Strong Evidence for Nucleon Resonances near 1900 MeV

Data on the reaction $\gamma p\to K^{+}\mathrm{\Lambda }$ from the CLAS experiments are used to derive the leading multipoles, $E_{0+}$, $M_{1-}$, $E_{1+}$, and $M_{1+}$, from the production threshold to 2180 MeV in 24 slices of the invariant mass. The four multipoles are determined without any constraints. The multipoles are fitted using a multichannel $L+P$ model that allows us to search for singularities and to extract the positions of poles on the complex energy plane in an almost model-independent method. The multipoles are also used as additional constraints in an energy-dependent analysis of a large body of pion and photoinduced reactions within the Bonn-Gatchina partial wave analysis. The study confirms the existence of poles due to nucleon resonances with spin parity $J^P=1/2^{-}$, $1/2^{+}$, and $3/2^{+}$ in the region at about 1.9 GeV.

Figure 1

Example of a fit to the data for the mass range from 1950 to 1970 MeV. $d\sigma /d\mathrm{\Omega }$: [40], $P$ [40], $\mathrm{\Sigma }$, $T$, $O_x$, $O_z$ [42]. The (red) dotted curve corresponds to the fit used to determine the multipoles of Fig. 2, the (black) solid curves to fits using $L+P$ for low-$L$ partial wave and BNGA for high-$L$, and the (green) dashed curves to the BNGA fit.

Figure 2

Real (red triangles) and imaginary (blue dots) part of the $E_{0+}$, $M_{1-}$, $E_{1+}$, and $M_{1+}$ multipoles for the reaction $\gamma p\to K^{+}\mathrm{\Lambda }$. The systematic errors are given at the top (real part) and bottom (imaginary part) of the subfigures. $E_{0+}$ excites the partial wave $J^P=1/2^{-}$, $M_{1-}$: $J^P=1/2^{+}$, $E_{1+}$ and $M_{1+}$, $J^P=3/2^{+}$. The solid curve shows the $L+P$ fit, the dashed curve the energy-dependent BNGA fit.