Transverse polarization of ${\Sigma }^{+}\left(1189\right)$ in photoproduction on a hydrogen target in CLAS

Experimental results on the ${\Sigma }^{+}\left(1189\right)$ hyperon transverse polarization in photoproduction on a hydrogen target using the CLAS detector at Jefferson Laboratory are presented. The ${\Sigma }^{+}\left(1189\right)$ was reconstructed in the exclusive reaction $\gamma +p\to K_S^0+{\Sigma }^{+}\left(1189\right)$ via the ${\Sigma }^{+}\to p{\pi }^0$ decay mode. The $K_S^0$ was reconstructed in the invariant mass of two oppositely charged pions with the ${\pi }^0$ identified in the missing mass of the detected $p{\pi }^{+}{\pi }^{-}$ final state. Experimental data were collected in the photon energy range $E_{\gamma }=1.0$–3.5 GeV ($\sqrt{s}$ range 1.66–2.73 GeV). We observe a large negative polarization of up to 95$\%$. As the mechanism of transverse polarization of hyperons produced in unpolarized photoproduction experiments is still not well understood, these results will help to distinguish between different theoretical models on hyperon production and provide valuable information for the searches of missing baryon resonances.

Figure 1

Different distances of closest approach and the collinearity angle, ${\theta }_{\text{collinearity}}$. DOCA(${\pi }^{+}{\pi }^{-}$) is the distance of closest approach between the two charged pions. DOCA($\gamma p$) is the distance of closest approach between the photon and the proton. “$p{K}_s^0$ distance” is the distance between the center of DOCA($\gamma p$) and the center of DOCA(${\pi }^{+}{\pi }^{-}$).

Figure 2

Distribution of cosine of the collinearity angle, $\cos {\theta }_{\text{collinearity}}$, with the ${\pi }^0$ and $K_S^0$ selected. This distribution results from the DOCA resolutions (Fig. 1), the $K_S^0$ decay distance distribution, and the nonresonant $\pi \pi$ continuum.

Figure 3

(a) Reconstructed $K_S^0$ signal in the invariant mass of the two charged pions with the collinearity angle cut. The shaded region shows the $\pm 3\sigma$ cut around the peak to select the $K_S^0$ and the striped region shows the $3\sigma$ sideband region of the $K_S^0$ taken to determine the background under the ${\Sigma }^{+}$ (see Sec. 4 for details), (b) Reconstructed ${\pi }^0$ signal in the missing mass of the detected proton and two oppositely charged pions with a $K_S^0$ selected and the collinearity angle cut. The shaded region shows the $\pm 3\sigma$ cut around the peak to select the ${\pi }^0$ events.

Figure 4

Reconstructed ${\Sigma }^{+}$ signal (dashed line) in the missing mass of the two charged pions, with the ${\pi }^0$ and the $K_S^0$ selected, and with the collinearity angle cut as described in the text. The dotted line is the $MM\left({\pi }^{+}{\pi }^{-}\right)$ distribution from the sidebands of the $K_S^0$ with a ${\pi }^0$ selected and with the same collinearity angle cut as above. No normalization or scaling is done. The fit (solid line) includes a Gaussian for the peak and a second order polynomial for the background. The shaded region shows the $\pm 3\sigma$ cut around the peak to select the ${\Sigma }^{+}$.

Figure 5

Coordinate system.

Figure 6

Transverse polarization versus $\cos {\left({\theta }_{{\Sigma }^{+}}\right)}_{\text{c.m.}}$ at different photon beam energies. The bands on the horizontal axis are the systematic uncertainties.

Figure 7

Transverse polarization versus photon beam energy at different ${\left({\theta }_{{\Sigma }^{+}}\right)}_{\text{c.m.}}$. The bands on the horizontal axis are the systematic uncertainties.

Figure 8

Comparison of polarization of ${\Sigma }^{+}$ between this result (circle) and SAPHIR (triangle) [17] for two different photon energy ranges. The photon energy range in (a) is from threshold to 1.55 GeV.

Figure 9

Comparison of ${\Sigma }^{+}$ polarization from this result (circle, in reaction $\gamma p\to {\Sigma }^{+}{K}^0$) with ${\Sigma }^0$ polarization from Ref. [10] (triangle, in reaction $\gamma p\to {\Sigma }^0{K}^{+}$) for four different angles.