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

Abstract

We present a measurement of the dipion mass spectrum in the decay X(3872)-J/ 7+i using a 360 pbsample of pp collisions at va = 1.96 TeV collected with the CDF II detector at the Fermnilab Tevatron Collider. As a benchmark, we also extract the dipion mass distribution for 4(2S) J/-+7r-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/po. We find that the measured spectrum is compatible with 3 S 1 charmonium decaying to J/+ 7+and with the X(3872)-J/bpO hypothesis. There is, however, no 3 S 1 charmonium state available for assignment to the X(3872). The multipole expansion calculations for 1 P 1 and 3 Dj states are in clear disagreement with the X(3872) data. For the 4(2S) the data agrees well with previously published results and to multipole expansion calculations for 3 S 1 charmonium. Other, non-charmonium, models for the X(3872) are described too. We conclude that since the dipion mass spectrum for X(3872) is compatible with J/p hypothesis, the X(3872) should be C-positive. This conclusion is supported by recent results from Belle Collaboration which observed X(3872)-+ J/V7y decay. We argue that if X(3872) is a charmonium, then it should be either 1 1 D 2-+ or 2 3 P 1 ++ state, decaying into J/ i+Tin violation of isospin conservation. A non-charmonium assignment, such as DD* molecule, is also quite possible.