Evidence for spin-one resonance production in the reaction γγ*→π+π-π0π0

Using data from the TPC/Two-Gamma experiment at the SLAC e+e- storage ring PEP, a C=+1 resonance has been observed in the π+π-π0γ final state resulting from the fusion of one nearly real and one quite virtual photon. The actual decay channel is probably π+π-π0π0, where one final-state photon is not detected, and the mass of the fully reconstructed state would be approximately 1525 MeV. A four-pion decay mode in turn implies that the resonance has even isospin. The nonobservation of this R(1525) when both initial-state photons are nearly real suggests a spin-1 assignment. Since the large measured value of the product of the branching ratio into π+π-π0π0 and the γγ coupling makes it unlikely that this state is the mostly s̄s f1(1510), its interpretation may lie outside of conventional meson spectroscopy. There is a second, less-significant enhancement observed in the same reaction at a four-pion mass centered around 2020 MeV.