dilepton and multilepton final states with the ATLAS experiment
The center-of-mass energy of the Large Hadron Collider (LHC) was increased to √s = 13 TeV in 2015, opening a unique window for searches with multiple high-mass particles in the final state. With a mass close to the scale of electroweak symmetry breaking, the top quark is predicted to have a very large coupling to the Higgs boson and to other new resonances hypothesized in many models beyond the Standard Model (BSM). Hence, an enhanced rate of events containing many top quarks, and in particular events containing four top quarks, t¯tt¯t, can be a sign of new phenomena.
This research project wants to focus on the search for four-top-quark production via Standard Model (SM) processes, allowing the possibility to recast relevant limits within several BSM models considered as benchmarks: t¯tt¯t production via four-top-quarks effective field theory model and from universal extra dimensions scenarios. The SM four-top-quark production cross section at next-to-leading order (NLO) accuracy in QCD is predicted to be 9.2 fb, with scale and parton distribution function (PDF) uncertainties of the order of 30% and 6%, respectively.
The final state topology of a four-top-quark event is determined by the decays of each of the W-bosons, assuming that the top quark decays exclusively through t → Wb. All four-top-quark decay topologies can therefore be divided in two subgroups, further characterized by the dominant backgrounds: top-quark pair production in association with jets (t¯t+jets) in events with one isolated charged lepton (electron or muon) or with two opposite-sign charged leptons; fake/non-prompt leptons, t¯t + Z/W and diboson production in events with two isolated same-sign leptons and in multi-leptonic events.
Previous searches for four-top-quark production using Run II data at √s = 13 TeV were performed both by the ATLAS and CMS collaborations, with no significant observed excess of data above the background expectation. Among them, the CMS search in the same-sign dilepton and multilepton final states has obtained an observed (expected) upper limit on the SM production cross-section for t¯tt¯t of 41.7 fb (20.8 fb) at the 95% confidence level.
In all decay channels a search for four-top-quark SM production can exploit the high multiplicity of jets identified as containing b-hadrons (b-tagged jets) and the high scalar sum of the jet transverse momenta. Further improvement in search sensitivity comes in the same-sign and multilepton final states also from cathegorization of events by lepton multiplicities.
With an estimated final integrated luminosity of 100 fb−1 at the end of Run II, the limit on t¯tt¯t production (or, equivalently, the total uncertainty on the measured cross section) is expected to be eventually dominated by systematic uncertainties (differently from previous searches), and in particular from modelling of the dominant backgrounds less accurately described in such extreme regimes by Monte Carlo techniques (e.g fake/non-prompt leptons). Therefore particular attention has to be devoted to study the possibility of designing or improving existing data-driven techniques, in order to estimate more accurately these backgrounds, in particular in high jet and b-jet multiplicity regions with higher sensitivity to this signal.