Search for massive, long-lived particles in events with displaced vertices and displaced muons in $pp$ collisions at $\sqrt{s}=13.6$ TeV with the ATLAS experiment

Abstract

A search is presented for massive long-lived particles in events featuring at least one displaced vertex and at least one displaced muon, using proton-proton collision data collected by the ATLAS detector at the Large Hadron Collider from 2022 to 2024 at a centre-of-mass energy of 13.6 TeV. The data sample corresponds to an integrated luminosity of 164 fb$^{-1}$. The analysis targets scenarios in which long-lived particles decay inside the ATLAS inner detector, resulting in a topology of at least one massive, displaced vertex (DV) with multiple associated tracks, and at least one muon with a large transverse impact parameter relative to the primary interaction point. The muon is not required to be associated with the DV. Two signal regions are defined by the transverse distance of the reconstructed DV from the interaction point. Background contributions are estimated by using fully data-driven techniques. No significant excess above the expected background is observed. Upper limits at 95% confidence level are set on the visible cross-section and on the production cross-sections of several benchmark models of $R$-parity-violating supersymmetry.

Figures (8)

• Figure 1: Illustrative diagrams for the three considered RPV SUSY simplified benchmark models, each assuming a single non-zero RPV term: \ref{['fig:feynman_diagrams_lqd']} higgsino pair production with $\lambda'_{211}$ decays, \ref{['fig:feynman_diagrams_udd']} higgsino production with $\lambda"_{323}$ decays, and \ref{['fig:feynman_diagrams_top']} top-squark--anti-squark pair production with $\lambda'_{233}$ decays. Equivalent diagrams with charge-conjugate processes are also considered.
• Figure 2: Event-level trigger efficiency for the triggers used in this analysis. The plots show separate curves for the MS-only triggers and the displaced-muon trigger, which exploits the dedicated large-impact-parameter track reconstruction introduced in the HLT in Run 3, and their combination. The efficiencies are evaluated as a function of \ref{['fig:trig_eff_muon_pT']} the highest muon $p_{\text{T}}\xspace$ in the event, and \ref{['fig:trig_eff_muon_d0']} the transverse impact parameter, $|d_0|$, of that muon. The benchmark sample used here consists of higgsino pair-production events with LNV $\lambda'_{211}$ decays ($m=150\text{Ge V}\xspace$, $\tau=0.1\ns$). For the MS-only triggers, the trigger plateau criteria are applied to the highest-$p_{\text{T}}$ muon in the event (63$\text{Ge V}$ for the $p_{\text{T}}\xspace > 60\text{Ge V}\xspace$, $|\eta| < 1.05$ trigger and 84$\text{Ge V}$ for the $p_{\text{T}}\xspace > 80\text{Ge V}\xspace$, $|\eta| < 2.4$ trigger with additional requirement of three hits in precision tracking layers for $|\eta| > 1.05$). The muon $p_{\text{T}}$ plot shows the combined trigger efficiency turn-on behaviour.
• Figure 3: Categorisation of events based on the presence and quality of DV (horizontal axis) and the background source $i$ of the highest-$p_{\text{T}}$ muon (vertical axis). The schematic is compressed vertically for simplicity. If a muon fails the veto for a specific muon background source, it is assigned that background category; if it passes all muon requirements, it is considered signal-like. The signal regions, $\mathrm{SR}_{\mathrm{far}}$ and $\mathrm{SR}_{\mathrm{near}}$, require at least one signal-like muon and at least one signal-like far or near DV, respectively. Additional validation regions are defined by dropping or inverting the $n_{\mathrm{track}}$ and $m_{\mathrm{DV}}$ requirements for signal-like DV. The regions $\mathrm{C}$ and $\mathrm{D}_i$ consist of the zero-DV and material-map-vetoed DV classes; the TF are derived from subsets of these regions by applying additional muon selections to enhance the purity for each source. The background predictions for $\mathrm{SR}_{\mathrm{far}}$ and $\mathrm{SR}_{\mathrm{near}}$ are then obtained by applying the TF to the $\mathrm{B}_{\text{far},i}$ and $\mathrm{B}_{\text{near},i}$ regions, respectively.
• Figure 4: Overview of the analysis regions with varying DV selection criteria. The predicted background yields are obtained via the TF method from the zero-DV and material-map-vetoed DV selections, with additional muon selection criteria to enhance the purity for each background. Good agreement is observed in all validation regions, except the far $n_{\mathrm{track}}\xspace \geq 4, m_{\mathrm{DV}}\xspace < 20\text{Ge V}\xspace$ region. An additional non-closure uncertainty is applied to the heavy-flavour estimate in that region and in $\mathrm{SR}_{\mathrm{far}}$. The hatched band represents the total uncertainty in the prediction. The predictions are overlaid with one representative higgsino pair production benchmark model, with $m = 1500\text{Ge V}\xspace$ and $\tau = 0.1\ns$, and LNV decays via the $\lambda'_{211}$ coupling. The uncertainty in the signal prediction includes statistical and systematic uncertainties.
• Figure 5: Distributions of the predicted background yields and the observed data for events with \ref{['fig:far-lowNTrk-mu-d0']} a far DV with a transverse distance from the primary vertex larger than 4 and $n_{\mathrm{track}}\xspace < 4$ as a function of the $|d_0|$ of the highest-$p_{\text{T}}$ muon, and \ref{['fig:near-lowNTrk-mu-pt']} a near DV with a transverse distance from the primary vertex between 1 and 4 and $n_{\mathrm{track}}\xspace < 4$ as a function of the $p_{\text{T}}$ of the highest-$p_{\text{T}}$ muon. The hatched band represents the total uncertainty in the prediction. Overflow events are included in the last bins. Good agreement is observed in both regions.