$N(1520)$ $3/2^{-}$ Helicity Amplitudes from an Energy-Independent Multipole Analysis Based on New Polarization Data on Photoproduction of Neutral Pions

New data on the polarization observables $T$, $P$, and $H$ for the reaction $\gamma p\to p{\pi }^0$ are reported. The results are extracted from azimuthal asymmetries when a transversely polarized butanol target and a linearly polarized photon beam are used. The data were taken at the Bonn electron stretcher accelerator ELSA using the CBELSA/TAPS detector. These and earlier data are used to perform a truncated energy-independent partial wave analysis in sliced-energy bins. This energy-independent analysis is compared to the results from energy-dependent partial wave analyses.

Figure 1

Event yield asymmetry as a function of $\varphi$, left: $\mathrm{\Delta }N(\varphi {)}_T$, right: $\mathrm{\Delta }N(\varphi {)}_{\mathit{\text{BT}}}$, fitted by the functions given in Eqs. (3) and (5), respectively. Both show data from the energy bin $W=1.524–1.542\mathrm{GeV}$.

Figure 2

The polarization observables $T$, $P$, and $H$ (energy bins in GeV). References to earlier data [gray triangles (red)] are given in Ref. [20], refs. [49–71] therein. The data are compared to predictions (dashed curves) from BnGa2011 (black), MAID [light gray (green)], and SAID CM12 [dark gray (blue)]. The BnGa refit (BnGa2014) is shown as a black solid line. Similar fits have been performed within the Bonn-Jülich dynamical coupled-channel model [29]. The systematic errors due to the uncertainty in the degrees of proton (2%) and photon (4%) polarizations, in the dilution factor (1%–4%), and the background contamination (0.01 absolute error) are shown as a dark gray band. An additional systematic error on the photon energy reaching from ${\sigma }_{E\gamma }^{\mathrm{sys}}=6.5\mathrm{MeV}$ at the lowest to 5.4 MeV at the highest energy bin plotted is not shown.

Figure 3

Magnitude (upper row) and phase (lower row) of multipoles derived from a fit to data in slices of energy. Gray (red) crosses show the results of an unbiased fit, and black crosses represent a fit with a penalty function (see text). The dashed lines show the new energy-dependent Bonn-Gatchina fit (BnGa2014), and the solid lines represent a Breit-Wigner plus background fit to the black crosses for $E_{2^{-}}$ and $M_{2^{-}}$. The largest contributions from higher waves (from BnGa2014) are shown as well. The $E_{3^{-}}$ and $M_{3^{-}}$ multipoles excite the close-by resonance $N(1680)5/2^{+}$, and $M_{3^{+}}$ excites $\mathrm{\Delta }(1950)7/2^{+}$.