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ATLAS probes uncharted territory with improved trigger

20 July 2024 | By

ATLAS Collaboration

The ATLAS Collaboration has released its first search for new physics phenomena at the highest-ever collision energy of 13.6 TeV, targeting exotic events with two “displaced” leptons.

ATLAS
Figure 1: Distributions of electron arrival time at the calorimeter relative to prompt electrons, for data in black and grey points and for three possible signal models in solid and dashed lines. (Image: ATLAS Collaboration/CERN)

The discovery of the Higgs boson by the ATLAS and CMS Collaborations completed the Standard Model of particle physics and led to the 2013 Nobel Prize in Physics. Despite being enormously successful at describing the world we live in, the Standard Model does not explain several outstanding mysteries of the universe, such as why the Higgs-boson mass is so light (the “hierarchy problem”) or the origin of dark matter. The search for new physics phenomena that go beyond the Standard Model is an important focus of the ATLAS physics programme.

Many searches for new physics phenomena look for new particles that would decay “promptly” and produce decay products that emanate from the LHC’s proton-proton interaction point. However, several beyond-the-Standard-Model physics models, including supersymmetry and models with extra dimensions, predict “long-lived particles” (LLPs) that could travel significant distances in the ATLAS experiment before decaying. LLPs would produce decay products away from the interaction point. Such particles require dedicated reconstruction techniques and may have eluded detection in prior searches.

These "displaced" tracks in the ATLAS experiment are rare signatures that could be indicative of new physics phenomena.

ATLAS
Figure 2: Distribution of the output of the Boosted Decision Tree (BDT), for the (solid blue line) expected background, (dashed lines) three example signal models, and (black points) data. A signal would manifest itself as an excess of the data above the background in the right-most bins. (Image: ATLAS Collaboration/CERN)

The ATLAS Collaboration has published a new result that looks for pairs of long-lived particles that each decay into an electron, muon or tau lepton, resulting in pairs of particle tracks that are “displaced” from the interaction point (see event display) – a rare signature that could be indicative of new physics phenomena. This is the first ATLAS search for new physics using the proton-proton collision data from Run 3 of the LHC (2022-ongoing), recorded at a record collision energy of 13.6 TeV. Prior to Run 3, researchers had improved ATLAS' event selection system (the "trigger") to allow them to reconstruct displaced tracks. This improvement enhanced their ability to select signal events with less energetic leptons, expanding their ability to search for beyond-the-Standard-Model physics.

In their new result, ATLAS physicists looked for a new signature where one of the LLP travels far enough before decaying so that only a single electron is detected. This analysis uses the Liquid Argon calorimeter's capability to measure the arrival time of particles relative to the time at which the collision occurred, which is typically larger for electrons from LLP decays than for Standard Model background processes (as shown in Figure 1). Researchers used a Boosted Decision Tree (BDT) machine-learning algorithm to help handle otherwise unmanageably large backgrounds, using events with a negative time-of-flight to predict the expected BDT output distribution. As shown in Figure 2, the data are consistent with the background prediction.

The event yields in all search regions matched Standard-Model expectations. These results set the strictest constraints yet on the long-lived supersymmetric partners of electrons, muons, and tau leptons. Future searches will consider additional signatures, studying the full Run-3 dataset and the significantly larger dataset expected from the High-Luminosity LHC upgrade.

About the event display: Display of an event recorded by the ATLAS detector in October 2022 at a collision energy of 13.6 TeV, featuring two candidate displaced electrons, triggered by the new large-radius-tracking-based electron trigger. On the right is a side view of the ATLAS detector with two electron candidates, each represented by a track (blue lines) that points to an energy deposit (green boxes) in the electromagnetic calorimeter (the grey layer). Additional energy deposits in the electromagnetic calorimeter are shown in green, as well as energy deposits in the hadronic calorimeter in yellow. In the bottom-left is an axial view of the ATLAS detector concentrating on the first two Pixel detector layers, illustrating the candidate electron tracks (blue lines) displaced from the interaction point (red circle) by a few mm. In both views, other, lower-transverse momentum tracks in the event are drawn as orange lines. (Image: ATLAS Collaboration/CERN)

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