Well-Established Nucleon Resonances Revisited by Double-Polarization Measurements

The first measurement is reported of the double-polarization observable $G$ in the photoproduction of neutral pions off protons, covering the photon energy range from 620 to 1120 MeV and the full solid angle. $G$ describes the correlation between the photon polarization plane and the scattering plane for protons polarized along the direction of the incoming photon. The observable is highly sensitive to contributions from baryon resonances. The new results are compared to the predictions from SAID, MAID, and BnGa partial wave analyses. In spite of the long-lasting efforts to understand $\gamma p\to p{\pi }^0$ as the simplest photoproduction reaction, surprisingly large differences between the new data and the latest predictions are observed which are traced to different contributions of the $N(1535)$ resonance with spin parity $J^P=1/2^{-}$ and $N(1520)$ with $J^P=3/2^{-}$. In the third resonance region, where $N(1680)$ with $J^P=5/2^{+}$ production dominates, the new data are reasonably close to the predictions.

Figure 1

(a) The central part of the detector system. The CsI(Tl) crystals (blue and green) are read out via wavelength shifters and photodiodes or photomultipliers, ${\mathrm{BaF}}_2$ crystals (yellow) in forward direction with photomultipliers. (b) The missing mass distribution, and (c) the ${\pi }^0$ angular distribution for reaction (1) for an incident photon energy of $E_{\gamma }=1000\pm 25\mathrm{MeV}$; butanol (black squares), hydrogen (blue triangle), carbon (red circle), and the sum of hydrogen and carbon data (green star). From these distributions, the dilution factor (d) is determined.

Figure 2

(a) A typical ${\varphi }_{\pi }$ distribution with a fit using Eq. (2). (b, c) The beam asymmetry $\Sigma$ as a function of $\cos {\theta }_{\pi }$ for $E_{\gamma }=800\mathrm{MeV}$ and for $E_{\gamma }=1100\mathrm{MeV}$. Black dots show our data, red squares GRAAL data [20]. The curves represent predictions from different partial wave analyses. Solid (black) curve: BnGa [24]; dashed (red): SAID [22]; long-dashed (black): BnGa with $E_{0^{+}}$ and $E_{2^{-}}$ amplitudes from SAID; dotted (blue): MAID [23]; dashed-dotted (black): BnGa with $E_{0^{+}}$ and $E_{2^{-}}$ amplitudes from MAID. Gray area shows the systematic error due to interactions on nuclei and uncertainty in the photon polarization.

Figure 3

The polarization observable $G$ as a function of $\cos {\theta }_{\pi }$ from $E_{\gamma }=800\mathrm{MeV}$ up to $E_{\gamma }=1100\mathrm{MeV}$. Systematic errors are shown in gray bars. Curves: see Fig. 2.

Figure 4

The double-polarization observable $G$ as a function of energy for two selected bins in ${\theta }_{\pi }$. Curves: see Fig. 2. For comparison the positions of several resonances are marked.