A new method to distinguish hadronically decaying boosted Z bosons from W bosons using the ATLAS detector

The distribution of particles inside hadronic jets produced in the decay of boosted W and Z bosons can be used to discriminate such jets from the continuum background. Given that a jet has been identified as likely resulting from the hadronic decay of a boosted W and Z boson, this paper presents a technique for further differentiating Z bosons from W bosons. The variables used are jet mass, jet charge, and a b-tagging discriminant. A likelihood tagger is constructed from these variables and tested in the simulation of W′ → W Z for bosons in the transverse momentum range 200 GeV < p_T <  400 GeV in √s = 8 TeV pp collisions with the ATLAS detector at the LHC. For Z-boson tagging efficiencies of ϵ_Z = 90, 50, and 10%, one can achieve W⁺-boson tagging rejection factors (1/ϵ_W+) of 1.7, 8.3 and 1000, respectively. It is not possible to measure these efficiencies in the data due to the lack of a pure sample of high p_T, hadronically decaying Z bosons. However, the modelling of the tagger inputs for boosted W bosons is studied in data using a tt¯-enriched sample of events in 20.3 fb⁻¹ of data at √s = 8 TeV. The inputs are well modelled within uncertainties, which builds confidence in the expected tagger performance.