Speaker
Description
Author: Bernhard Ketzer
The unambiguous detection of exotic non-qqbar states and their identification in terms of quark and gluonic degrees of freedom is one of the most ambitious goals of hadron spectroscopy. In the charm- and bottom-quark regime, many of the new $X, Y, Z$ states are believed to be of exotic nature, but until now the internal structure of almost all of them is still a matter of debate. One of the reasons for this is that all of the newly discovered states have quantum numbers that can in principle also be formed by a qqbar pair.
States with spin-exotic quantum numbers provide a smoking gun of non-qqbar configurations. In the light-quark sector, though generally believed to be more challenging both experimentally and theoretically because of the wide and overlapping nature of states, several signals with spin-exotic quantum numbers have been claimed in the past. High-statistics data from the COMPASS-experiment have settled a decade-long dispute on the resonant nature of some of these. Together with theory predictions, the $\pi_1(1600)$ is now generally accepted as the lightest hybrid meson in nature. Very recently, BESIII has announced the discovery of the $\eta_1(1855)$, which may be the isoscalar partner of the $\pi_1(1600)$. In order to fully understand the spectrum of light hybrid mesons, further work on branching ratios and the identification of possible strange partners in the multiplet are needed. The talk will summarize the status of exotic-meson searches in the light-quark sector and discuss future plans in this direction.
