The H1 Collaboration has observed anti-deuterons produced in the collisions of electrons and protons at HERA. An anti-deuteron is an anti-nucleus which consists of a bound state of an anti-proton and an anti-neutron. This is the anti-particle of the deuteron, a form of heavy hydrogen. If such an anti-nucleus meets a nucleus it can annihilate into pure energy. In the science fiction story StarTrek, the USS Enterprise had anti-matter engines to drive Captain Kirk and his crew to their adventures. Presumably, in this advanced society a way had been found to store the anti-matter and to allow it to annihilate to give the energy which drove the USS Enterprise.

At first sight, it is surprising to observe such objects being emitted from the very high energy fireball created when an electron and proton collide at HERA, since no less than six anti-quarks have to be produced from the vacuum, close enough to one another to form the bound state. Furthermore, the binding forces of the anti-deuteron are weak, so it is not obvious how such a state can be easily produced. It is a bit like a coal being spat out of a very hot furnace which is so cool that it can be handled immediately.

Nevertheless anti-deuterons have been observed by H1. They are recognised on the basis of the quantity of ionisation that they deposit as they pass through the Central Drift Chambers of the experiment. This ionisation loss depends on the mass of the particle and its velocity, so given measurements of the particle velocity and ionisation loss, the mass and hence the particle type can be determined. The measured rate of production of the anti-deuterons has been interpreted using a theoretical model (the coalescence model). In this model an anti-deuteron is formed from an anti-proton and an anti-neutron created in the collision providing that these are produced at nearly the same point in space and with a small relative velocity between them. In this case the nuclear force can bind them together to form an anti-deuteron. At the temperature of the fireball produced in HERA the nuclear force is weak relative to the forces which are capable of breaking the anti-deuteron apart. Hence our surprise when first observing these particles. However, the coalescence model offers and explanation for the mechanism which forms the anti-deuterons.

We have compared the measurements at HERA with similar measurements of anti-deuterons in high energy nucleus-nucleus collisions. The comparison clearly shows that anti-deuteron production is inhibited in such nuclear collisions relative to our observations at HERA. Within the coalescence model, this suggests that the anti-protons and anti-neutrons are formed relatively far apart in nucleus-nucleus collisions so that they are less likely to stick together to form an anti-deuteron. Hence the size of the fireball in nucleus-nucleus collisions must be much larger than that produced in an electron-proton collision at HERA. This result has interesting possible implications for heavy ion physics, where it is believed that at sufficiently large temperatures, i.e. in the initial stages after the collision, a quark-gluon plasma is formed. The production of hadrons such as anti-protons and anti-neutrons happens only later, after the fireball has expanded in size and cooled. So perhaps the larger size of the fireball at the point where the anti-deuterons are made in heavy ion collisions provides some evidence for the initial quark-gluon plasma phase.