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Searches for unusual signatures from dark sectors with the ATLAS Experiment

Danielle Wilson-Edwards, ATLAS Collaboration

TL;DR

The paper tackles searches for unusual dark-sector signatures at the LHC with ATLAS, focusing on two complementary signatures: emerging jets from a confining dark sector and a single displaced vertex in the muon spectrometer. It combines a cut-based and a transformer-based jet tagging approach for emerging jets, with data-driven ABCD and mistag-rate background estimation, while also probing scalar, Higgs-portal, ALP, and dark-photon models via a one-displaced-vertex LLP search in the muon spectrometer using two channels and NN-based background control. No significant excess is observed in either analysis, and the resulting limits push mediator masses into the multi-TeV range (e.g., $m_{Z'} \lesssim 2.5$ TeV and $m_{\Phi} \lesssim 1.35$ TeV) and constrain LLP scenarios, including excluding Higgs-portal BRs above $\sim 1\%$ for lifetimes in the $5\ \mathrm{cm}$ to $40\ \mathrm{m}$ range; the one-DV search extends lifetime sensitivity beyond prior two-DV results. These results demonstrate ATLAS's capability to probe wide swaths of the dark-sector parameter space and motivate continued exploration with Run-2 and Run-3 data.

Abstract

Various theories beyond the Standard Model predict unusual signatures, including new, long-lived particles decaying at a significant distance from the collision point. These unique signatures are difficult to reconstruct and face unusual and challenging backgrounds. This contribution will focus on recent results from searches for unusual signatures motivated by dark sector models, utilising pp collision data collected by the ATLAS experiment at the Large Hadron Collider.

Searches for unusual signatures from dark sectors with the ATLAS Experiment

TL;DR

The paper tackles searches for unusual dark-sector signatures at the LHC with ATLAS, focusing on two complementary signatures: emerging jets from a confining dark sector and a single displaced vertex in the muon spectrometer. It combines a cut-based and a transformer-based jet tagging approach for emerging jets, with data-driven ABCD and mistag-rate background estimation, while also probing scalar, Higgs-portal, ALP, and dark-photon models via a one-displaced-vertex LLP search in the muon spectrometer using two channels and NN-based background control. No significant excess is observed in either analysis, and the resulting limits push mediator masses into the multi-TeV range (e.g., TeV and TeV) and constrain LLP scenarios, including excluding Higgs-portal BRs above for lifetimes in the to range; the one-DV search extends lifetime sensitivity beyond prior two-DV results. These results demonstrate ATLAS's capability to probe wide swaths of the dark-sector parameter space and motivate continued exploration with Run-2 and Run-3 data.

Abstract

Various theories beyond the Standard Model predict unusual signatures, including new, long-lived particles decaying at a significant distance from the collision point. These unique signatures are difficult to reconstruct and face unusual and challenging backgrounds. This contribution will focus on recent results from searches for unusual signatures motivated by dark sector models, utilising pp collision data collected by the ATLAS experiment at the Large Hadron Collider.
Paper Structure (4 sections, 2 figures)

This paper contains 4 sections, 2 figures.

Table of Contents

  1. Introduction
  2. Search for emerging jets
  3. Search for events with one displaced vertex from long-lived neutral particles decaying into hadronic jets in the muon spectrometer
  4. Summary

Figures (2)

  • Figure 1: The 95% CL exclusion limits on (a, c) $\sigma(qq \rightarrow Z') \times \mathrm{Br}(Z' \rightarrow q_\mathrm{D} q_\mathrm{D})$ and (b, d) $\sigma(pp \rightarrow q_\mathrm{D} q_\mathrm{D})$ for $m_{\pi_\mathrm{D}}=10$ GeV EJ_ATLAS:2025bsz. The cut-based and ML-based strategies are shown in triangles and circles, respectively. Theoretical cross-section predictions are shown as solid lines.
  • Figure 2: Summary of the expected and observed $95 \%$ CL upper limits for the $\Phi / H \rightarrow ss$ model 1DV_MS_ATLAS:2025pak. (a) Comparison of the one DV 1DV_MS_ATLAS:2025pak and two DV 2DV_MS_ATLAS:2022gbw results and their combined limits on $(\sigma / \sigma_{ggH}) \times B$ for the Higgs-portal mediator with $m_s = 35$ GeV. (b) Upper limits on the $Z$+ALP production cross-section for $m_a = 40$ GeV compared with previous limits from HCal_LLP_ATLAS:2024ocv and ID_LLP_ATLAS:2024qoo. (c) Observed $95 \%$ CL upper limits on $\sigma \times B(Z_\mathrm{D} \rightarrow f\bar{f})$ for the dark photon $Z_\mathrm{D}$ model.