Features of ω photoproduction off nucleon target at forward angles: dominance of π exchange with Regge cuts and scaling of differential cross-sections

Abstract

We investigate photoproduction of ω off a nucleon target γN → ωN by using a Reggeized model where π(135) + σ(500) + f1(1285) + f2(1270)+Pomeron exchanges are included in the t-channel for the reaction at forward angles. The reaction mechanism at low energy is featured by the dominance of the π exchange with the absorptive cuts introduced to modulate the pion contribution to both γp → ωp and γn → ωn reactions. Necessity of σ exchange is illustrated in the analysis of the cross section from natural parity exchanges. Cross sections for differential, total, and spin density matrix with parity and beam polarization asymmetries are reproduced and compared with existing data on γp → ωp. Scaled differential cross sections of Jefferson Lab data on γp → ωp are investigated at the production angle θ = 90 • with the nonlinear trajectories for a saturation. Differential and total cross sections for the γn → ωn reaction are analyzed to compare with recent experimental data at the CBELSA/TAPS Collaboration. An application to the γp → ω∆ + reaction is presented to demonstrate the significance of the π exchange in the total and differential cross sections. PACS numbers: 11.55.Jy, 13.40.-f, 13.60.Le for unnatural parity (P = −(−1) J ) exchanges.The Regge propagator is given bywritten in the collective form for the ϕ meson of spin-J which stands for all the mesons considered here. s 0 = 1 GeV 2 . The phase factor is, in general, taken to be of the canonical form, 1 2 [(−1) J + e −iπαJ (t) ], for each meson exchange. Photon and vector meson polarizations

Vector meson photoproduction with a linearly polarized beam

“…Regarding axial vector exchanges, since the decay widths of f 1 , a 1 → γV are not known, 1 their contribution is difficult to evaluate. Within a specific quark model [20], the contribution of the f 1 to ω photoproduction is found to be negligible. As we will show, it is possible to saturate the unnatural components of the SDMEs by pseudoscalar exchanges.…”

“…In the ω photoproduction model from [20], a scalar exchange representing a σ meson trajectory was also considered. Since the σ meson trajectory is below the (leading) f 2 trajectory, we do not include it here.…”

“…To be consistent with factorization, and to reduce the number of parameters, we assume that the couplings β N 1 and β N 2 are the same for all vector mesons. The steep falloff of the forward differential cross section is well described by exponential factors, gamma functions [18,28] or dipole form factors [17,19,20,29,30]. All of these models can be approximated by an exponential function of the form e b N t [12,[14][15][16].…”

“…In our model we added a helicity-independent exponential factor b N to reproduce the energy-independent shrinkage of the differential cross section. This feature is generally described by exponential factors [12,[14][15][16], gamma functions [18,28] or dipole form factors [17,19,20,29,30]. This t-dependence originates from the energy dependence of the nearest cross-channel singularity.…”

“…In this paper, we discuss the SLAC data in the context of a Regge-pole exchange model, which allows us to assess contributions of individual exchanges to the SDMEs. Various models have been proposed in the past [11][12][13][14][15][16][17][18][19][20], with different descriptions of the momentum-transfer dependence of the helicity amplitudes. In general these 3 GeV.…”