Search for new phenomena in a lepton plus high jet multiplicity final state with the ATLAS experiment using s = 13 $$ \sqrt{s}=13 $$ TeV proton-proton collision data

Abstract

A search for new phenomena in final states characterized by high jet multiplicity, an isolated lepton (electron or muon) and either zero or at least three b-tagged jets is presented. The search uses 36.1 fb −1 of √ s = 13 TeV proton-proton collision data collected by the ATLAS experiment at the Large Hadron Collider in 2015 and 2016. The dominant sources of background are estimated using parameterized extrapolations, based on observables at medium jet multiplicity, to predict the b-tagged jet multiplicity distribution at the higher jet multiplicities used in the search. No significant excess over the Standard Model expectation is observed and 95% confidence-level limits are extracted constraining four simplified models of R-parity-violating supersymmetry that feature either gluino or top-squark pair production. The exclusion limits reach as high as 2.1 TeV in gluino mass and 1.2 TeV in top-squark mass in the models considered. In addition, an upper limit is set on the cross-section for Standard Model tttt production of 60 fb (6.5 × the Standard Model prediction) at 95% confidence level. Finally, model-independent limits are set on the contribution from new phenomena to the signal-region yields. Conclusion 25The ATLAS collaboration 34 IntroductionThe ATLAS experiment at the Large Hadron Collider (LHC) has carried out a large number of searches for beyond the Standard Model (BSM) physics. These searches cover a broad range of different final-state particles and kinematics. However, one gap in the search coverage, as pointed out in refs. [1,2], is in final states with one or more leptons, many jets and no-or-little missing transverse momentum (the magnitude of which is denoted by -1 - JHEP09(2017)088E miss T ). Such a search is presented in this article, considering final states with an isolated lepton (electron or muon), at least eight to twelve jets (depending on the jet transverse momentum threshold), either zero or many b-tagged jets, and with no requirement on E miss T . This search has potential sensitivity to a large number of BSM physics models. In this article, model-independent limits on the possible contribution of BSM physics to several single-bin signal regions are presented. In addition, four R-parity-violating (RPV) supersymmetric (SUSY [3-8]) benchmark models are used to interpret the results. In this case, a multi-bin fit to the two-dimensional space of jet-multiplicity and b-tagged jet multiplicity is used to constrain the models. The dominant Standard Model (SM) background arises from top-quark pair production and W/Z+jets production, with at least one lepton produced in the vector boson decay. The theoretical modelling of these backgrounds at high jet multiplicity suffers from large uncertainties, so they are estimated from the data by extrapolating the b-tagged jet multiplicity distribution extracted at moderate jet multiplicities to the high jet multiplicities of the search region. Previous searches targeting similar RPV SUSY models have been carried out by the ATLAS and CMS collabo...

Least constrained supersymmetry with R -parity violation

“…Even though most of our specific signals have not been searched at the LHC yet, there are some existing searches for the similar final states which could already constrain our signal processes. We will recast three relevant RPV SUSY searches from ATLAS: a lepton plus high jet multiplicity search [16], pair-produced resonances in four-jet final states [19], and multijet final states [54]. The validations of our recasting are provided in appendixes.…”

“…The signature of a pure hadronic multijet [15] in the final state has been searched to constrain the gluino pair production if λ ijk, i =3 is nonzero. When λ 3jk = 0, there could be top quarks from the gluino decay, the leptonic decay of which gives leptons + multijet final state [16][17][18]. Searches for the same final state are also constraining the L i Q j D c k operator.…”

“…In particular, the bound on the stop pair production with the subsequent decay through intermediate Bino or Higgsino, which further decays into jets by a U c 2 D c 1 D c 3 operator, are considered. And a similar process with Bino/Higgsino decays through the U c 3 D c 2 D c 3 operator is searched by the ATLAS Collaboration [16]. The sensitivity of an upgraded LHC on those RPV operators were studied in Ref.…”

Quark jet rates and quark-gluon discrimination in multijet final states

“…The data-driven method is often used for the analysis of multi-jet final states. A typical analysis is the micro black hole search [73][74][75][76][77][78][79]. In the analysis, phase space is divided by a variable related to the hard scale, e.g., the scalar sum of jet transverse momenta H T , the scalar sum of the vi The one problem on the topic of quark/gluon discrimination is Monte-Carlo uncertainties of the predictions, and known that experimental data on certain observables related to quark/gluon tagging lie in between the predictions of the two MC generators Pythia and Herwig [54,56,57].…”

“…Many new physics models form multi-jet final states. For example, heavy colored resonances in each model, like gluino or squarks in SUSY, could emit many partons via their cascade decays, and there are several studies for these at the LHC [73][74][75][76][77][78][79]. Another example is searches of low-scale gravity which deals with the hierarchy problem concerned with the difference between the electroweak and Plack scale.…”