In a paper published by the journal JHEP, the MoEDAL experiment at CERN narrows the window of where to search for a hypothetical particle, the magnetic monopole. Such particles were first predicted by physicist Paul Dirac in the 1930s, but have never been observed so far.

Just as electricity comes with two charges, positive and negative, so magnetism comes with two poles, North and South. The difference is that while it’s easy to isolate a positive or negative electric charge, nobody has ever seen a solitary magnetic charge, or monopole. If you take a bar magnet and cut it in half, you end up with two smaller bar magnets, each with a North and South pole. Yet theory suggests that magnetism could be a property of elementary particles. So just as electrons carry negative electric charge and protons carry positive charge, so magnetic monopoles could in theory carry a North or a South pole.

If monopoles exist, they are believed to be very massive. As the LHC produces collisions at unprecedented energy, physicists may be able to observe such particles if they are light enough to be in the LHC’s reach. For instance, high-energy photon–photon interactions could produce pairs of North and South monopoles. Monopoles could manifest their presence via their magnetic charge and through their very high ionizing power, estimated to be about 4700 times higher than that of the protons. The MoEDAL experiment at the LHC is designed specifically to look at these effects.

Although showing no evidence for trapped monopoles, the results have allowed the MoEDAL collaboration to place new mass limits, assuming a simple production mode of these hypothetical particles.