ATLAS explores uncharted territory with LHC Run 3 data

The ATLAS Collaboration has announced the first results of its search for new physics phenomena using data from LHC Run 3.

Representation of a collision event recorded by the ATLAS detector at an energy of 13.6 TeV. The displaced electrons of the two candidates are shown with trajectories (blue lines) pointing to the energy deposits in the ATLAS calorimeter (green). The inset shows an axial view of the detector, showing the trajectories of the electrons displaced a few mm from the interaction point (red circle). (Image: ATLAS/CERN) (Image: ATLAS/CERN)

Despite its great success in describing the fundamental building blocks of matter and their interactions, the Standard Model of particle physics is known to be incomplete, so experiments around the world and in space are searching for signs of new physical phenomena that could lead physicists towards a more comprehensive theory.

At the biennial ICHEP meeting in Prague earlier this week, the ATLAS collaboration presented first results from its search for new physics phenomena at record collision energies, targeting magnetic monopoles produced in heavy ion collisions and long-lived particles produced in proton collisions.

A magnetic monopole is a hypothetical particle with a single magnetically charged north or south pole. Its existence proves the perfect symmetry of electricity and magnetism, and confirms an aspect of a grand unified theory beyond the Standard Model that unifies the strong, weak, and electromagnetic forces at very high energies.

Researchers at the Large Hadron Collider (LHC) are searching for monopoles created in high-energy collisions that are so ionizing that they can strip electrons from atoms and leave significant energy deposits in particle detectors.

In a new search for magnetic monopoles, the ATLAS Collaboration analyzed the first heavy ion (Pb) collision data from LHC Run 3, collected in fall 2023 at an unprecedented high energy of 5.36 TeV per pair of nucleons (protons or neutrons). Specifically, ATLAS researchers focused on ultra-peripheral collisions, where ions do not collide in the center via a short-range strong interaction, but pass close enough to interact via weaker but long-range electromagnetic forces. Collisions between Pb ions can generate the largest magnetic fields in the Universe, with strengths of up to 1016 Tesla.

If such an interaction produced a pair of magnetic monopoles, it would be the only particle system found in the detectors in the sky, appearing as a concentrated cloud of ionizing electrons. ATLAS looked for unique signal characteristics and analyzed the backgrounds that could mimic them, but found no signs of monopoles in the heavy ion data from Run 3.

As a result, the results set the world's highest limit on the production rate of monopoles produced in ultra-peripheral heavy ion collisions for monopole masses below 120 GeV. Furthermore, the analysis introduces a methodology for studying highly ionized particles in heavy ion data from the LHC and beyond.

Most searches for new physical phenomena are looking for new particles that decay to produce decay products quickly away from the proton-proton interaction point at the LHC. However, physics theories beyond the Standard Model, including supersymmetry, also predict long-lived particles that produce decay products far from the interaction point. Such particles require specialized techniques to reconstruct their trajectories and may have gone undetected in previous searches.

ATLAS has published results from a new search for pairs of long-lived particles, each of which decays into an electron, muon, or tau lepton, finding trajectories of the two particles (see image above) that are offset from ATLAS's point of interaction – an unusual feature that could be a sign of new physics. In particular, ATLAS looked for a new signature where one of the long-lived particles travels far enough before decaying that only one electron is detected.

This is the first ATLAS search of its kind, using 13.6 TeV proton collision data from LHC Run 3. In preparation for Run 3, ATLAS researchers enhanced their online collision event selection “trigger” by reconstructing displaced tracks, enabling the current search for new long-lived particles.

The event production in all search regions was consistent with Standard Model predictions. These results set the most stringent constraints to date on the long-lived supersymmetric partners of the electron, muon and tau lepton.

With more data from the LHC and the future upgraded High Luminosity LHC, ATLAS physicists will continue their search for long-lived particles, magnetic monopoles and other hypothetical particles, further refining search techniques and developing new strategies.

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