Photoproduction of $\mathrm{\Lambda }$ and ${\mathrm{\Sigma }}^0$ hyperons off protons with linearly polarized photons at $E_{\gamma }=1.5–3.0$ GeV

We report the measurement of the $\gamma p\to K^{+}\mathrm{\Lambda }$ and $\gamma p\to K^{+}{\mathrm{\Sigma }}^0$ reactions at SPring-8. The differential cross sections and photon-beam asymmetries are measured at forward $K^{+}$ production angles using linearly polarized tagged-photon beams in the range of $E_{\gamma }=1.5–3.0$ GeV. With increasing photon energy, the cross sections for both $\gamma p\to K^{+}\mathrm{\Lambda }$ and $\gamma p\to K^{+}{\mathrm{\Sigma }}^0$ reactions decrease slowly. Distinct narrow structures in the production cross section have not been found at $E_{\gamma }=1.5–3.0$ GeV. The forward peaking in the angular distributions of cross sections, a characteristic feature of $t$-channel exchange, is observed for the production of $\mathrm{\Lambda }$ in the whole observed energy range. A lack of similar feature for ${\mathrm{\Sigma }}^0$ production reflects a less dominant role of $t$-channel contribution in this channel. The photon-beam asymmetries remain positive for both reactions, suggesting the dominance of $K^{*}$ exchange in the $t$ channel. These asymmetries increase gradually with the photon energy, and have a maximum value of +0.6 for both reactions. Comparison with theoretical predictions based on the Regge trajectory in the $t$ channel and the contributions of nucleon resonances indicates the major role of $t$-channel contributions as well as non-negligible effects of nucleon resonances in accounting for the reaction mechanism of hyperon photoproduction in this photon energy regime.

Figure 1

Missing mass spectrum of $\gamma p\to K^{+}X$ reaction $\left[{\mathrm{MM}}_X(\gamma p,K^{+})\right]$ at $E_{\gamma }=1.5–3.0$ GeV.

Figure 2

Azimuthal angle (${\varphi }_{K^{+}}$) dependence of the ratio $(n{N}_{\mathrm{V}}-N_{\mathrm{H}})/(n{N}_{\mathrm{V}}+N_{\mathrm{H}})$ in Eq. (1) for the (a) $K^{+}\mathrm{\Lambda }$ and (b) $K^{+}{\mathrm{\Sigma }}^0$ channels at $E_{\gamma }=2.7$ GeV and $cos{\theta }_{\mathrm{c}.\mathrm{m}.}^{K^{+}}=0.9$. The solid lines are the fit results using a function of $cos2{\varphi }_{K^{+}}$.

Figure 3

Differential cross sections for the $p$($\gamma ,K^{+}$)$\mathrm{\Lambda }$ reaction (a)–(d) and the $p$($\gamma ,K^{+}$)${\mathrm{\Sigma }}^0$ reaction (e)–(h) as a function of photon energy $E_{\gamma }$ for the kaon c.m. production polar angle $0.6<cos{\theta }_{\mathrm{c}.\mathrm{m}.}^{K^{+}}<1.0$. The curves denote the predictions of Regge-model [43] calculation with (RPR) and without resonances (RPR-Regge), and Bonn-Gatchina model (BG). The hatched histograms indicate the systematic uncertainty.

Figure 4

Differential cross sections for the $p$($\gamma ,K^{+}$)$\mathrm{\Lambda }$ reaction as a function of $|t-t_{\min }|$ for $1.5<E_{\gamma }<3.0$ GeV. The results of this measurement are shown by solid black circles. The results of LEPS 2006 [10], CLAS 2006 [13], CLAS 2010 [14], and SAPHIR 2004 [8] are also shown. The notations of curves are the same as those in Fig. 3. The shaded histograms show the systematic uncertainty.

Figure 5

Same as Fig. 4, but for the $p$($\gamma ,K^{+}$)${\mathrm{\Sigma }}^0$ reaction.

Figure 6

Photon-beam asymmetries (${\mathrm{\Sigma }}_{\gamma }$) for the $p$($\gamma ,K^{+}$)$\mathrm{\Lambda }$ reaction (a)–(c) and for $p$($\gamma ,K^{+}$)${\mathrm{\Sigma }}^0$ reaction (d)–(f) with systematic uncertainty plotted in hatched histogram as a function of photon energy $E_{\gamma }$ for the kaon c.m. production polar angle $0.6<cos{\theta }_{\mathrm{c}.\mathrm{m}.}^{K^{+}}<1.0$. The curve notations are the same as those in Fig. 3.

Figure 7

Photon-beam asymmetries (${\mathrm{\Sigma }}_{\gamma }$) for the $p$($\gamma ,K^{+}$)$\mathrm{\Lambda }$ reaction (a) and $p$($\gamma ,K^{+}$)${\mathrm{\Sigma }}^0$ reaction (b) with systematic uncertainty plotted in hatched histogram as a function of $E_{\gamma }$ for $0.6<cos{\theta }_{\mathrm{c}.\mathrm{m}.}^{K^{+}}<1.0$. The results of this measurement and LEPS 2006 are shown.

Figure 8

Photon-beam asymmetries (${\mathrm{\Sigma }}_{\gamma }$) for the $p$($\gamma ,K^{+}$)$\mathrm{\Lambda }$ reaction (a)–(f) and $p$($\gamma ,K^{+}$)${\mathrm{\Sigma }}^0$ reaction (g)–(l) with systematic uncertainty plotted in hatched histogram as a function of $cos{\theta }_{\mathrm{c}.\mathrm{m}.}^{K^{+}}$for $1.8<E_{\gamma }<3.0$ GeV. The results of this measurement and LEPS 2006 are shown. The curve notations are the same as those in Fig. 3.