Speaker
Dr
Mikhail Barabanov
(Joint Institute for Nuclear Research)
Description
The spectroscopy of charmonium-like mesons with masses above the 2mD open charm threshold has been full of surprises and remains poorly understood. The currently most compelling theoretical descriptions of the mysterious XYZ mesons attribute them to hybrid structure with a tightly bound cc\bar diquark or cq(cq')\bar tetraquark core that strongly couples to S-wave DD*\bar molecular-like structures. In this picture, production of a XYZ particle in high energy hadron collisions and its decays into light hadron plus charmonium final states proceed via the core component of the meson, while decays to pairs of open charmed mesons proceed via the DD*\bar component. These ideas have been applied with some success to the X(3872), where a detailed calculation finds a cc\bar core component that is only above 5% of the time with the DD*\bar component accounting for the rest. In this picture, the X(3872) is compose of three rather disparate components: a small charmonium-like cc\bar core with r_rms < 1 fm, a larger D^+D^*- component with r_rms ≈ 1.5 fm and a dominant component D^0D^*0\bar with a huge, r_rms > 9 fm spatial extent. The experiments with pp and pA collisions (p ≤ 26 Gev/c and L ≤ 10^32 cm-2s-1) are well suited to test this picture for X(3872) and, possibly, other XYZ mesons. In near threshold production experiments (√SpN ≈ 8 GeV), X(3872) mesons can be produced with typical kinetic energies of a few hundred MeV. In the case of X(3872), its decay length will be greater than 50 fm while the distance scale for the cc\bar → D^0D^0*\bar transition would be 2 ~ 3 fm. Since the survival probability of an r_rms ~ 9 fm “molecular” inside nuclear matter should be very small, X(3872) meson production on a nuclear target with r_rms ~ 5 fm or more (A ~ 60 or larger) should be strongly quenched. Thus, if the hybrid picture is correct, the atomic number dependence of X(3872) production at fixed √SpN should have a dramatically different behavior than that of the ψ', which is long lived compact charmonium state.
The current experimental status of XYZ mesons together with hidden charm tetraquark candidates and present simulations what we might expect from A-dependence of X(3872) in pp and pA collisions are summarized.
