  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.
