Abstract
A measurement is presented of the cross section for electroweak production of a Z boson and a photon in association with two jets (Zγjj) in proton-proton collisions. The Z boson candidates are selected through their decay into a pair of electrons or muons. The process of interest, electroweak Zγjj production, is isolated by selecting events with a large dijet mass and a large pseudorapidity gap between the two jets. The measurement is based on data collected at the CMS experiment at sqrt{s} = 13 TeV, corresponding to an integrated luminosity of 35.9 fb−1. The observed significance of the signal is 3.9 standard deviations, where a significance of 5.2 standard deviations is expected in the standard model. These results are combined with published results by CMS at sqrt{s} = 8 TeV, which leads to observed and expected respective significances of 4.7 and 5.5 standard deviations. From the 13 TeV data, a value is obtained for the signal strength of electroweak Zγjj production and bounds are given on quartic vector boson interactions in the framework of dimension-eight effective field theory operators.
Highlights
Background estimationThe dominant source of background to the EW signal stems from quantum chromodynamics (QCD)-induced Zγjj production, such as the Feynman diagram in figure 1
We present a study of EW production of Zγjj that includes a measurement of the production cross section and limits on aQGCs at 13 TeV
The cross sections for EW Zγjj and EW+QCD Zγjj production are measured in a fiducial region designed to approximate the acceptance of the CMS detector and the signal selection requirements based on the particle-level objects: (i) electrons and muons are required to be prompt, and those from τ lepton decays are excluded; (ii) the momenta of prompt photons with ∆R γ < 0.1 are added to the lepton momenta to correct for final-state photon radiation, referred to as “dressing”; (iii) nonprompt photons are excluded; and (iv)
Summary
The central feature of the CMS [14] apparatus is a superconducting solenoid of 6 m internal diameter, providing a magnetic field of 3.8 T. Forward calorimeters extend the coverage provided by the barrel and endcap detectors up to pseudorapidities of |η| = 5. Muons are measured in gas-ionization detectors embedded in the steel flux-return yoke outside the solenoid. Events of interest are selected using a two-level trigger system [15]. The first level (L1), composed of specialized hardware processors, uses information from the calorimeters and muon detectors to select events of interest with a maximum rate of 100 kHz. A highlevel trigger processor farm decreases this rate to 1 kHz before storage. A more detailed description of the CMS detector, together with a definition of the coordinate system and kinematic variables, can be found in ref. [14]
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