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A resonance interpretation for the nonmonotonic behavior of the phi-photoproduction cross section near threshold

We study whether the nonmonotonic behavior found in the differential cross section of the phi-meson photoproduction near threshold can be described by a resonance. The resonant contribution is evaluated by using an effective Lagrangian approach. We find that, with the assumption of a J^P=3/2^- resonance with mass of 2.10 \pm 0.03 GeV and width of 0.465 \pm 0.141 GeV, LEPS data can indeed be well described. The ratio of the helicity amplitudes A_1/2/A_3/2 calculated from the resulting coupling constants differs in sign from that of the known D13(2080). We further find that the addition of this postulated resonance can substantially improve the agreement between the existing theoretical predictions and the recent omega photoproduction data if a large value of the OZI evading parameter x_OZI = 12 is assumed for the resonance.

Recently, a local maximum in the differential cross sections (DCS) of φ photoproduction on protons at forward angles at around E γ ∼ 2.0 GeV, has been observed by the LEPS collaboration [1]. Models which consist of t-channel exchanges Refs. [2]- [7] have not been able to account for such a nonmonotonic behavior.
Here, we study whether the nonmonotonic behavior found in Ref. [1] can be described by a resonance. Namely, we will add a resonance to a model consisting of Pomeron and (π, η) exchange [8,9] by fiat and see if, with a suitable assignment of spin and parity, mass and width, as well as the coupling constants, one would be able to obtain a good description of all the data reported by the LEPS collaboration. Since the local maximum appears quite close to the threshold, we will investigate, as a first step, the possibility of the spin of the resonance being either 1/2 or 3/2. Similar analysis was carried out in a coupled-channel model [10]. However, the analysis was marred by a confusion in the phase of the Pomeron-exchange amplitude [11].
In tree-level approximation, only the mass, width, and the products of coupling constants enter in the invariant amplitudes. The details of the amplitudes can be found in Ref. [12]. They are determined with the use of MINUIT, by fitting to the LEPS experimental data [1].
One might be tempted to identify the 3/2 − as the D 13 (2080) as listed in PDG [15]. However, with the coupling constants given above, we obtain a value of A 1 2 /A 3 2 = 1.16 which differ from −1.18 for D 13 (2080) [15] in relative sign.
In general, we find that the effects of the resonance are substantial in many of the polarization observables [3]. Results for single and double polarization observables Σ x , T y , C BT yz , and C BT zx are shown in left panel of Fig. 3. In the same figure, the results using a 3/2 + resonance are also shown by dash-dotted curve. We see that measurements of these polarization observables would help to resolve the question of the parity of the resonance. Left: Single and double polarization observables Σ x , T y , C BT yz , and C BT zx taken at photon laboratory energy E γ = 2 GeV. The solid and dash-dotted lines correspond to our results with the choices of J P = 3/2 − and J P = 3/2 + , respectively, while the dotted lines denote the nonresonant contribution. Right: DCS of ω photoproduction as a function of |t| at W = 2.105 GeV. Solid and dashed lines represent the model predictions of Ref. [16] without and with the addition of our resonance with x OZI = 12. Data are from Ref. [17].
From the φ − ω mixing, one would expect that a resonance in φ N channel would also appear in ωN channel. The conventional "minimal" parametrization relating φ NN * and ωNN * is g φ NN * = − tan ∆θ V x OZI g ωNN * , with ∆θ V ≃ 3.7 • corresponds to the deviation from the ideal φ − ω mixing angle. The larger value of the OZI-evading parameter x OZI would indicate larger strangeness content of the resonance.
By adding the resonance postulated here to the model of Ref. [16] with x OZI = 12, whose prediction is given in the dashed line in right panel of Fig. 3, we see that the DCS at W = 2.105 GeV can be reproduced with roughly the correct strength. The large value of x OZI = 12 would imply that the resonance we propose here contains a considerable amount of strangeness content.
In summary, we study the possibility of accounting for the nonmonotonic behavior as observed by the LEPS collaboration at energies close to threshold as a possible manifestation of a resonance. We confirm that nonresonant contribution alone cannot describe the LEPS data, as well as the addition of a resonance with J = 1/2. However, with an assignment of J = 3/2 − , a nice agreement with most of the LEPS data can be achieved, with a greater stability with respect to changes in the low-energy Pomeron parameters, compared to J = 3/2 + . The obtained resonance mass and width are 2.10 ± 0.03 and 0.465 ± 0.141 GeV, respectively. The resulting coupling constants give rise to a ratio of the helicity amplitudes that differs from that of the known D 13 (2080) in sign. Furthermore, we find that the postulated resonance gives substantial contribution to the polarization observables, which can also be used to determine the parity of the resonance if it indeed exists. The addition of a J = 3/2 − resonance to the model of Ref. [16] with a choice of a large value of OZI-evading parameter x OZI = 12 could indeed considerably improve the agreement of the model prediction with the most recent data. That would imply the resonance postulated here does contain considerable amount of strangeness content.