups suggests that the pomeron, if it exists, also has its own quarks.

He says that previously it had been thought that the pomeron, if it existed, would show itself only when the interaction between particles was very weak and not during the highly energetic events particles are forced to undergo in the HERA collider.

Dr Foster stresses that much more data needs to be collected at HERA before physicists can make a final judgement on the existence of the pomeron. If they gather irrefutable proof that it does exist, then scientists will want to know why protons throw them out sometimes during collision experiments. "If the pomeron is really there we will need to understand its structure in detail. We are now able to study particles inside the proton that have energies of 10 to the minus 5 of that of the proton as a whole. We need the same level of sensitivity in understanding the pomeron. It may of course be that something completely different is going on so we need to look at various other possibilities."

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He says that the answers to the mystery could throw light on how the strong nuclear force behaves, helping to solve puzzles that particle physicists have struggled with for decades. The strong nuclear force is the strongest of the four fundamental forces of nature, the others being gravity, electromagnetism and the weak nuclear force, and acts within the nucleus of an atom.