Measurement of $K^{-}p$ radiative capture to $\gamma \Lambda$ and $\gamma {\Sigma }^0$ for $p_{K^{-}}$ between 514 and 750 MeV/$c$

Differential cross sections for $K^{-}$ radiative capture in flight on the proton, leading to the $\gamma \Lambda$ and $\gamma {\Sigma }^0$ final states, have been measured at eight $K^{-}$ momenta between 514 and 750 MeV/$c$. The data were obtained with the Crystal Ball multiphoton spectrometer installed at the separated $K/\pi$ beam line C6 of the BNL Alternating Gradient Synchrotron. The results substantially improve the existing experimental data available for studying radiative decays of excited hyperon states. An exploratory theoretical analysis is performed within the Regge-plus-resonance approach. According to this analysis, the $\gamma {\Sigma }^0$ final state is dominated by hyperon-resonance exchange and hints at an important role for a resonance in the mass region of 1700 MeV. In the $\gamma \Lambda$ final state, in contrast, the resonant contributions account for only half the strength, and the data suggest the importance of a resonance in the mass region of 1550 MeV.

Figure 1

Acceptance for the production angle ${\theta }^{*}$ of the outgoing photon in the c.m. frame for the reactions $K^{-}p\to \gamma \Lambda$ and $K^{-}p\to \gamma {\Sigma }^0$ at beam momenta of 514 and 750 MeV/$c$.

Figure 2

Our differential cross sections for $K^{-}p\to \gamma \Lambda$ at the eight beam momenta. The curves are the Legendre polynomial fits to our data.

Figure 3

Our differential cross sections for $K^{-}p\to \gamma {\Sigma }^0$ at the eight beam momenta. The curves are the Legendre polynomial fits to our data.

Figure 4

Comparison of our differential cross sections for $K^{-}p\to \gamma \Lambda$ with RPR model calculations. The solid line is the Regge-2 model prediction consisting solely of nonresonant diagrams. The dashed, dotted, and dashed-dotted curves represent fits to our data of the Regge-2 model supplemented with the $\Lambda (1520)D_{03}$, $\Sigma (1660)P_{11}$, or $\Sigma (1670)D_{13}$ resonance, respectively.

Figure 5

Comparison of our differential cross sections for $K^{-}p\to \gamma {\Sigma }^0$ with RPR model calculations. The solid line is the Regge-3 model prediction consisting solely of nonresonant diagrams. The dashed, dotted, and dashed-dotted curves represent fits to our data of the Regge-3 model supplemented with the $\Lambda (1600)P_{01}$, $\Lambda (1670)S_{01}$, or $\Lambda (1690)D_{03}$ resonance, respectively.

Figure 6

Our total cross sections as a function of the center-of-mass energy $W$ for $K^{-}p\to \gamma \Lambda$ (top) and $K^{-}p\to \gamma {\Sigma }^0$ (bottom). The solid lines are the Regge-2 model prediction for $K^{-}p\to \gamma \Lambda$ (top) and the Regge-3 model prediction for $K^{-}p\to \gamma {\Sigma }^0$ (bottom). The dashed, dotted, and dashed-dotted curves shown for $K^{-}p\to \gamma \Lambda$ represent the Regge-2 model supplemented with the $\Lambda (1520)D_{03}$, $\Sigma (1660)P_{11}$, or $\Sigma (1670)D_{13}$ resonance, respectively. The dashed, dotted, and dashed-dotted curves shown for $K^{-}p\to \gamma {\Sigma }^0$ represent the Regge-3 model supplemented with the $\Lambda (1600)P_{01}$, $\Lambda (1670)S_{01}$, or $\Lambda (1690)D_{03}$ resonance, respectively.

Figure 7

Our $K^{-}p\to \gamma {\Sigma }^0$ total cross sections as a function of the beam momentum compared to the V-A results from Ref. [19]. The uncertainties plotted for our results are the sum of statistical and systematic uncertainties. The uncertainties plotted for V-A results are statistical only. The V-A points shown by arrows are upper limits.