Measurement of the jet mass in high transverse momentum $Z(\rightarrow b\overline{b})γ$ production at $\sqrt{s}= 13$ TeV using the ATLAS detector

Abstract: The integrated fiducial cross-section and unfolded differential jet mass spectrum of high transverse momentum $Z\rightarrow b\overline{b}$ decays are measured in $Z\gamma$ events in proton-proton collisions at $\sqrt{s} = 13$ TeV. The data analysed were collected between 2015 and 2016 with the ATLAS detector at the Large Hadron Collider and correspond to an integrated luminosity of 36.1 fb$^{-1}$. Photons are required to have a transverse momentum $p_{\mathrm{T}}>175$ GeV. The $Z\rightarrow b\overline{b}$ decay is reconstructed using a jet with $p_{\mathrm{T}} > 200$ GeV, found with the anti-$k_{t}$ $R = 1.0$ jet algorithm, and groomed to remove soft and wide-angle radiation and to mitigate contributions from the underlying event and additional proton-proton collisions. Two different but related measurements are performed using two jet grooming definitions for reconstructing the $Z\rightarrow b\overline{b}$ decay: trimming and soft drop. These algorithms differ in their experimental and phenomenological implications regarding jet mass reconstruction and theoretical precision. To identify $Z$ bosons, $b$-tagged $R = 0.2$ track-jets matched to the groomed large-$R$ calorimeter jet are used as a proxy for the $b$-quarks. The signal yield is determined from fits of background templates extracted from the data to the different jet mass distributions for the two grooming methods. Integrated fiducial cross-sections and unfolded jet mass spectra for each grooming method are compared with leading-order theoretical predictions. The results are found to be in good agreement with Standard Model expectations within the current statistical and systematic uncertainties.