Evidence for a Spin-1 Particle Produced by Two Photons

Two-photon production of ${K}_{S}^{0}{K}^{\ifmmode\pm\else\textpm\fi{}}{\ensuremath{\pi}}^{\ensuremath{\mp}}$ states has been studied by the TPC/Two-Gamma experiment at the SLAC storage ring PEP. A resonance of mass 1.42 GeV was seen when one of the photons was quite virtual but not when both photons were nearly real. Production of a spin-1 meson, which cannot be made by two real photons, would fit these observations. The ${Q}^{2}$ dependence of the data in the resonance region agrees with this spin assignment and is incompatible with a spin-0 hypothesis. The mass and width of the resonance are similar to those of the $E$ meson, which has been assigned ${J}^{P}={0}^{\ensuremath{-}}$ and ${J}^{P}={1}^{+}$ by different experiments.