A Direct Search for Stable Magnetic Monopoles Produced in Positron-Proton Collisions at HERA

Magnetism has always in the past been observed to be produced by electric currents. Isolated magnetic poles, although a useful aid to understanding, have never been observed. A consequence of the production of magnetism by electric currents is that if we think of a system in terms of "magnetic poles" such poles always appear in pairs of opposite charge. For example the needle of a compass can be thought of as a pair of oppositely charged magnetic poles residing at the ends. One pole is attracted to the magnetic North pole of the Earth and one attracted to the Earth's South magnetic pole. These are not magnetic poles at all but they are generated by the electric currents in the atoms of the material so that a pair of poles appears. Similarly the Earth's magnetism is thought to be generated by electric currents in its core so that a pair of poles (the North and South) appears.

In the 1930s Paul Dirac showed that he could relate quantum mechanics and the observation of the unique unit of electric charge on the electron and proton providing that an isolated magnetic pole could exist on its own i.e. a magnetic monopole. Since then there have been many searches for isolated magnetic monopoles and none have been found. Despite the failure to find them physicists continue to believe that they exist and many theories put forward to explain various aspects of cosmology (e.g. string theory) predict their existence as well. One such theory to explain the binding of quarks in the proton uses the presence of a soup of magnetic monopoles, a so called monopole condensate. If this is the case monopoles should be ejected from the proton by electrons in the collisions such as those which occur at HERA. There have been no previous searches in such collisions and this is the reason we undertook this particular search.

If monopoles are produced in the very energetic electron-proton collisions at the HERA collider at DESY they should fly away from the collision region and come to rest in the material surrounding this region. In 1997 the aluminium vacuum pipe, which surrounded the interaction region, was upgraded to a lighter and sturdier structure made of carbon fibres. We took the old aluminium pipe and cut it into pieces each of which was examined by a sensitive magnetometer to see if we could detect the characteristic magnetic field produced by any monopoles lodged in it. The magnetometer was sensitive to monopoles of strength more than about 1/10 of that predicted by Dirac's original theory. Unfortunately no monopoles were found. This shows that if they exist they must have a higher mass than that which can be created by the energy available in a HERA collision.