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 or 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 $W'\rightarrow WZ$ for bosons in the transverse momentum range 200 GeV $<p_\text {T}<$ 400 GeV in $\sqrt{s}=8$ TeV pp collisions with the ATLAS detector at the LHC. For Z-boson tagging efficiencies of $\epsilon _Z=90$ , 50, and $10\,\%$ , one can achieve $W^+$ -boson tagging rejection factors ( $1/\epsilon _{W^+}$ ) 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 $p_\text {T}$ , hadronically decaying Z bosons. However, the modelling of the tagger inputs for boosted W bosons is studied in data using a $t\bar{t}$ -enriched sample of events in 20.3 fb ${}^{-1}$ of data at $\sqrt{s}=8$ TeV. The inputs are well modelled within uncertainties, which builds confidence in the expected tagger performance.