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A study of the $Z$ production cross-section in $pp$ collisions at $\sqrt{s}$ = 7 TeV using tau final states

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Abstract

A measurement of the inclusive $Z \to \tau\tau$ cross-section in $pp$ collisions at $\sqrt{s} = 7$ TeV is presented based on a dataset of 1.0 fb$^{-1}$ collected by the LHCb detector. Candidates for $Z \to \tau\tau$ decays are identified through reconstructed final states with two muons, a muon and an electron, a muon and a hadron, or an electron and a hadron. The production cross-section for $Z$ bosons, with invariant mass between 60 and 120 GeV/$c^2$, which decay to $\tau$ leptons with transverse momenta greater than 20 GeV/$c$ and pseudorapidities between 2.0 and 4.5, is measured to be $\sigma_{pp \to Z \to \tau\tau} = 71.4 \pm 3.5 \pm 2.8 \pm 2.5$ pb; the first uncertainty is statistical, the second uncertainty is systematic, and the third is due to the uncertainty on the integrated luminosity. The ratio of the cross-sections for $Z \to \tau\tau$ to $Z \to \mu\mu$ is determined to be $0.93 \pm 0.09$, where the uncertainty is the combination of statistical, systematic, and luminosity uncertainties of the two measurements.

Figures and captions

Invariant mass distributions for the \subref{fig:mumu} $\tau_\mu\tau_\mu$ , \subref{fig:mue} $\tau_\mu\tau_e$ , \subref{fig:emu} $\tau_e\tau_\mu$ , \subref{fig:muh} $\tau_\mu\tau_ h $ , and \subref{fig:eh} $\tau_e\tau_ h $ candidates with the excluded mass range indicated for $\tau_\mu\tau_\mu$ . The $ Z \to \tau\tau $ simulation (solid red) is normalised to the number of signal events. The $\mathrm{QCD}$ (horizontal green), electroweak (vertical blue), and $ Z$ (solid cyan) backgrounds are estimated from data. The $ {t\bar{t}}$ (vertical orange) and $ {WW}$ (horizontal magenta) backgrounds are estimated from simulation and generally not visible.

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Measured cross-sections for the $ Z$ decaying to the final states $\tau_\mu\tau_\mu$ , $\tau_\mu\tau_e$ , $\tau_e\tau_\mu$ , $\tau_\mu\tau_ h $ , and $\tau_e\tau_ h $ (open points) compared with theory (yellow band) and the combined $ Z \to \tau\tau $ and LHCb $ Z \to \mu\mu $ measurements (closed points) where $ p_\mathrm{T}$ and $\eta$ are the transverse momentum and pseudorapidity of the leptons, and $M$ is the di-lepton invariant mass. The inner error bars represent statistical uncertainty while the outer error bars represent combined statistical, systematic, and luminosity uncertainties. The central theory value is given by the light yellow line while the associated uncertainty by the orange band.

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Animated gif made out of all figures.

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Tables and captions

Acceptance factors, branching fractions, selection efficiencies, numbers of background and observed events for each $ Z \to \tau\tau $ analysis stream.

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Systematic uncertainties expressed as a percentage of the cross-section for each $ Z \to \tau\tau $ analysis stream. Contributions from acceptance $\mathcal{A}$ , branching fractions $\mathcal{B}$ , number of background events $N_\mathrm{bkg} $, reconstruction efficiencies $ {\varepsilon_{\mathrm{rec} }}$ , and selection efficiencies $ {\varepsilon_{\mathrm{sel} }}$ are listed. The superscripts on $ {\varepsilon_{\mathrm{trk} }} ^{(i)}$ and $ {\varepsilon_{\mathrm{id} }} ^{(i)}$ indicate the first or second $\tau$ lepton decay product candidate. The percentage uncertainties on the cross-section for $N_\mathrm{bkg} $ are quoted for each individual background, as well as the total background. The efficiency uncertainties are split in a similar fashion.

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Created on 09 December 2018.Citation count from INSPIRE on 18 December 2018.