Boosted Higgs ($\rightarrow b\bar{b}$) Boson Identification with the ATLAS Detector at $\sqrt{s} = 13$ TeV

This document summarises the baseline boosted Higgs boson identification strategy for Run 2 data collected by the ATLAS detector at a centre-of-mass energy of $\sqrt{s} = 13$ TeV, as updated for the analysis of the 2015 and 2016 data. Boosted $H\rightarrow b\bar{b}$ decays are reconstructed from jets found with the anti-$k_{t}$ $R = 1.0$ jet algorithm, that are trimmed using a subjet radius of $R_{\text{subjet}} = 0.2$ and a minimum transverse momentum fraction of $f_{\text{cut}} = 5\%$. To tag Higgs bosons, requirements are applied on the following quantities: $b$-jets identified using $R = 0.2$ track jets matched to the large-$R$ calorimeter jet, the trimmed jet mass and the trimmed jet energy correlation ratio $D_2^{\beta=1}$. The Higgs boson tagging efficiency and corresponding multi-jet and hadronic top background rejections in simulated events are presented. Several benchmark tagging selections are defined based on specific signal efficiency targets. Systematic uncertainties on the Higgs tagging efficiency and background rejection resulting from uncertainties on the $b$-tagging efficiency, mass and $D_2^{\beta=1}$ variables are provided. In addition, the $b$-tagging performance at small $b$-quark angular separations and the modelling of the jet properties, including jet substructure variables, are tested in a high $p_\text{T}$ $g\rightarrow b\bar{b}$ rich sample of large-$R$ jets from $pp$ collisions at $\sqrt{s} = 13$ TeV recorded in 2015.