Measurement of the jet mass in high transverse momentum Production at using the ATLAS detector

The integrated fiducial cross-section and unfolded differential jet mass spectrum of high transverse momentum $Z\to b\stackrel{‾}{b}$ decays are measured in Zγ events in proton–proton collisions at $\sqrt{s}=13\phantom{\rule{0.25em}{0ex}}\text{TeV}$. 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 $36.1\phantom{\rule{0.25em}{0ex}}{\text{fb}}^{-1}$. Photons are required to have a transverse momentum ${p}_{T}>175\phantom{\rule{0.25em}{0ex}}\text{GeV}$. The $Z\to b\stackrel{‾}{b}$ decay is reconstructed using a jet with ${p}_{T}>200\phantom{\rule{0.25em}{0ex}}\text{GeV}$, 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 $Z\to b\stackrel{‾}{b}$ 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 the data-driven background templates 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.