On the Spin of the X(3872)

TL;DR

This paper re-analyzes experimental data on the X(3872) resonance to determine whether it is an axial or tensor state, using a comprehensive statistical approach to interpret angular and mass distributions from multiple decay channels.

Contribution

It provides a rigorous combined analysis of decay data to distinguish the quantum numbers of X(3872), clarifying the viability of 1++ versus 2-+ hypotheses.

Findings

The 1++ hypothesis has a 5.5% agreement probability with data.

The 2-+ hypothesis has a 0.1% agreement probability with data.

Separate decay channels suggest different preferred quantum states.

Abstract

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 2-+ state based on the 3 pions spectrum in the X -> J/psi omega decays. Furthermore, the Belle collaboration published the 2 pions invariant mass and spin-sensitive angular distributions in X -> J/psi rho decays. Starting from a general parametrization of the decay amplitudes for the axial and tensor quantum numbers of the X, we re-analyze 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/psi rho (angular and mass distributions) and J/psi omega (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.