Search for emerging jets in $pp$ collisions at $\sqrt{s} = 13$ TeV with the ATLAS experiment

TL;DR

The paper presents a search for emerging jets arising from a dark QCD-like sector in $pp$ collisions at $\sqrt{s}=13~\text{TeV}$ using $140~\mathrm{fb}^{-1}$ of ATLAS Run 2 data. A two-stage BDT framework—jet-level and event-level—plus a data-driven ABCD background estimation isolates EJ signals characterized by multiple displaced vertices within jets. No excess is observed, and 95% CL limits are placed on the pair-production cross-section of a heavy scalar mediator $\phi$, excluding mediator masses up to about $2~\text{TeV}$ for certain dark-pion lifetimes and masses. These results constrain dark-QCD-like scenarios and complement CMS and other ATLAS searches in probing long-lived dark-sector phenomenology with displaced-vertex signatures.

Abstract

A search is presented for emerging jets using 140 fb$^{-1}$ of proton-proton collision data at $\sqrt{s} = 13$ TeV, collected by the ATLAS experiment between 2015 and 2018. The search looks for the existence of a dark sector with symmetries similar to those in quantum chromodynamics. This dark sector is populated with dark quarks, which undergo showering similar to quarks in the Standard Model, leading to a high multiplicity of long-lived dark hadrons within a dark jet. These dark hadrons subsequently decay to Standard Model particles via a new heavy scalar mediating particle $φ$. This results in jets which contain multiple displaced vertices, known as emerging jets. This analysis targets four-jet topologies, with two emerging jets and two Standard Model jets, resulting from the decay of pair-produced scalar mediators. No significant excess above the Standard Model background is observed. For dark pion proper decay lengths of 20 mm, mediator masses are excluded between 1 TeV and 2 TeV assuming a dark pion mass of 20 GeV.

Figures (12)

• Figure 1: Feynman diagram of the pair-production of dark scalar mediators, $\phi$, via quark--antiquark annihilation. The resulting four jets are shown as cones for illustrative purposes. Dashed lines within the cones represent the trajectories of dark pions, solid lines within the cones represent the trajectories of SM particles.
• Figure 2: Normalized event-level BDT response distribution for the QCD multijet background, with three selected signal samples for comparison.
• Figure 3: Distributions of events in the ABCD plane for (left) Run 2 data and (right) an example signal model with parameters: $m_{\pi_{d}} = 20~\text{Ge V}\xspace$, $M_{\phi} = 1.8~\text{Te V}\xspace$, and $c\tau_{\pi_{d}} = 20~\mathrm{mm}$. The signal events are scaled to the pair-production cross-section for $pp \to \phi^{\dagger}\phi$ at $M_{\phi} = 1.8~\text{Te V}\xspace$, and normalized to the integrated luminosity of the dataset.
• Figure 4: Schematic diagram of the Shifted-ABCD VR constructed in CRs C and D. The four new regions are shown in red, and labeled A', B', C', and D'. The actual ABCD plane regions are shown in black for comparison.
• Figure 5: Comparison of the data with $N_{\mathrm{DV}} > 1$ and the estimated background in the SR using the modified ABCD method. The division between the SR and CR is shown by the vertical dashed line. In the final fit, the bins with $R > 0.4$ are combined into a single bin. Three selected signal samples are included for comparison.