  The first model-independent measurement of the charm mixing parameters in the
decay $D^0 \to K_S \pi^+ \pi^-$ is reported, using a sample of $pp$ collision
data recorded by the LHCb experiment, corresponding to an integrated luminosity
of 1.0 fb$^{-1}$ at a centre-of-mass energy of 7 TeV. The measured values are
\begin{eqnarray*} x &=& (-0.86 \pm 0.53 \pm 0.17) \times 10^{-2}, \\ y &=&
(+0.03 \pm 0.46 \pm 0.13) \times 10^{-2}, \end{eqnarray*} where the first
uncertainties are statistical and include small contributions due to the
external input for the strong phase measured by the CLEO collaboration, and the
second uncertainties are systematic.
