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

The integrated fiducial cross-section and unfolded differential jet massspectrum of high transverse momentum $Z\rightarrow b\overline{b}$ decays aremeasured in $Z\gamma$ events in proton-proton collisions at $\sqrt{s} = 13$TeV. The data analysed were collected between 2015 and 2016 with the ATLASdetector at the Large Hadron Collider and correspond to an integratedluminosity of 36.1 fb$^{-1}$. Photons are required to have a transversemomentum $p_{\mathrm{T}}>175$ GeV. The $Z\rightarrow b\overline{b}$ decay isreconstructed using a jet with $p_{\mathrm{T}} > 200$ GeV, found with theanti-$k_{t}$ $R = 1.0$ jet algorithm, and groomed to remove soft and wide-angleradiation and to mitigate contributions from the underlying event andadditional proton-proton collisions. Two different but related measurements areperformed using two jet grooming definitions for reconstructing the$Z\rightarrow b\overline{b}$ decay: trimming and soft drop. These algorithmsdiffer in their experimental and phenomenological implications regarding jetmass reconstruction and theoretical precision. To identify $Z$ bosons,$b$-tagged $R = 0.2$ track-jets matched to the groomed large-$R$ calorimeterjet are used as a proxy for the $b$-quarks. The signal yield is determined fromfits of background templates extracted from the data to the different jet massdistributions for the two grooming methods. Integrated fiducial cross-sectionsand unfolded jet mass spectra for each grooming method are compared withleading-order theoretical predictions. The results are found to be in goodagreement with Standard Model expectations within the current statistical andsystematic uncertainties.