Measurement of the Inclusive Z Cross Section via Decays to Tau Pairs in pp Collisions at sqrt(s)=7 TeV

TL;DR

The paper reports a CMS measurement of the inclusive Z boson production cross section times the branching ratio to tau pairs in pp collisions at 7 TeV, using four final states that include hadronically decaying taus. By combining data-driven background estimation with a fit to the visible mass distributions, the analysis achieves a precise σ(pp → Z X) × B(Z → τ^+τ^-) = 1.00 ± 0.05 (stat) ± 0.08 (syst) ± 0.04 (lumi) nb, in agreement with NNLO QCD predictions. It validates the hadronic tau reconstruction efficiency and provides a cross-check against Z decays to e^+e^- and μ^+μ^-, contributing to a more precise understanding of τ identification in CMS. The results demonstrate the viability of tau-based Z measurements and improve the calibration of tau reconstruction in the CMS detector.

Abstract

A measurement of inclusive Z to tau^+ tau^- production in pp collisions is presented, in the final states mu+hadrons, e+hadrons, e+mu, and mu+mu. The data sample corresponds to an integrated luminosity of 36 inverse picobarns collected with the CMS detector at the LHC. The measured cross section is sigma(pp to Z) B(Z to tau^+ tau^-) = 1.00 +/- 0.05 (stat.) +/- 0.08 (syst.) +/-0.04 (lumi.) nb, which is in good agreement with the next-to-next-to-leading order QCD prediction and with previous measurements in the Z to e^+e^- and mu^+ mu^- channels. The reconstruction efficiency for hadronic tau decays is determined with a precision of 7%.

Figures (3)

• Figure 1: Muon+${E\!\!\!/}_{\mathrm{T}}\xspace$ transverse mass (left) for the $\tau_{\mu}\tau_{\rm had}$ final state. Likelihood discriminant (right) for the $\tau_{\mu}\tau_{\mu}$ final state. The arrows show the final selection criteria applied.
• Figure 2: Visible mass distributions of the $\tau_{\mu}\tau_{\rm had}$ (top left), $\tau_\mathrm{e}\tau_{\rm had}$ (top right), $\tau_\mathrm{e}\tau_{\mu}$ (bottom left), and $\tau_{\mu}\tau_{\mu}$ (bottom right) final states.
• Figure 3: Likelihood contours for the joint parameter estimation of the cross section and the $\tau$-identification (left). The fitted central values (solid line) and their estimated 1$\sigma$ uncertainties (dashed lines) are also shown. Summary of the measured $\mathrm{Z}\rightarrow\tau^+\tau^-$ cross sections in the $\tau_{\mu} \tau_{\rm had}$, $\tau_\mathrm{e} \tau_{\rm had}$, $\tau_\mathrm{e} \tau_{\mu}$, and $\tau_{\mu} \tau_{\mu}$ final states, in the invariant mass range of $60 < M_{\tau^+\tau^-} < 120$$\,\text{Ge\spaceV}$ (right). The inner error bar represents the statistical uncertainty. The extracted cross section from the combined fit and the NNLO theoretical prediction are also shown.