Spin of the X(3872)

Whether the much-studied X(3872) is an axial or tensor resonance makes an important difference to its interpretation. A recent paper by the BABAR Collaboration raised the viable hypothesis that it might be a JPC=2 -+ state based on the π +π -π0 spectrum in the X→J/ψω decays. Furthermore, the Belle Collaboration published the π +π - invariant mass and spin-sensitive angular distributions in X→J/ψρ decays. Starting from a general parametrization of the decay amplitudes for the axial and tensor quantum numbers of the X, we reanalyze the whole set of available data. The level of agreement of the two spin hypotheses with data is interpreted with a rigorous statistical approach based on Monte Carlo simulations in order to be able to combine all the distributions regardless of their different levels of sensitivity to the spin of the X. Our analysis returns a probability of 5.5% and 0.1% for the agreement with data of the 1 ++ and 2 -+ hypotheses, respectively, once we combine the whole information (angular and mass distributions) from both channels. On the other hand, the separate analysis of J/ψρ (angular and mass distributions) and J/ψω (mass distribution) indicates that the 2 -+ assignment is excluded at the 99.9% C.L. by the former case, while the latter excludes at the same level the 1 ++ hypothesis. There are therefore indications that the two decay modes behave in a different way. © 2012 American Physical Society.