Search for electroweak production of vector-like leptons in $τ$-lepton and $b$-jet final states in $pp$ collisions at $\sqrt{s}$ = 13 TeV with the ATLAS detector

TL;DR

The paper presents ATLAS results searching for electroweak production of vector-like leptons (VLLs) in the 4321 UV-complete framework, where VLLs decay via a vector leptoquark U1 into third-generation fermions, yielding final states with multiple $\tau$ leptons and $b$-quarks. The analysis leverages the full Run 2 ATLAS dataset at $\sqrt{s}=13$ TeV (~$140~\mathrm{fb}^{-1}$) and employs neural-network classifiers to optimize signal discrimination across five signal regions, supplemented by data-driven background estimates for $t\bar{t}$+jets, $Z$+jets, fake $\tau_{had}$, and multijet processes. No significant excess over the SM background is observed; 95% CL upper limits on $\sigma_{\mathrm{VLL}}\times\mathrm{BR}$ as a function of $m_{\mathrm{VLL}}$ yield a observed lower limit of 910 GeV and an expected limit of 970 GeV for the VLL mass, with compatible SUSY RPV interpretations giving $m_{\mathrm{higgsino}} > 880$ GeV and $m_{\mathrm{wino}} > 1170$ GeV. The results constitute the first ATLAS constraints on VLLs decaying through $U_1$ to third-generation SM fermions and demonstrate the effectiveness of NN-based event-classification in channels with high jet and heavy-flavour multiplicities.

Abstract

A search for pair-production of vector-like leptons is presented, considering their decays into a third-generation Standard Model (SM) quark and a vector leptoquark ($U_1$) as predicted by an ultraviolet-complete extension of the SM, referred to as the '4321' model. Given the assumed decay of $U_1$ into third-generation SM fermions, the final state can contain multiple $τ$-leptons and $b$-quarks. This search is based on a dataset of $pp$ collisions at $\sqrt{s}=13$ TeV recorded with the ATLAS detector during Run 2 of the Large Hadron Collider, corresponding to an integrated luminosity of up to 140 fb$^{-1}$. No significant excess above the SM background prediction is observed, and 95% confidence level limits on the cross-section times branching ratio are derived as a function of the vector-like lepton mass. A lower observed (expected) limit of 910 GeV (970 GeV) is set on the vector-like lepton mass. Additionally, the results are interpreted for a supersymmetric model with an $R$-parity violating coupling to the third-generation quarks and leptons. Lower observed (expected) limits are obtained on the higgsino mass at 880 GeV (940 GeV) and on the wino mass at 1170 GeV (1170 GeV).

Figures (14)

• Figure 1: Vector-like lepton pair production and decays in the '4321' model for (a) $pp\rightarrow E^{+}N$, (b) $pp\rightarrow E^+E^-$, (c) $pp\rightarrow N\overline{N}$.
• Figure 2: Diagrams for SUSY $R$-parity violation model with $\lambda'_{333}$ coupling for wino or higgsino production of (a) $pp\rightarrow \tilde{\chi}^{\pm}_{1} \tilde{\chi}^{0}_{1,2}$, (b) $pp\rightarrow \tilde{\chi}^{\pm}_{1} \tilde{\chi}^{\mp}_{1}$, (c) $pp\rightarrow \tilde{\chi}^{0}_{1} \tilde{\chi}^{0}_{2}$.
• Figure 3: Trigger selection strategy for $0\ell$ channels using multiple triggers to maximise signal acceptance, where the kinematic selections in the analysis are chosen in the plateau region of the trigger efficiency, followed by corresponding trigger matching and trigger decisions. The (MET) trigger category targets events with $\geq\!200$$\text{Ge V}$, the single-$\tau_\mathrm{had}$ (STT) triggers select events with single $\tau_\mathrm{had}$ of above the thresholds indicated in the diagram, the di-$\tau_\mathrm{had}$ (DTT) triggers recover events with two $\tau_\mathrm{had}$ with relatively lower thresholds, and the (BJET) triggers recover events that have low $\tau_\mathrm{had}$ but a large multiplicity of . The MST category refers to events that pass either the STT or the MET trigger chain. Similarly, events that belong to the STT, the DTT, or the MET trigger chains are collectively labelled as MSDT. Variations in the offline thresholds are due to changes in trigger thresholds across different data-taking periods.
• Figure 4: A sketch of the analysis regions considered in this '4321' VLL search. The regions labelled with 'SR' ('CR') refer to the signal (control) regions, as summarised in Table \ref{['tab:tauonly_srs']} (\ref{['tab:taucrs']}). They are used in the simultaneous profile likelihood fit, as discussed in Section \ref{['sec:result']}. The regions labelled with 'SF' are used for the derivation of correction factors which depend on event kinematics for the $t\bar{t}+\mathrm{jets}$, $Z+\mathrm{jets}$, fake $\tau_\mathrm{had}$ and multijet background. The validation region 'VR' is used to verify the modelling of the fake $\tau_\mathrm{had}$ background mainly originating from $t\bar{t}+\mathrm{jets}$ and $W/Z+\mathrm{jets}$ processes. The selections for 'SF' and 'VR' regions are summarised in Table \ref{['tab:ttbarZjetsFakeTauCRs']}. The $Z+\mathrm{LF}$ ($Z+\mathrm{HF}$) region is enriched in $Z$ boson events without (with) additional $b$-tagged jets. A few analysis regions with different trigger selections are labelled with abbreviated notations: MST for the combined MET and STT trigger categories, MSDT for the combined MET, STT and DTT trigger categories, and MLT for the combination of MET triggers with SLT or DLT triggers.
• Figure 5: The expected acceptance times efficiency (including object identification and reconstruction, trigger selection, and event selection) for VLL signal as a function of $m_{\mathrm{VLL}}$ for the combined signal regions as well as for three signal region categories: $1\tau_\mathrm{had}\xspace\geq\!3b$ MST, $1\tau_\mathrm{had}\xspace\geq\!3b$ BJET and $\geq\!2\tau_\mathrm{had}\xspace\geq\!3b$ MSDT. The region $1\tau_\mathrm{had}\xspace\geq\!3b$ MST refers to the combination of the $1\tau_\mathrm{had}\xspace 3b$ MST and $1\tau_\mathrm{had}\xspace \geq\!4b$ MST signal regions, while the region $1\tau_\mathrm{had}\xspace\geq\!3b$ BJET refers to the combination of the $1\tau_\mathrm{had}\xspace 3b$ BJET and $1\tau_\mathrm{had}\xspace\geq\!4b$ BJET signal regions.