Nature, as described by the Standard Model (SM), has decreed that stable or pseudo-stable subnuclear charged particles can only have an observable electrical charge of ±1e, where e is the charge of an electron and that there is no magnetic charge. Thus, searching for anomalously electrically charged and magnetically charged particles (Magnetic Monopoles, Dyons, etc.) is an outstanding way to search for physics Beyond the SM (BSM).

However, the LHC’s general-purpose experiments, ATLAS and CMS, are not optimized to detect non-standard electrical or magnetic charges. The MoEDAL-MAPP experiment is designed to extend the discovery horizon of the LHC in this arena. The MoEDAL detector’s task is to search for Highly Ionizing Particles (HIPs), and MAPP’s (MoEDAL Apparatus for Penetrating Particles) purpose includes the quest for fractionally charged particles with charge as small as 10^-3e.

A prime example of a HIP is the magnetic monopole (MM) magnetic monopoles or a Dyon, a particle with electric and magnetic charge. Of course, HIPs can also be electrically charged objects that range from singly charged mass slow-moving particles to Highly Electrically Charged Objects (HECOs). Such particles can arise from interactions involving either protons or heavy ions. In proton-proton collisions, HIP pairs can originate from either a single virtual photon, via the Drell–Yan (DY) mechanism, or the fusion of two virtual photons, referred to as the photon-fusion. MoEDAL, was the first experiment to explore MM production from photon-fusion at the LHC (PRL 123 (2019) 2, 021802).

Additionally, MM pairs could emerge from the vacuum within the hyper-strong magnetic fields generated during ultraperipheral collisions of heavy ions via the Schwinger mechanism, which derives its name from Julian Schwinger, who won the Nobel Prize in 1965 for his contributions to Quantum Electrodynamics. In 1951, Schwinger predicted that electron-positron pairs would be produced ``out of the vacuum’’ in a strong electric field. If MMs exist, then electromagnetic duality requires that MMs would be produced by the Schwinger mechanism in a super-strong magnetic field. Such magnetic fields, amazingly as high as 1016 T, are fleetingly generated in the ultraperipheral collisions of heavy ions at the LHC. The vital importance of the MM production via the Schwinger Mechanism is that it is not exponentially suppressed due to the finite size of the MMs, as is the case with DY and photon-fusion production. Thus, if MMs are not point-like, this may have been the first-ever chance to observe them. Another positive benefit is that cross-sections for Schwinger MM production are valid. On the other hand, DY and photon fusion MM production cross-sections are non-perturbative due to the very large MM coupling. Schwinger also hypothesized the existence of the Dyon, a hypothetical particle bearing both electric and magnetic charges. MoEDAL-MAPP accomplished the pioneering task of conducting the inaugural direct search for Dyons in 2021 (PRL. 126 (2021) 7, 071801).

The vital importance of the MM production via the Schwinger Mechanism is that it is not exponentially suppressed due to the finite size of the MMs, as is the case with DY and photon-fusion production.  Thus, if MMs are not point-like, this may have been the first-ever chance to observe them. Another positive benefit is that cross-sections for Schwinger MM production are valid. On the other hand, DY and photon fusion MM production cross-sections are non-perturbative due to the very large MM coupling.

Schwinger also hypothesized the existence of the Dyon, a hypothetical particle bearing both electric and magnetic charges. MoEDAL-MAPP accomplished the pioneering task of conducting the inaugural direct search for Dyons in 2021 (PRL. 126 (2021) 7, 071801).

In the past few months, the MoEDAL-MAPP collaboration has published three results that delineate its search for anomalously charged avatars of new physics. The first result we consider involves the search for HIPs using the full MoEDAL detector. The MoEDAL detector consists of two main detector systems, the first of which is a Nuclear Track Detector (NTD) array of NTD stacks only sensitive to HIPs. This array can permanently register the tracks of MMs and Highly Electrically Charged Objects (HECOs) with no SM physics backgrounds. Chemical etching enhances and reveals such tracks, which are then registered using optical scanning microscopes at INFN Bologna and the University of Helsinki.

The second detector system is a trapping array designed to capture MMs. This is the first of its kind, made-to-purpose, apparatus to be deployed at a collider.  The MMT volumes are monitored offline using a SQUID Magnetometer situated at ETH Zurich. MoEDAL’s MMT detector system makes it the only collider experiment in the world that can definitively and directly identify the MM’s magnetic charge.

