Measurement of the jet mass in high transverse momentum $Z(\to b\overline{b})\gamma$ 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 $Z\to b\overline{b}$ 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 $Z\to b\overline{b}$ 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 $Z\to b\overline{b}$ 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.

37 pages in total, author list starting page 21, 3 figures, 5 tables, submitted to Phys. Lett. B. All figures including auxiliary figures are available at https://atlas.web.cern.ch/Atlas/GROUPS/PHYSICS/PAPERS/STDM-2017-14/

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