The Monopole and Exotics Detector at the Large Hadron Collider (MoEDAL) does what it says on the tin. It searches for magnetic monopoles – hypothetical particles with either a “north” or a “south” magnetic charge instead of both – and other exotic theoretical particles. These searches have so far come up empty-handed, but they have delivered crucial information to help guide future searches. Now, in a first for an experiment at a particle collider, MoEDAL has searched for magnetic monopoles produced through a process called the Schwinger mechanism.
Nobel Prize winner Julian Schwinger showed that pairs of particles with electrical charge can be spontaneously created in a strong electric field. Similarly, pairs of magnetic monopoles could be spontaneously created in a strong magnetic field. Compared to other means of producing magnetic monopoles, this process, known as the Schwinger mechanism, has advantages, including that the monopoles should be created at a greater rate, thus increasing the chances of spotting them.
The MoEDAL team usually looks for magnetic monopoles by exposing the experiment’s “magnetic monopole trappers”, which consist of 800 kg of aluminium blocks, to proton–proton collisions produced at the Large Hadron Collider (LHC). To search for Schwinger magnetic monopoles, however, the team exposed the blocks to lead–lead collisions produced by the LHC in November 2018, just before the collider was shut down for maintenance.
Lead–lead collisions at the LHC generate extremely strong magnetic fields, and the November 2018 run generated a maximum magnetic field that was more than ten thousand times stronger than the strongest magnetic fields in the cosmos, which are found on the surfaces of fast-spinning neutron stars called magnetars, and ten million times stronger than the field strength required to create Schwinger monopoles. Therefore, these collisions could have produced such monopoles.
After exposing the blocks to the lead–lead collisions, the MoEDAL researchers used a device called a SQUID magnetometer to scan the blocks for any trapped magnetic charges belonging to Schwinger monopoles. The researchers found no signs of such monopoles in the blocks, but the lead–lead collision data allowed them to rule out the existence of Schwinger monopoles that have masses up to 75 GeV/c2, where c is the speed of light, for magnetic charges ranging from 1 to 3 base units of magnetic charge.
“A unique feature of the Schwinger monopoles is that they are not point-like, they have a finite size,” explains MoEDAL spokesperson James Pinfold. “Our mass bound is the first lower mass limit for finite-size monopoles from a collider search, and it’s tighter than previous similar mass bounds, such as that obtained from neutron-star data.”
The MoEDAL team will continue its searches during the next run of the LHC, which will start in 2022 and deliver more proton–proton and lead–lead collision data for analysis.