Photoproduction of the $\omega$ meson on the proton near threshold

An experimental study of $\omega$ photoproduction on the proton was conducted by using the Crystal Ball and TAPS multiphoton spectrometers together with the photon tagging facility at the Mainz Microtron MAMI. The $\gamma p\to \omega p$ differential cross sections are measured from threshold to the incident-photon energy $E_{\gamma }=1.40$ GeV $(W=1.87$ GeV for the center-of-mass energy) with 15-MeV binning in $E_{\gamma }$ and full production-angle coverage. The quality of the present data near threshold gives access to a variety of interesting physics aspects. As an example, an estimation of the $\omega N$ scattering length ${\alpha }_{\omega p}$ is provided.

Figure 1

$m\left({\pi }^0\gamma \right)$ invariant-mass distributions obtained for $E_{\gamma }=1176$ MeV and $cos\theta =0$. (a) MC simulation of $\gamma p\to \omega p\to {\pi }^0\gamma p$ with a Gaussian fit $(\sigma =8.2$ MeV and FWHM = 19.3 MeV). (b) MC simulation of the background reaction $\gamma p\to {\pi }^0{\pi }^{0}p$ fitted with a polynomial of order eight. (c) Measured spectrum fitted with the sum of a Gaussian and a polynomial of order eight, where $N_{\omega }$ is the number of $\omega$ mesons found from this fit.

Figure 2

Same as Fig. 1, but for $cos\theta =0.933$. A Gaussian fit to the $\omega$ peak results in $\sigma =12.4$ MeV (FWHM = 29.2 MeV). A polynomial of order seven used to fit the background.

Figure 3

Same as Fig. 1, but for $E_{\gamma }=1325$ MeV. A Gaussian fit to the $\omega$ peak results in $\sigma =13.6$ MeV (FWHM = 32.0 MeV). A polynomial of order eight used to fit the background.

Figure 4

Differential cross sections for $\gamma p\to \omega p$ as a function of $cos\theta$, where $\theta$ is the angle between the directions of the $\omega$ meson and the incident photon in the c.m. frame. The results of this work are shown by blue dots, SAPHIR [6] results by open triangles, and CLAS [13] results by open circles. The vertical error bars of all data points include statistical uncertainties only. The horizontal error bars are shown only for SAPHIR data, reflecting their nonidentical bins in $cos\theta$. The incident-photon energies of the data points shown for previous experiments are within $\pm 5$ MeV of $E_{\gamma }$ indicated in each panel. Model calculations from Ref. [7] are shown by black short-dashed lines and from Ref. [11] by magenta long-dashed lines.

Figure 5

Results of this work (blue dots) for the $\gamma p\to \omega p$ total cross sections ${\sigma }_t$ are shown in (a) for ${\sigma }_t^2$ as a function of the incident-photon energy $E_{\gamma }$ and in (b) for ${\sigma }_t$ as a function of the c.m. momentum $q$ of the final-state particles. The ${\sigma }_t$ results from SAPHIR [6] are depicted by open triangles. The vertical error bars represent the total uncertainties of the results. The horizontal error bars reflect the energy binning. The red solid line shows the fit of the present data (a) with Eq. (1) and (b) with Eq. (2). The result from the calculation of Ref. [7] is shown by a black short-dashed line, of Ref. [11] by a magenta long-dashed line, and of Ref. [12] by a green dashed line.