Issues in light hadron spectroscopy

Abstract

A high priority in light spectroscopy is to seek out and characterize various types of non-$(Q\bar{Q}$) meson. The large quantity of new data now appearing will present a great opportunity. To identify the non-$(Q\bar{Q}$) intruders one needs to know the regular $(Q\bar{Q}$) pattern well; whole meson families thus become a target for close investigation. A powerful discovery strategy is to observe the same meson in a variety of reactions. Because mesons appear as resonances, other dynamics can distort the signal in a particular decay channel. Unitarity is the master principle for co- ordinating various sightings of the same resonance. Much of the new spectroscopic information in prospect will come from inferring two-body dynamics from three-body final states. Conventional methods of analysis via the isobar model use approximations to unitarity that need validation. Of all the meson families, the scalars should be a prime hunting ground for non-$(Q\bar{Q}$)'s. Even before the advent of the new results, some revisions of the `official' classifications are urged. In particular, it is argued that the lightest broad $I=0$ scalar is a very broad $f_0$ (1000). One unfinished task is to decide whether $f_0$ (975) and $a_0$ (980) are alike or different; several non-$(Q\bar{Q}$) scalar scenarios hinge on this. To settle this, much better data