Measurement of $Z\rightarrow\tau^+\tau^-$ production in proton-proton collisions at $\sqrt{s} = 8$ TeV

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A measurement of $Z\rightarrow\tau^+\tau^-$ production cross-section is presented using data, corresponding to an integrated luminosity of 2 fb$^{-1}$, from $pp$ collisions at $\sqrt{s}=8$ TeV collected by the LHCb experiment. The $\tau^+\tau^-$ candidates are reconstructed in final states with the first tau lepton decaying leptonically, and the second decaying either leptonically or to one or three charged hadrons. The production cross-section is measured 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. The cross-section is determined to be $\sigma_{pp\rightarrow{}Z\rightarrow{}\tau^+\tau^-} = 95.8 \pm 2.1 \pm 4.6 \pm 0.2 \pm 1.1 \mathrm{pb}$, where the first uncertainty is statistical, the second is systematic, the third is due to the LHC beam energy uncertainty, and the fourth to the integrated luminosity uncertainty. This result is compatible with NNLO Standard model predictions. The ratio of the cross-sections for $Z\rightarrow\tau^+\tau^-$ to $Z\rightarrow\mu^+\mu^-$ ($Z\rightarrow{}e^+e^-$), determined to be $1.01 \pm 0.05$ ($1.02 \pm 0.06$), is consistent with the lepton-universality hypothesis in $Z$ decays.

Figures and captions

Distributions of invariant (dashed line) and corrected (full line, shaded) mass of $\tau_{h Z \rightarrow \tau ^+\tau ^- }$ (blue) events and the $ V j$ (red) background.

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Invariant-mass distributions for (a) $\tau_{\mu}\tau_{\mu}$ {} , (b) $\tau_{e}\tau_{e}$ {} , (c) $\tau_{\mu}\tau_{h}$ candidates with the excluded mass ranges indicated by the gray areas. The $ Z \rightarrow \tau ^+\tau ^- $ simulation (red) is normalised to the observed signal. The $ Z $ (blue), QCD (brown), and electroweak (magenta) backgrounds are estimated from data. The $ t \overline t $ , $ VV$ backgrounds and cross-feed (green) are estimated from simulation (see text) and generally not visible.

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Summary of the measurements of $ Z \rightarrow l^+l^-$ production cross-section inside the LHCb acceptance region from $pp$ collisions at 8$\mathrm{ Te V}$. The error bar represents the total uncertainty. The dotted inner error bar corresponds to the statistical contribution. The coloured band corresponds to the combined measurement of $ Z \rightarrow \tau ^+\tau ^- $ from this analysis. The last 7 rows represent the NNLO predictions with different parameterizations of the PDFs.

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Animated gif made out of all figures.


Tables and captions

Expected backgrounds yields and total number of candidates observed. In the last row the uncertainties are the statistical and systematic contributions combined.

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Relative uncertainties of the various contributions affecting the cross-section measurement, given in percent. The uncertainties are correlated between streams, except in rows denoted with $^\dagger$.

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Created on 20 February 2021.