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Search for Chargino and Neutralino Production in Final States with a Higgs Boson and Missing Transverse Momentum at √S = 13 Tev with the ATLAS Detector

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Search for Chargino and Neutralino Production in Final States with a Higgs Boson and Missing Transverse Momentum at √S = 13 Tev with the ATLAS Detector UvA-DARE (Digital Academic Repository) Search for chargino and neutralino production in final states with a Higgs boson and missing transverse momentum at √s = 13 TeV with the ATLAS detector Aaboud, M.; The ATLAS Collaboration DOI 10.1103/PhysRevD.100.012006 Publication date 2019 Document Version Final published version Published in Physical Review D. Particles and Fields License CC BY Link to publication Citation for published version (APA): Aaboud, M., & The ATLAS Collaboration (2019). Search for chargino and neutralino production in final states with a Higgs boson and missing transverse momentum at √s = 13 TeV with the ATLAS detector. Physical Review D. Particles and Fields, 100(1), [012006]. https://doi.org/10.1103/PhysRevD.100.012006 General rights It is not permitted to download or to forward/distribute the text or part of it without the consent of the author(s) and/or copyright holder(s), other than for strictly personal, individual use, unless the work is under an open content license (like Creative Commons). Disclaimer/Complaints regulations If you believe that digital publication of certain material infringes any of your rights or (privacy) interests, please let the Library know, stating your reasons. In case of a legitimate complaint, the Library will make the material inaccessible and/or remove it from the website. Please Ask the Library: https://uba.uva.nl/en/contact, or a letter to: Library of the University of Amsterdam, Secretariat, Singel 425, 1012 WP Amsterdam, The Netherlands. You will be contacted as soon as possible. UvA-DARE is a service provided by the library of the University of Amsterdam (https://dare.uva.nl) Download date:23 Sep 2021 PHYSICAL REVIEW D 100, 012006 (2019) Search for chargino and neutralino production in finalpffiffi states with a Higgs boson and missing transverse momentum at s =13 TeV with the ATLAS detector M. Aaboud et al.* (ATLAS Collaboration) (Received 27 December 2018; published 30 July 2019) A search is conducted for the electroweak pair production of a chargino and a neutralino Æ 0 Æ 0 Æ pp → χ˜1 χ˜2, where the chargino decays into the lightest neutralino and a W boson, χ˜1 → χ˜1W , while the neutralino decays into the lightest neutralino and a Standard Model-like 125 GeV Higgs 0 0 boson, χ˜2 → χ˜1h. Fully hadronic, semileptonic, diphoton, and multilepton (electrons, muons) final states with missing transverse momentum are considered in this search. Higgs bosons in the final state are identified by either two jets originating from bottom quarks (h → bb¯), two photons (h → γγ), → → → ττ 36 1 −1 porffiffiffi leptons from the decay modes h WW, h ZZ or h . The analysis is based on . fb of s ¼ 13 TeV proton-proton collision data recorded by the ATLAS detector at the Large Hadron Collider. Observations are consistent with the Standard Model expectations, and 95% con- Æ 0 fidence-level limits of up to 680 GeV in χ˜1 =χ˜2 mass are set in the context of a simplified supersymmetric model. DOI: 10.1103/PhysRevD.100.012006 I. INTRODUCTION conservation hypothesis, the lightest supersymmetric Theoretical and experimental arguments suggest that particle (LSP) is stable. If the LSP is weakly interacting, the Standard Model (SM) is an effective theory valid up it may provide a viable DM candidate. The nature of the to a certain energy scale. The observation by the ATLAS LSP is defined by the mechanism that spontaneously and CMS collaborations of a particle consistent with the breaks supersymmetry and the parameters of the chosen SM Higgs boson [1–4] has brought renewed attention to theoretical framework. the mechanism of electroweak symmetry breaking and In the SUSY scenarios considered as benchmarks in this χ˜0 the hierarchy problem [5–8]: the Higgs boson mass is paper, the LSP is the lightest of the neutralinos ( ) which, χ˜Æ strongly sensitive to quantum corrections from physics at together with the charginos ( ), represent the mass γ very high energy scales and demands a high level of eigenstates formed from the mixture of the , W, Z and Higgs bosons’ superpartners (the higgsinos, winos and fine-tuning. Supersymmetry (SUSY) [9–14] resolves the binos). The neutralinos and charginos are collectively hierarchy problem by introducing for each known boson referred to as electroweakinos. Specifically, the electro- or fermion a new partner (superpartner) that shares the weakino mass eigenstates are designated in order of same mass and internal quantum numbers if supersym- increasing mass as χ˜Æ (i ¼ 1, 2) (charginos) and χ˜0 metry is unbroken. However, these superpartners have i j (j ¼ 1, 2, 3, 4) (neutralinos). In the models considered not been observed, so SUSY must be a broken symmetry in this paper, the compositions of the lightest chargino (χ˜Æ) and the mass scale of the supersymmetric particles is as 1 χ˜0 yet undetermined. The possibility of a supersymmetric and next-to-lightest neutralino ( 2) are wino-like and the dark matter (DM) candidate [15,16] is related closely to two particles are nearly mass degenerate, while the lightest χ˜0 the conservation of R-parity [17]. Under the R-parity neutralino ( 1) is assumed to be bino-like. Naturalness considerations [18,19] suggest that the lightest of the charginos and neutralinos have masses near *Full author list given at the end of the article. the electroweak scale. Their direct production may be the dominant mechanism at the Large Hadron Collider (LHC) Published by the American Physical Society under the terms of if the superpartners of the gluon and quarks are heavier than the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to a few TeV. In SUSY models where the masses of the the author(s) and the published article’s title, journal citation, heaviest (pseudoscalar, charged) MSSM Higgs boson and and DOI. Funded by SCOAP3. the superpartners of the leptons have masses larger than 2470-0010=2019=100(1)=012006(37) 012006-1 © 2019 CERN, for the ATLAS Collaboration M. AABOUD et al. PHYS. REV. D 100, 012006 (2019) FIG. 1. Diagrams illustrating the signal scenarios considered for the pair production of chargino and next-to-lightest neutralino targeted by the (a) hadronic (0lbb¯) and (b) 1lbb¯, (c) 1lγγ, (d) lÆlÆ, 3l leptonic channel selections. In (a) and (b) the Higgs boson decays into two b-quarks. In (c), the diphoton channel is shown with h → γγ. In (d), the visible multilepton final state of the Higgs boson is shown. Leptons are either electrons or muons. those of the lightest chargino and next-to-lightest neutra- set to 125 GeV and its branching ratios are assumed to be 0 lino, the former might decay into the χ˜1 and a W boson the same as in the SM. The Higgs boson candidate can be Æ 0 0 ¯ ¯ (χ˜1 → Wχ˜1), while the latter could decay into the χ˜1 and fully reconstructed with 0lbb, 1lbb and 1lγγ signatures, the lightest MSSM Higgs boson (h, SM-like), or Z boson while lÆlÆ and 3l final states are sensitive to decays 0 0 (χ˜2 → h=Zχ˜1) [17,20,21]. The decay via the Higgs boson is h → WW, h → ZZ and h → ττ. Previous searches for dominant for many choices of the parameters as long as the charginos and neutralinos at the LHC targeting decays via mass-splitting between the two lightest neutralinos is larger the Higgs boson into leptonic final states have been than the Higgs boson mass and the higgsinos are heavier reported by the ATLAS [28] and CMS [29] collabora- than the winos. SUSY models of this kind, where sleptons tions; a search in the hadronic channel is also reported in Æ are not too heavy although with masses above that of χ˜1 this paper. 0 and χ˜2, could provide a possible explanation for the discrepancy between measurements of the muon’s anoma- II. ATLAS DETECTOR lous magnetic moment g − 2 and SM predictions [22–25]. The ATLAS detector [30] is a multipurpose particle This paper presents a search in proton-proton collision pffiffiffi detector with a forward-backward symmetric cylindrical produced at the LHC at a center-of-mass energy s ¼ geometry and nearly 4π coverage in solid angle.1 The inner 13 TeV for the direct pair production of mass-degenerate tracking detector consists of pixel and microstrip silicon charginos and next-to-lightest neutralinos that promptly detectors covering the pseudorapidity region jηj < 2.5, χ˜Æ → χ˜0 χ˜0 → χ˜0 decay as 1 W 1 and 2 h 1. The search targets surrounded by a transition radiation tracker which enhances hadronic and leptonic decays of both the W and Higgs electron identification in the region jηj < 2.0. A new inner bosons. Three Higgs decay modes are considered: decays pixel layer, the insertable B-layer [31,32], was added at a into a pair of b-quarks, a pair of photons, or a pair of W or mean radius of 3.3 cm during the period between Run 1 and Z bosons or τ-leptons, where at least one of the W=Z=τ Run 2 of the LHC. The inner detector is surrounded by a decays leptonically. Four signatures are considered, illus- thin superconducting solenoid providing an axial 2 T trated in Fig. 1. All final states contain missing transverse magnetic field and by a fine-granularity lead/liquid-argon ⃗miss miss jηj 3 2 momentum (pT , with magnitude ET ) from neutrali- (LAr) electromagnetic calorimeter covering < . nos, and in some cases neutrinos. Events are characterized A steel/scintillator-tile calorimeter provides hadronic cov- by the various decay modes of the W and Higgs bosons.
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