Hadron production in two-photon collisions

Abstract

The contributions of two-photon physics to meson spectroscopy are first of all the measured γγ widths in Table 1. Further results and conclusions can be summarized as follows: •• Pseudoscalars: the pseudoscalar mixing angle is φp = (−22.2 ±1.2)°; non-qq admixtures, like gluonium, are not needed; no other light 0− states, like radial excitations, are observed; the measured γγ width of the ηc is consistent with the charmonium model.•• Tesor Mesons: Well measured, well understood multiplet; mixing angle ≈ 28°.•• Axial vectors: φ1(1285) and φ1(1420) could be members of the same nonet with mixing angle 45–57°; an exotic explanation is not required by the γγ data.•• Scalar Mesons: the measured γγ width of α0(980) and φ0(975) are smaller than expected for qq states, but consistent with four-quark or KK molecule models; no other I=0 state has been observed.•• D-wave qq(2−+): γγ → π2(1680) → π0π0π0 observed, Γγγ = (1.4±0.3) keV.•• Glueballs: Non-observation of η(1440), φ2(1720) in γγ rreactions supports glueball hypotheses.•• Vector meson pairs: All channels involving ϱ,ω,φ,K∗ are studied; contributions from four-quark states not yet clear; no other good explanation for γγ → ϱϱ near threshold.