Search for displaced decays of long-lived particles in events with missing transverse momentum in $\sqrt{s} = 13$ TeV $pp$ collisions with the ATLAS detector

Abstract

A search for long-lived particles in events with significant missing transverse momentum and at least one displaced vertex is presented. This analysis is performed using 137 $\text{fb}^{-1}$ of $pp$ collision data collected between 2016--2018 during Run 2 of the Large Hadron Collider by the ATLAS detector. Displaced vertices are identified using two different secondary vertexing algorithms, including a novel ``fuzzy'' vertexing algorithm optimized for identifying displaced decays of heavy quarks. Separate searches are performed using each algorithm, and the expected Standard Model background is independently estimated for each search using a data-driven procedure. No significant excess is observed over the background in either case. The results are used to set 95% confidence-level limits on potential beyond-the-Standard Model physics that could produce this final state. Results are interpreted in the context of four models: long-lived gluinos that form $R$-hadrons before decaying, neutralinos decaying via Higgs-mediated channels in the Bino-Wino coannihilation model, long-lived Higgsinos decaying to axinos, and an exotic Higgs portal model predicting displaced decays of light pseudoscalars.

Figures (12)

• Figure 1: Illustrative diagrams for the four signal interpretations considered in this analysis; the \ref{['fig:feynman-RHadron']} Gluino $R$-hadron model described in Section \ref{['sec:rhadron']}; \ref{['fig:feynman-BinoWino']} the Bino-Wino coannihilation model described in Section \ref{['sec:bino_wino']}; \ref{['fig:feynman-Axino']} the DFSZ axino model described in Section \ref{['sec:axino']}; and \ref{['fig:feynman-HiggsPortal']} the Higgs portal described in Section \ref{['sec:higgs_portal']}. Red lines indicate BSM particles, and the small gray circles indicate displaced vertices.
• Figure 2: Vertex reconstruction efficiency for the standard and fuzzy vertexing methods for \ref{['fig:vertex_eff_rhadron']} Gluino $R$-hadron and \ref{['fig:vertex_eff_binowino']} Bino-Wino coannihilation models. $L_{xy}$ is the radial displacement of the truth position of the DV from the beamline axis. The spike around $L_{xy}$ = 260 mm is due to statistical fluctuations, as confirmed by looking at other signal models.
• Figure 3: Map showing a cross-section at $z=0$ of the physical material comprising the ATLAS detector's beam pipe and tracking detector, used to define the "material map veto" SUSY-2018-33.
• Figure 4: Estimated and observed events in the sideband VRs of the SDV SR. The gray hashed band shows the background estimate uncertainties from the CR's statistics and from the distributions of the number of pileup collisions between events in the CR and the events that pass common event-level selections.
• Figure 5: Estimated and observed events in the sideband VRs of the 1FDV VRs. The gray hashed band shows the background estimate uncertainties from the CR's statistics and from the distributions of the number of pileup collisions between events in the CR and the events that pass common event-level selections and the strict NCB veto. The non-closure uncertainty discussed in the text is not included here.