Formation of spin-one mesons by photon-photon fusion

We have observed the formation of two spin-one mesons in γγ fusion reactions in which one photon was highly virtual. The first, consistent with being the JPC=1++ f1(1285), was seen in the final state ηπ+π- (η→γγ), as was the η’(958). We have previously reported the observation of the second spin-one state, the X(1420), in the final state K±KS0π∓ (KS0→π+π-). The formation of this state, which may be the f1(1420) observed in hadronic interactions, is reanalyzed using a new model and more data. We consider whether the X(1420) could be the partner of the f1(1285) in the JPC=1++ meson nonet. The γγ* width of each resonance was determined in several Q2 bins. Using a model due to Cahn with a ρ form factor, we obtained for the coupling parameter Γ̃γγ(f1(1285)) the value 2.4±0.5±0.5 keV. We also found B(X(1420)→KK¯π)Γ̃γγ(X(1420))=1.3±0.5±0.3 keV using a ρ form factor, or 0.63±0.24±0.15 keV using a φ form factor. The decay distributions of the observed X(1420) events are consistent with a decay proceeding via K*(892)K, and favor positive, but do not exclude negative, parity. Assuming that the X(1420) and the f1(1285) are members of the same qq¯ meson nonet and that B(X(1420)→KK¯π)≊1, and using a φ form factor in the model for X(1420) formation, we determined the singlet-octet mixing angle of that nonet. The result, (45.4±6.2)°, agrees well with the 42.2° implied by the Gell-Mann–Okubo quadratic mass formula; it is farther from the 35.3° ideal mixing angle. Our results are thus consistent with the X(1420) being the mostly ss¯ isoscalar member of the axial-vector qq¯ meson nonet; however, we have no evidence contrary to hypotheses that the X(1420) is an exotic q2q¯ 2 or qq¯g state.