Glueballs, Hybrids and the Standard Model: Hadron '97 Summary (experiment) I Introduction

HADRON '97 covered a wide spectrum of particle physics, from high-Q 2 events in e +-proton collisions at DESY to the recent discovery of 1 ?+ exotic mesons. This paper reviews progress reported at the conference, in particular the status of glueballs and hybrids. Personal views and interpretations are oofered, sometimes at variance with accepted views. The role of gluons in hadron spectroscopy is the continuing theme of the HADRON conferences. Also after HADRON '97 this role is still controversial. Many speakers expressed their believe that evidence for the lowest-mass glueball is overwhelming, that it is a scalar state and that it mixes with the isoscalar 1 3 P 0 mesons. But there is no agreement on a number of questions which are very important for a real understanding of how a primordial glueball enters the world of quarkonia: Is the Crystal-Barrel f 0 (1500) the glueball with some qq component mixed in or does the f 0 (1710), the old (1690) with ambiguous spin assignment, play this eminent role? Are we missing a s s scalar meson and why did LASS not nd it? Can we resolve the discrepant results on radiative J== decays? Is the evidence for the f 0 (1370) convincing enough to provide a cornerstone in our understanding of glueballs? Problems of mixing with ordinary mesons do not occur for hybrids carrying exotic quantum numbers. Mesons of qq structure with orbital angular momentum l and total spin s must have parity P = (?1) l+1 and C-parity C = (?1) l+s. Mesons with quantum numbers J P C = 1 ?+ can thus not be oridinary qq system; they are exotic. We shall discuss evidence for the J P C = 1 ?+ wave as evidence for hybrids, for qqg structure. Of course, a interpretation as qq qq state is not excluded.