On the near-threshold peak structure in the differential cross section of \phi-meson photoproduction: indication of a missing resonance with non-negligible strangeness content

摘要

The details of the analysis of the near-threshold bump structure in theforward differential cross section of the \phi-meson photoproduction todetermine whether it is a signature of a resonance are presented. The analysisis carried out in an effective Lagrangian approach which includes Pomeron and(\pi,\eta) exchanges in the t channel, and contributions from the s- andu-channel excitations of a postulated nucleon resonance. In addition to thedifferential cross sections, we use the nine spin-density matrix elements(SDME) as recently measured, instead of the phi-meson decay angulardistributions which depend only on six SDMEs as was done before, to constrainthe resonance parameters. We conclude that indeed the nonmonotonic behavior,along with the other experimental data as reported by LEPS, can only beexplained with an assumption of the excitation of a resonance of spin 3/2, aspreviously reported. However, both parities can account for the data equallywell with almost identical mass of 2.08 +/- 0.04 GeV and width of 0.501 +/-0.117 and 0.570 +/- 0.159 for 3/2^+ and 3/2^-, respectively. The ratio of thehelicity amplitudes A_{1/2}/A_{3/2} calculated from the resulting couplingconstants differs in sign from that of the known D_{13}(2080). Moreexperimental data on single and double polarization observables are needed toresolve the parity. We further find that with an assumption of large values ofthe OZI-evading parameters x_{OZI} = 12 for J^P = 3/2^- and x_{OZI} = 9 for J^P= 3/2^+, the discrepancy between the recent experimental data on \omega-mesonphotoproduction and theoretical model can be considerably reduced. We arguethat the large value of x_{OZI} indicates that the postulated resonancecontains non-negligible amount of strangeness content.