Early on in school, students learn that a magnet always has a north and a south pole. But nothing in classical electrodynamic theory or quantum mechanics says that magnetic monopoles can’t exist. They’re the hypothetical analogues to electric charges in Maxwell’s equations. In fact, their existence would make the equations more symmetrical: Electric terms could be transformed to magnetic ones and vice versa (see the article by Arttu Rajantie, Physics Today, October 2016, page 40).
The search for magnetic monopoles, however, has failed to find any so far. Most experiments have focused on elementary-particle collisions that could produce monopoles that are point-like particles. Theorists don’t expect those monopoles to suffer from an exponential suppression of their production cross section, unlike the composite monopoles predicted in various theories that consider physics beyond the standard model. But point-like and composite monopoles are expected to strongly couple to photons. That issue has previously prevented researchers from reliably calculating their production cross sections (see Physics Today, July 2006, page 16).
The collaboration known as MoEDAL—the Monopole and Exotics Detector at the Large Hadron Collider (LHC)—pursued a different strategy with heavy-ion collisions. In November 2018 a lead–lead collision experiment at the LHC succeeded in producing a magnetic field with a strength of 1016 T, the strongest ever observed in the universe. MoEDAL has now published its results, and although no magnetic monopoles were observed, the team did exclude the possibility of monopoles with masses smaller than 75 GeV, which is roughly 80 times as large as the mass of the proton.
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