First measurement of the Σ beam asymmetry in η photoproduction on the neutron

We present the first measurement of the Σ beam asymmetry in η photoproduction on the neutron in the photon energy range from threshold to 1.5 GeV. Data have been collected by using the tagged and linearly polarized photon beam and the large solid angle apparatus of the GRAAL facility. Neutron data have been selected among η photoproduction events on the deuteron in the quasi-free kinematics approximation. Proton data, extracted in the same way, are within errors identical to those previously obtained on free protons at the GRAAL facility, except for a few points falling in certain energy bins. This difference is, however, well understood in terms of the Fermi motion of the nucleons in the deuteron. We take the consistency of free and quasi-free proton data as an indication that nuclear effects are negligible also for neutron data. The ETAMAID model, in which a strong coupling of the $D_{15}(1675)$ resonance to the $\eta$-neutron channel is introduced, can explain the proton data very well, but it cannot describe the beam asymmetry for the neutron.

Figure 1

Schematic view (not to scale) of the LAγRANGE detector: (1) BGO calorimeter, (2) plastic scintillator barrel, (3) cylindrical multiwire proportional counters (MWPCs), (4) target, (5) plane MWPCs, (6) double plastic scintillator hodoscope, and (7) shower detector.

Figure 2

(a) The correlation $\Delta \theta$ vs $\Delta \varphi -{180}^{°}$ in a three-dimensional view and (b) its level curve projection. (c) Bidimensional Gaussian fit and (d) its level curve projection.

Figure 3

(a) The correlation $E_{\eta }^{\mathrm{calc}}/E_{\eta }^{\mathrm{meas}}$ vs $M_X-M_N$ in a three-dimensional view. (b) Level curve projection.

Figure 4

The η invariant mass without cuts (solid line) and with the kinematical cuts (dotted line) for (a) a proton and (b) a neutron in the central region and for (c) a proton and (d) a neutron in the forward direction.

Figure 5

The azimuthal distribution of the ratio [Eq. (4)] for the quasi-free proton (left panel) and the quasi-free neutron (right panel) data for the values of $E_{\gamma }$ and ${\theta }_{\eta }^{\mathrm{c}.\mathrm{m}.}$ indicated inside the figures.

Figure 6

(Left) Coplanarity between the η and the quasi-free proton (dotted line) and the free proton (solid line): The broadening of one distribution with respect to the other is due to the Fermi motion. (Right) The Fermi momentum distribution before (solid line) and after (dashed line) bidimensional cuts are applied (see text for details).

Figure 7

Beam asymmetry Σ in η photoproduction on quasi-free protons (open squares) in the deuteron and on free protons (full circles) [16]. The energy values outside and inside parentheses indicate the mean value of the bin for quasi-free and free protons, respectively. Dotted lines are the predictions of MAID2001 [4] for the free proton; solid and dashed lines are for the quasi-free proton given by MAID2001 [4] and the reggeized model [33], respectively (see text for details).

Figure 8

Energy dependence of the differential cross section for $\gamma p\to \eta p$ calculated at ${\theta }_{\eta }^{\mathrm{c}.\mathrm{m}.}={50}^{°}$. The unpolarized cross section $d\sigma /d\Omega$ [Eq. (8)], the polarized cross section $\Sigma (d\sigma /d\Omega )$ [Eq. (9)], and the beam asymmetry Σ are presented. For each observable, the curves for a free proton (dashed curve) and a quasi-free proton (solid curve) are shown.

Figure 9

Beam asymmetry Σ in η photoproduction on quasi-free neutrons in the 11 bins in which the energy range has been divided, plotted as a function of ${\theta }_{\eta }^{\mathrm{c}.\mathrm{m}.}$. In each plot, the mean γ energy of the bin is also indicated. Solid and dashed lines illustrate the predictions for neutrons of MAID2001 [4] and of the reggeized model [33], respectively (see text for details).

Figure 10

Comparison between the beam asymmetry Σ in η photoproduction on quasi-free protons (open squares) and quasi-free neutrons (full triangles) in the 11 bins in which the energy range has been divided, plotted as a function of ${\theta }_{\eta }^{\mathrm{c}.\mathrm{m}.}$. See text for details.

Figure 11

Comparison between the beam asymmetry Σ in η photoproduction on quasi-free protons (open squares) and quasi-free neutrons (full triangles) in the seven bins in which the angle range has been divided, plotted as a function of the γ energy.