The search for the A 1 meson

Abstract

We review the problem of the apparent experimental absence of the A 1 meson. Although it is required to exist, with rather definite properties, by both the quark model and chiral symmetry, it has not been found. We give a unified set of cross-section predictions for its production in forward and backward non-diffractive πN and KN reactions, e +e − → τ + τ − → μ ±A 1 ± + neutrals, and νN → μ + A 1 + X. Most predictions are comparable to or exceed current sensitivity limits, so a signal could have been seen. Recent indirect evidence about the A 1, such as the relevance of other axial vector particles, is discussed. In particular recent measurements with polarized beam and target imply that A 1 reggeon exchange in pp → pp does occur with the strength expected from meson dominance of the axial vector weal current. All our results are summarized in table 1.