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The quantum numbers of the Xð3872Þ meson are determined to be J PC ¼ 1 þþ based on angular correlations in B þ ! Xð3872ÞK þ decays, where Xð3872Þ ! þ À J=c and J=c ! þ À . The data correspond to 1:0 fb À1 of pp collisions collected by the LHCb detector. The only alternative assignment allowed by previous measurements J PC ¼ 2 Àþ is rejected with a confidence level equivalent to more than 8 Gaussian standard deviations using a likelihood-ratio test in the full angular phase space. This result favors exotic explanations of the Xð3872Þ state.

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