Despite its success in explaining the fundamental components of matter and their interactions, the Standard Model of particle physics is acknowledged to be incomplete. Consequently, experiments worldwide, both terrestrial and extraterrestrial, are actively searching for signs of new phenomena that could lead to a more comprehensive theory beyond the Standard Model.

Magnetic monopoles are theoretical particles possessing a single magnetic pole, either north or south, making them magnetically charged. Their discovery would demonstrate a complete symmetry between electricity and magnetism and validate aspects of “grand unified theories” that aim to unify the strong, weak, and electromagnetic forces at extremely high energies. Researchers at the Large Hadron Collider (LHC) are investigating the potential creation of monopoles in high-energy collisions. If they exist, monopoles would be highly ionizing, stripping electrons from atoms and leaving substantial energy deposits in particle detectors.

At the recent International Conference on High Energy Physics (ICHEP) in Prague, the ATLAS collaboration unveiled its initial findings from searches for new physics using record collision energies from LHC Run 3. These efforts are focused on detecting magnetic monopoles produced in heavy-ion collisions and long-lived particles generated in proton-proton collisions.

In their latest search for magnetic monopoles, the ATLAS collaboration analyzed its first heavy-ion (lead-lead) collision data from LHC Run 3, gathered in the autumn of 2023 at an unprecedented energy level of 5.36 TeV per pair of nucleons (protons or neutrons). The researchers focused on ultraperipheral collisions, where ions do not collide directly but pass close enough to interact via the long-ranged electromagnetic force. These lead-ion collisions can generate the strongest magnetic fields in the Universe, up to 10^16 Tesla. If magnetic monopoles were produced in these interactions, they would appear as the only particles in an otherwise empty detector, manifested as a concentrated cloud of ionization electrons. By identifying unique signal features and analyzing potential background noise, ATLAS found no evidence of monopoles in their Run 3 heavy-ion data. As a result, this study establishes the world’s best limits on the production rate of monopoles in ultraperipheral heavy-ion collisions for monopole masses below 120 GeV. Furthermore, this analysis introduces a new methodology for studying highly ionizing particles in heavy-ion data from the LHC and other facilities.

Traditionally, searches for new physics focus on particles that decay “promptly,” producing decay products directly from the LHC’s proton-proton interaction points. However, beyond-the-Standard-Model theories, such as supersymmetry, also predict “long-lived particles” that decay away from the interaction point. Detecting these particles requires specialized techniques to reconstruct particle tracks, as they may have evaded detection in previous searches. ATLAS has now released results from a new search for long-lived particles that decay into an electron, muon, or tau lepton, resulting in two displaced particle tracks from the ATLAS interaction point. This rare signature could be a sign of new physics. Specifically, ATLAS sought a scenario where one of the long-lived particles travels far before decaying, so that only a single electron is detected.

This search, using 13.6 TeV proton-proton collision data from LHC’s Run 3, marks the first of its kind for ATLAS. In preparation for Run 3, ATLAS researchers improved the online collision-event selection (“trigger”) to include the reconstruction of displaced tracks, enabling this search for new long-lived particles. The event yields in all search regions aligned with Standard Model expectations. These findings establish the strictest limits yet on the long-lived supersymmetric partners of electrons, muons, and tau leptons.

With more data from the LHC and its future upgrade, the High-Luminosity LHC, ATLAS physicists will continue their pursuit of long-lived particles, magnetic monopoles, and other hypothetical particles. They will also refine their search techniques and develop new experimental strategies. Moreover, other experiments worldwide, such as those at Fermilab and various cosmic ray observatories, are contributing to the search for magnetic monopoles and long-lived particles. These collaborative efforts across different facilities and detector technologies enhance the overall sensitivity and coverage of new physics searches, bringing us closer to potential breakthroughs in our understanding of the universe.