Abstract
The integrated fiducial cross-section and unfolded differential jet mass spectrum of high transverse momentum Z→bb‾ decays are measured in Zγ events in proton–proton collisions at s=13TeV. 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 36.1fb−1. Photons are required to have a transverse momentum pT>175GeV. The Z→bb‾ decay is reconstructed using a jet with pT>200GeV, found with the anti-ktR=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→bb‾ 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.
Highlights
This Letter presents a measurement of the fiducial and differential jet mass cross-sections of high transverse momentum Z bosons that decay into bbpairs and are produced in association with a photon, denoted by Z (→ bb)γ
A detailed study of the Z → bbsignal is important for assessing systematic uncertainties and identification techniques for the measurement of H → bbin the high-pT range, as well as for potential TeV-scale resonances decaying into dibosons, one of them being a Z boson or a Higgs boson decaying into bb [3,4]
The signal yield is extracted by simultaneously fitting the signal and the background templates described in Section 5 to the observed Z → bbcandidate jet mass distribution
Summary
This Letter presents a measurement of the fiducial and differential jet mass cross-sections of high transverse momentum (pT) Z bosons that decay into bbpairs and are produced in association with a photon, denoted by Z (→ bb)γ. Proton (pp) collision data collected in 2015 and 2016 by the ATLAS detector [1√] at the Large Hadron Collider (LHC) at a center-of-mass energy of s = 13 TeV. This measurement of the unfolded jet mass spectrum of hadronically decaying Z bosons at the LHC explores the experimental features and phenomenological implications of techniques used to reconstruct boosted bosons – colour singlets – decaying into bb. A detailed study of the Z → bbsignal is important for assessing systematic uncertainties and identification techniques for the measurement of H → bbin the high-pT range, as well as for potential TeV-scale resonances decaying into dibosons, one of them being a Z boson or a Higgs boson decaying into bb [3,4]
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