Large Hadron Collider experiment targets magnetic monopoles

The late physicist Joseph Polchinski once said that the existence of magnetic monopoles is “one of the safest bets one can make about physics that has not yet been seen.” In the search for these particles, which carry a magnetic charge and are predicted by several theories of the extended Standard Model, MoEDAL’s collaboration at the Large Hadron Collider (LHC) has yet to prove Polchinski right, but its latest discovery marks a momentous Progress forward.

The results are published in two papers published in the journal Science arXiv Preprint servers greatly narrow the search window for these hypothetical particles.

In the Large Hadron Collider, interactions between protons or heavy ions can create pairs of magnetic monopoles. In collisions between protons, they can be formed from a single virtual photon (Drell-Yan mechanism) or the fusion of two virtual photons (photon fusion mechanism). Through a process called the Schwinger mechanism, pairs of magnetic monopoles can also be created in a vacuum within the huge magnetic fields generated by heavy ion collisions.

Since it began collecting data in 2012, MoEDAL has achieved a number of firsts, including the first search for magnetic monopoles produced through the photon fusion mechanism and the Schwinger mechanism at the Large Hadron Collider.

In the first new study, MoEDAL collaborates to search for monopoles and highly charged objects (HECOs) produced through Drell-Yan and photon fusion mechanisms. The search is based on proton-proton collision data collected during the second run of the Large Hadron Collider, using the full MoEDAL detector for the first time.

The complete detector includes two main systems sensitive to magnetic monopoles, HECO and other highly ionizing hypothetical particles. The first one to permanently record the trajectories of magnetic monopoles and HECOs without background signals from Standard Model particles. The trajectories were measured using an optical scanning microscope from INFN Bologna.

The second system consists of approximately a large number of capture volumes designed to capture magnetic monopoles. These capture volumes – which make MoEDAL the only collider experiment in the world to be able to unambiguously and directly identify the magnetic monopole charges – are scanned at ETH Zurich using a special type of magnetometer called SQUID. Look for any trapped monopoles they may contain.

In its latest scan of the trap volume, the MoEDAL team found no magnetic monopoles or HECOs, but it set limits on the mass and productivity of these particles for different values ​​of particle spin, an intrinsic form of angular momentum.

For magnetic monopoles, the mass limit of magnetic charge is set to 1 to 10 times the Dirac charge (gD), the basic unit of magnetic charge, and excludes the existence of monopoles with masses up to about 3.9 TeV.

For HECO, the mass limit is established for charges from 5e to 350e, where e is the electron charge, and excludes the existence of HECO in the mass range up to 3.4 TeV.

“The scope of MoEDAL’s search for monopoles and HECOs enables the collaboration to survey large swathes of the theoretical ‘discovery space’ of these hypothetical particles,” said MoEDAL spokesman James Pinfold.

In a second new study, the MoEDAL team focused on looking for monopoles produced through the Schwinger mechanism in heavy ion collision data acquired during the first run of the Large Hadron Collider. In a unique effort, it scanned a decommissioned portion of the CMS experiment’s beam tube rather than the capture volume of the MoEDAL detector, looking for trapped monopoles.

Once again, the team found no monopoles, but it set the strongest mass limit yet for Schwinger monopoles with charges between 2gD and 45gD, ruling out the existence of monopoles with masses up to 80 GeV.

“The crucial thing about the Schwinger mechanism is that the production of composite monopoles is not suppressed compared to elementary monopoles, as is the case with Drell-Yan and photon fusion processes,” explains Pinfold. “So if monopoles are composite particles, this and our previous Schwinger monopole searches may be the first opportunities to observe them.”

The MoEDAL detector will soon be joined by the MoEDAL Penetrating Particle Apparatus, or MAPP, which will allow the experiment to cast a wider net in the search for new particles.