Fig. 1: Z → μμ background normalization.
From: Test of the universality of τ and μ lepton couplings in W-boson decays with the ATLAS detector
A region enriched in Z → μμ events is defined by selecting events with two muons. This control region is used to extract the Z → μμ normalization, which is applied in the signal region. A histogram with a fixed bin width of 2.5 GeV of the di-muon invariant mass (mμμ) distribution is shown in the figure. The data are shown by black markers and the different components contributing to this region, taken from simulation, are shown by stacked histograms. The different contributions are the primary process of interest (Z → μμ), along with the main backgrounds from events involving top quarks (Top), a pair of W and/or Z bosons (Di-boson processes) and the grouping of all remaining SM processes (Other SM processes). The bottom panel shows the ratio of the data to the predicted expectation after the fit. The uncertainties on the data are the Poisson errors due to the limited size of the data sample. Blue bands indicate the ±1σ systematic uncertainties on the prediction, with the constraints from the analysis fit applied.
