Measurement of the Dipion Mass Spectrum in the Decay X(3872) → J/Ψ π<sup>+</sup> π<sup>-</sup> at the CDF II Experiment

Abstract

The author presents a measurement of the dipion mass spectrum in the decay X(3872) → J/Ψπ+ π- using a 360 pb-1 sample of p$\bar{p}$ collisions at √s = 1.96 TeV collected with the CDF II detector at the Fermilab Tevatron Collider. As a benchmark, they also extract the dipion mass distribution for Ψ(2S) → J/Ψπ+ π- decay. The X(3872) dipion mass spectrum is compared to QCD multipole expansion predictions for various charmonium states, as well as to the hypothesis X(3872) → J/Ψρ0. They find that the measured spectrum is compatible with 3S1 charmonium decaying to J/Ψπ+ π- and with the X(3872) → J/Ψρ0 hypothesis. There is, however, no 3S1 charmonium state available for assignment to the X(3872). The multipole expansion calculations for 1P1 and 3DJ states are in clear disagreement with the X(3872) data. For the Ψ(2S) the data agrees well with previously published results and to multipole expansion calculations for 3S1 charmonium. Other, non-charmonium, models for the X(3872) are described too. They conclude that since the dipion mass spectrum for X(3872) is compatible with J/Ψρ0 hypothesis, the X(3872) should be C-positive. This conclusion is supported by recent results from Belle Collaboration which observed X(3872) → J/Ψγ decay. They argue that if X(3872) is a charmonium, then it should be either 1D2± or 23P1++ state, decaying into J/Ψπ+ π- in violation of isospin conservation. A non-charmonium assignment, such as D$\bar{D}$* molecule, is also quite possible.