Break a magnet in two, no matter how small, and you'll get two magnets each with a south and a north pole of opposite magnetic nature. However, some theories predict particles with an isolated magnetic pole, which would carry a magnetic charge analogous to a positive or negative electric charge. But despite many searches, such magnetic monopoles have never been spotted at particle colliders. A new search by the ATLAS collaboration at CERN places some of the tightest bounds yet on the production rate of these hypothetical particles. These results are complementary to those from CERN's MoEDAL experiment, which is specifically designed to search for magnetic monopoles.
Originally proposed in 1931 by physicist Paul Dirac, magnetic monopoles have since been shown to be an outcome of so-called grand unified theories (GUTs) of particle physics, which connect fundamental forces at high energies into a single force. Such GUT monopoles typically have masses that are too high for them to be spotted at particle colliders, but some extensions of the Standard Model predict monopoles with masses that could be in a range accessible to colliders.
The latest ATLAS search is based on data from proton-proton collisions produced at the Large Hadron Collider at an energy of 13 TeV. The collaboration looked for signs in the data of large energy deposits that would be left behind by the magnetic monopoles in the ATLAS particle detector. The energy deposits would be proportional to their magnetic charge squared. Such large deposits are also an expected signature of high-electric-charge objects (HECOs), which may include mini black holes, so the search was also sensitive to HECOs.
The team found no sign of magnetic monopoles or HECOs in the data but improved previous work on several fronts. Firstly, the search achieves improved limits on the production rate of monopoles that carry one or two units of a fundamental magnetic charge called Dirac charge. The new limits surpass those from MoEDAL, although MoEDAL is sensitive to a larger range of magnetic charge - up to five Dirac charges - and can probe monopoles produced by two mechanisms, whereas ATLAS probed only one. MoEDAL researchers are also working towards pushing the experiment to probe monopoles with magnetic charges well beyond five Dirac charges.
In addition, the ATLAS search improves limits on the production of HECOs with electric charge between 20 and 60 times the charge of the electron. Finally, the search is the first to probe HECOs with charges greater than 60 times the electron charge, surpassing the charge probed by previous searches by ATLAS and also by the CMS collaboration.