Unfortunately, this most recent search, using all the Run-2 data, failed to reveal the presence of HIPs in 6.46 fb^-1 of proton-proton collisions. However, this non-observation allowed us to place unprecedented bounds on MM production via the DY mechanism or photon-fusion, for magnetic charges from 1 to 10_gD, and for MM masses as high as about 3.9 TeV.  For HECOs, limits were established for electric charges from 5e to 350e and masses as high as 3.4 TeV. The corresponding ATLAS sensitivity for MMs reaches only to a magnetic charge of 2_gD for MMs and for HECOs from 20e to 100e. A comparison of MoEDAL and ATLAS limits on MM and HECO production is shown below.   

In the second study, the MoEDAL team continued their groundbreaking work published in Nature (Nature 602 (2022) 7895, 63-67) on searching for MM produced via the Schwinger mechanism in heavy-ion collisions during LHC’s Run-1.  

In this study, the  MoEDAL collaboration utilized a decommissioned interaction region beampipe donated by the CMS Collaboration, instead of its own MMT array detector (CERN Courier “CMS beam pipe to be mined for monopoles, Searches for New Physics| News, 8th March 2019). The $1M beryllium beam pipe was broken into small shards, which were then passed through a SQUID magnetometer at ETH Zurich to search for the tell-tale magnetic fields of trapped monopoles.  

Once again, the MoEDAL team found no evidence of MM monopoles trapped in the CMS beam pipe, allowing mass limits on Schwinger-production of MMs of 80 GeV for a charge between 2g_D and 45g_D. As we discussed above, for the first time in the extensive history of accelerator searches for MMs this limit also applies to non-pointlike MMs.

The third study (JHEP 04 (2024) 137) involves Phase-1 of the unfurling development of the MoEDAL-MAPP Experiment at LHC’s Run-3 and beyond. That is the MAPP-1 detector currently being installed in the UA83 tunnel some 100m from IP8. Its primary physics goal is to search for weakly ionizing messengers of BSM physics such as millicharged particles (mCPs). A cutaway picture of MAPP-1 is shown below. It consists of 400 plastic scintillator bars of size 10 cm x 10 cm x 75cm arranged in four collinear compartments each with 10 x 10 bars. Each bar is readout by a 3-inch PMT. So, a mCP traversing the detector would see 3m of scintillator triggered by a coincidence of 4 PMTs.

The study of the MAPP-1 detector includes a realistic assessment of detector efficiency and detector-generated backgrounds and presents an analysis of MAPP-1’s sensitivity to milli-charged particles (mCPs) arising from Drell–Yan pair production, direct decays of heavy quarkonia and light vector mesons, as well as single Dalitz decays of pseudoscalar mesons. It also includes estimated constraints on the milli-charged strongly interacting dark matter.  Our results indicate that MAPP-1 exhibits sensitivity to sizable regions of unconstrained parameter space and can probe effective charges as low as 6 × 10⁻⁴ e,  when deployed at the High Luminosity LHC.

The MoEDAL-MAPP collaboration has also presented plans to the LHC Committee to install an “Outrigger” detector for MAPP-1, designed to improve the acceptance for mCPs with mass above a few GeV/c2. This is an important region, as lighter mCPs are strongly constrained by measurements of the Cosmic Microwave background made by the Planck Surveyor Satellite (Astron. Astrophys. 641, A6 (2020)). If approved, the Outrigger Detector will be housed near MAPP-1 in a duct, connecting the UA83 tunnel with the beam tunnel, and would consist of eighty 30 cm x 60 cm x 5 cm overlapping scintillating plates arranged in four overlapping layers, each readout by a single PMT.

The three recent papers discussed above demonstrate that the MoEDAL-MAPP experiment has an unprecedented sensitivity to electrically charged particles ranging from 350e to 10^-3e, which is over five orders of magnitude! It also has a unique sensitivity to magnetic charge, extending from 1g_d to 10g_d and beyond. To date, MoEDAL-MAPP has already explored a vast swathe of previously unexplored “discovery phase space” in its search for anomalously charged and magnetically charged particles. Its future is now to also push that search to include fractional charge with MAPP-1, and, with a proposed future detector, MAPP-2, to expand the search to include very long-lived particle emissaries of BSM physics.