[Paper Review] Searches for new phenomena in events with two leptons, jets, and missing transverse momentum in 139 fb$^{-1}$ of $ \sqrt{s}=13$ TeV $ pp$ collisions with the ATLAS detector

ATLAS Collaboration|arXiv (Cornell University)|Apr 27, 2022

Particle physics theoretical and experimental studies5 citations

TL;DR

This paper presents a search for new physics beyond the Standard Model using 139 fb⁻¹ of 13 TeV proton-proton collisions collected by the ATLAS detector, focusing on final states with two same-flavor opposite-sign leptons, jets, and missing transverse momentum. Using recursive-jigsaw reconstruction, electroweak, and strong production searches, the study sets new exclusion limits up to 900 GeV for electroweakinos, 1550 GeV for squarks, and 2250 GeV for gluinos, improving previous limits by 200–400 GeV.

ABSTRACT

Searches for new phenomena inspired by supersymmetry in final states containing an $e^+e^-$ or $μ^+μ^-$ pair, jets, and missing transverse momentum are presented. These searches make use of proton-proton collision data with an integrated luminosity of 139 $ ext{fb}^{-1}$, collected during 2015-2018 at a centre-of-mass energy $\sqrt{s}=13 $TeV by the ATLAS detector at the Large Hadron Collider. Two searches target the pair production of charginos and neutralinos. One uses the recursive-jigsaw reconstruction technique to follow up on excesses observed in 36.1 $ ext{fb}^{-1}$ of data, and the other uses conventional event variables. The third search targets pair production of coloured supersymmetric particles (squarks or gluinos) decaying through the next-to-lightest neutralino $( ildeχ_2^0)$ via a slepton $( ilde\ell)$ or $Z$ boson into $\ell^+\ell^- ildeχ_1^0$, resulting in a kinematic endpoint or peak in the dilepton invariant mass spectrum. The data are found to be consistent with the Standard Model expectations. Results are interpreted using simplified models and exclude masses up to 900 GeV for electroweakinos, 1550 GeV for squarks, and 2250 GeV for gluinos.

Motivation & Objective

• To search for new physics in final states with two same-flavor opposite-sign leptons, jets, and missing transverse momentum in 139 fb⁻¹ of 13 TeV proton-proton collisions.
• To test supersymmetric models involving electroweak and strong production of sparticles, particularly charginos, neutralinos, squarks, and gluinos.
• To improve sensitivity to simplified models of supersymmetry by leveraging a larger dataset and optimized analysis techniques.
• To investigate the persistence of small excesses observed in earlier 36 fb⁻¹ data, particularly in the two-lepton final state.
• To extend sensitivity to gauge-mediated SUSY breaking models with higgsino next-to-lightest SUSY particles.

Proposed method

• Utilizes recursive-jigsaw reconstruction (RJR) to enhance sensitivity to electroweak SUSY production, particularly targeting previously observed excesses in 36.1 fb⁻¹ of data.
• Employs conventional event variables and optimized selection criteria for electroweak and strong SUSY searches, including kinematic binning to improve sensitivity.
• Applies model-independent upper limits in single-bin signal regions and model-dependent profile likelihood fits in simplified SUSY models.
• Estimates backgrounds using data-driven techniques and Monte Carlo simulations, with systematic uncertainties evaluated for all components.
• Uses the dilepton invariant mass spectrum to identify kinematic endpoints or peaks from decays involving the next-to-lightest neutralino (˜𝜒₀₂) into ℓ⁺ℓ⁻˜𝜒₀₁.
• Performs background estimation for three distinct search channels: RJR, electroweak, and strong production, with dedicated control regions and validation techniques.

Experimental results

Research questions

• RQ1 Do the small excesses observed in the two-lepton final state in the 36.1 fb⁻¹ dataset persist with the full 139 fb⁻¹ dataset?
• RQ2 What are the improved exclusion limits on electroweak and strong SUSY production following optimization of analysis selections and use of a larger dataset?
• RQ3 Can the sensitivity to gauge-mediated SUSY breaking models with higgsino NLSPs be extended to higher masses and branching ratios?
• RQ4 How do the new limits compare to previous ATLAS searches in similar final states, particularly in the context of gluino and squark mass reach?
• RQ5 To what extent do the recursive-jigsaw reconstruction and optimized selection criteria enhance sensitivity compared to previous analyses?

Key findings

• The data are consistent with Standard Model expectations, with no significant excesses observed in the recursive-jigsaw reconstruction search, indicating that prior small excesses in the 36.1 fb⁻¹ data did not persist.
• Exclusion limits for electroweakinos reach up to 900 GeV, representing an improvement of approximately 200 GeV over previous results.
• Exclusion limits for squarks reach 1550 GeV, an improvement of 300 GeV compared to earlier analyses.
• Exclusion limits for gluinos reach 2250 GeV, an improvement of 400 GeV over previous results, demonstrating enhanced sensitivity to strong SUSY production.
• The electroweak search achieves improved sensitivity to GMSB models with higgsino NLSPs, covering a mass range between previous four-lepton and all-hadronic search limits.
• The strong search achieves sensitivity to higher ˜𝜒₀₁ masses than previous ATLAS searches in the same final state, demonstrating enhanced performance from dataset size and analysis optimization.