Determination of the X(3872) meson quantum numbers

The quantum numbers of the X(3872) meson are determined to be $J^{PC }= 1^{++}$ based on angular correlations in $B^+ \to X(3872) K^+$ decays, where $X(3872) \to \pi^+\pi^- J/\psi$ and $ J/\psi \to \mu^+\mu^-$. The data correspond to 1.0 fb$^{-1}$ of $pp$ collisions collected by the LHCb detector. The only alternative assignment allowed by previous measurements, $J^{PC}=2^{-+}$, is rejected with a confidence level equivalent to more than eight Gaussian standard deviations using the likelihood-ratio test in the full angular phase space. This result favors exotic explanations of the X(3872) state.