Abstract
A search for the pair production of heavy leptons as predicted by the type-III seesaw mechanism is presented. The search uses proton–proton collision data at a centre-of-mass energy of 13 TeV, corresponding to 139,{text {fb}}^{-1} of integrated luminosity recorded by the ATLAS detector during Run 2 of the Large Hadron Collider. The analysis focuses on the final state with two light leptons (electrons or muons) of different flavour and charge combinations, with at least two jets and large missing transverse momentum. No significant excess over the Standard Model expectation is observed. The results are translated into exclusion limits on heavy-lepton masses, and the observed lower limit on the mass of the type-III seesaw heavy leptons is 790 GeV at 95% confidence level.
Highlights
Background composition and estimationThe final-state topologies of the six analysis channels have different background compositions
The effect of multiple interactions in the same and neighbouring bunch crossings was modelled by overlaying the original hard-scattering event with simulated inelastic pp events generated with Pythia 8.186 using the NNPDF2.3lo set of parton distribution function (PDF) and the A3 set of tuned parameters [52]
Correction factors to account for differences in the identification and selection efficiency, reconstructed energy and energy resolution between data and Monte Carlo (MC) simulation are applied to the selected electrons, muons and jets, and when calculating ETmiss in MC simulation, as described in Refs. [55,57,59,61]
Summary
The ATLAS detector [14] at the LHC is a multipurpose particle detector with a forward–backward symmetric cylindrical geometry and a nearly 4π coverage in solid angle around the collision point It consists of an inner tracking detector surrounded by a thin superconducting solenoid, electromagnetic and hadronic calorimeters, and a muon spectrometer incorporating three large superconducting toroidal magnets. The high-granularity silicon pixel detector covers the vertex region and typically provides four measurements per track, the first hit being normally in the insertable B-layer, which was installed before Run 2 [15,16]. It is followed by the silicon microstrip tracker which usually provides four measurements per track. This is followed by a softwarebased trigger which further reduces the event rate to approximately 1 kHz
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