  The CP-violating weak phase $\phi_\mathrm{s}$ and the decay width difference
$\Delta\Gamma_{\mathrm{s}}$ of $\mathrm{B}^{0}_{\mathrm{s}}$ mesons are
measured by the CMS experiment at the LHC using a data sample of
$\mathrm{B}^{0}_{\mathrm{s}} \! \to \mathrm{J} / \psi\,\phi$ decays. The
analysed dataset corresponds to an integrated luminosity of about
$20~\mathrm{fb}^{-1}$ collected in $pp$ collisions at a centre-of-mass energy
$\sqrt{s} = 8~\mathrm{TeV}$. A total of $49\,000$ reconstructed
$\mathrm{B}^{0}_{\mathrm{s}}$ decays are used to extract the values
$\phi_\mathrm{s}$ and $\Delta\Gamma_\mathrm{s}$ by performing a time-dependent
and flavour-tagged angular analysis of the $\mu^+\mu^-\mathrm{K}^+\mathrm{K}^-$
final state.The weak phase is measured to be $\phi_\mathrm{s} = -0.03 \pm
0.11~\mathrm{(stat.)} \pm 0.03~\mathrm{(syst.)}~\mathrm{rad}$, and the decay
width difference between the $\mathrm{B}^{0}_{\mathrm{s}}$ mass eigenstates is
$\Delta\Gamma_\mathrm{s} = 0.096 \pm 0.014~\mathrm{(stat.)} \pm
0.007~\mathrm{(syst.)}~\mathrm{ps}^{-1}$.
