  Angular distributions of the decay $\mathrm{B}^0 \to \mathrm{K}^{*0} \mu^+
\mu^-$ are studied using a sample of proton-proton collisions at $\sqrt{s} = $
8 TeV collected with the CMS detector at the LHC, corresponding to an
integrated luminosity of 20.5 fb$^{-1}$. An angular analysis is performed to
determine the $P_1$ and $P_5'$ parameters, where the $P_5'$ parameter is of
particular interest because of recent measurements that indicate a potential
discrepancy with the standard model predictions. Based on a sample of 1397
signal events, the $P_1$ and $P_5'$ parameters are determined as a function of
the dimuon invariant mass squared. The measurements are in agreement with
predictions based on the standard model.
