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COSMOS searches for magnetic monopoles

COSMOS searches for magnetic monopoles

Artist's impression of a monopole-antimonopole pair production event in the MoEDAL experiment (Copyright: Heikka Valja / MoEDAL)

Magnets always have two poles, north and south . . . or do they? The existence of magnetic monopoles, elementary particles with a single magnetic north or south pole, was postulated in 1931 by Paul Dirac who found that they are compatible with quantum mechanics and that their existence would explain the quantisation of electric charge. This July, Arttu Rajantie from the COSMOS consortium led an exhibit called Monopole Quest ( at the Royal Society Summer Science Exhibition, explaining the physics of magnetic monopoles and the MoEDAL experiment (Monopole and Exotics Detector at the LHC) at CERN, which is searching for them.

In spite of extensive searches, magnetic monopoles have not been found in experiments. If they exist, they should have been produced in the very early universe, and as stable particles they would have survived until today. In that case we would expect to find them in cosmic rays, and also indirectly through their astrophysical effects. This monopole problem was one of the original arguments for cosmological inflation. It should also be possible to produce monopole-antimonopole pairs in particle collider experiments. The new MoEDAL experiment ( at the Large Hadron Collider has been designed for this purpose. The experiment started operation in June 2015, and it will publish its first results soon.

To interpret the results of the MoEDAL experiment and derive constraints on theories, one needs reliable and accurate theoretical predictions. This is a major challenge because the Dirac quantisation condition shows that the magnetic charge of a monopole would be very strong, and therefore the usual perturbation theory techniques based on Feynman diagrams, which are used to describe the production of other types of particles, are not applicable.

Rajantie and his collaborators have been developing numerical lattice field theory simulation methods for calculating the properties and behaviour of magnetic monopoles in quantum field theory. By using Monte Carlo simulation algorithms together with topologically non-trivial “twisted” boundary conditions, they can go beyond the semiclassical approximation and calculate fully quantum mechanical observables. These simulations will be crucial for understanding the physics implications of the MoEDAL results.