Charmonium spectrum in an unquenched quark model

TL;DR

This study refines the charmonium spectrum by incorporating virtual light quark pairs using a novel unquenched quark model, leading to improved agreement with experimental data and insights into the nature of the X(3872).

Contribution

It introduces a new unquenched quark model with feedback fitting and sums over intermediate states up to 2S, enhancing the accuracy of charmonium mass predictions.

Findings

Mass shifts are generally small after refitting.

The $ ext{chi}_{cJ}(2P)$ multiplet experiences significant upward shifts.

The $car{c}$ ${2}^3P_1$ state is pushed above the $D^*ar{D}$ threshold.

Abstract

The effects of virtual light quark pairs on the charmonium spectrum are studied. Pair creation is modelled with a ``$^{3}P_{0}$" vertex and intermediate states are summed up to 2S excitations. Quark model parameters are obtained by fitting to 12 well-known charmonium states, allowing for feedback between the decaying particle and the induced mass shifts. Both of these technical steps are new and improve agreement with the experimental spectrum. In general, the masses receive small shifts once model parameters are refit. This is true in almost cases except the $\chi_{cJ}(2P)$ multiplet, which experiences upwards mass shifts of order 150 MeV, has the ordering of the multiplet rearranged, and pushes the erstwhile $c\bar{c}$ ${2}^3P_1$ state well above $D^*\bar{D}$ threshold--observations that clarify the nature of the enigmatic $X(3872)$