$Z$ cross-section measurement at $\sqrt{s}=7$ TeV using the channel $Z \rightarrow\tau\tau$

This note presents preliminary results for the measurement of the inclusive cross section for production of $Z$ bosons in proton proton collisions at $\sqrt{s}=$7 TeV using decays of the $Z$ to tau lepton final states at LHCb, with up to 248 pb$^{-1}$ of data taken in 2010 and 2011. One tau is identified through its decay to a muon and neutrinos; the other through its decay to an electron or muon and neutrinos. We obtain a result of \begin{align} \sigma(Z \rightarrow \tau \tau; p_T^\tau > 20 ~{\rm GeV}; 2< \eta^\tau <4.5); 60 < m_Z < 120 ~{\rm GeV}) = 82 \pm 8 \pm 7 \pm 4 ~{\rm pb}\end{align} Where the first uncertainty is statistical, the second is systematic, and the third is due to the luminosity. The measurement is compatible with LHCb's measurement in the $Z \rightarrow \mu \mu$ analysis of $74.9 \pm 1.6 \pm 3.8 \pm 2.6$ pb. The ratio of these two numbers allows a comparison of the $Z$ coupling to taus and muons, and gives a value of \begin{equation*} \frac{ \Gamma (Z\rightarrow \tau \tau)}{ \Gamma (Z\rightarrow \mu \mu)} = 1.09\pm0.17 \end{equation*} consistent with lepton universality.