Abstract
A search for pair production of third-generation scalar leptoquarks decaying into a top quark and a τ-lepton is presented. The 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 139 fb−1. Events are selected if they have one light lepton (electron or muon) and at least one hadronically decaying τ -lepton, or at least two light leptons. In addition, two or more jets, at least one of which must be identified as containing b-hadrons, are required. Six final states, defined by the multiplicity and flavour of lepton candidates, are considered in the analysis. Each of them is split into multiple event categories to simultaneously search for the signal and constrain several leading backgrounds. The signal-rich event categories require at least one hadronically decaying τ-lepton candidate and exploit the presence of energetic final-state objects, which is characteristic of signal events. No significant excess above the Standard Model expectation is observed in any of the considered event categories, and 95% CL upper limits are set on the production cross section as a function of the leptoquark mass, for different assumptions about the branching fractions into tτ and bν. Scalar leptoquarks decaying exclusively into tτ are excluded up to masses of 1.43 TeV while, for a branching fraction of 50% into tτ, the lower mass limit is 1.22 TeV.
Highlights
Background estimationBackgrounds are categorised into irreducible and reducible backgrounds
The muon spectrometer measures the trajectories of muons with |η| < 2.7 using multiple layers of high-precision tracking chambers located in a toroidal field of approximately 0.5 T and 1 T in the central and endcap regions of ATLAS, respectively
The level of agreement found between observed and predicted yields is within 25%, which is assigned as a systematic uncertainty associated with the extrapolation of the estimate from the 2 internal conversion (IntC) and 3 IntC CRs to the other event categories
Summary
The ATLAS detector [24] at the LHC covers almost the entire solid angle around the collision point, and consists of an inner tracking detector surrounded by a thin superconducting solenoid producing a 2 T axial magnetic field, electromagnetic and hadronic calorimeters, and a muon spectrometer (MS) incorporating three large toroidal magnet assemblies. The inner detector contains a high-granularity silicon pixel detector, including the insertable B-layer [25, 26], and a silicon microstrip tracker, together providing a precise reconstruction of tracks of charged particles in the pseudorapidity range |η| < 2.5. The muon spectrometer measures the trajectories of muons with |η| < 2.7 using multiple layers of high-precision tracking chambers located in a toroidal field of approximately 0.5 T and 1 T in the central and endcap regions of ATLAS, respectively. A two-level trigger system [27], consisting of a hardware-based first-level trigger followed by a software-based high-level trigger (HLT), is used to reduce the event rate to a maximum of around 1 kHz for offline storage
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