Measurement of the $\gamma p\to K^0{\Sigma }^{+}$ reaction with the Crystal Ball/TAPS detectors at the Mainz Microtron

The $\gamma p\to K^0{\Sigma }^{+}$ reaction has been measured from threshold to $E_{\gamma }=1.45$ GeV ($W_{\mathrm{CM}}=1.9$ GeV) using the Crystal Ball and TAPS multiphoton spectrometers together with the photon tagging facility at the Mainz Microtron MAMI. In the present experiment, this reaction was searched for in the $3{\pi }^{0}p$ final state, by assuming $K_S^0\to {\pi }^0{\pi }^0$ and ${\Sigma }^{+}\to {\pi }^{0}p$. The experimental results include total and differential cross sections as well as the polarization of the recoil hyperon. The new data significantly improve empirical knowledge about the $\gamma p\to K^0{\Sigma }^{+}$ reaction in the measured energy range. The results are compared to previous measurements and model predictions. It is demonstrated that adding the present $\gamma p\to K^0{\Sigma }^{+}$ results to existing data allowed a better description of this reaction with various models.

Figure 1

Comparison of the invariant-mass distributions (a) $m\left(3{\pi }^0\right)$, (b) $m\left({\pi }^{0}p\right)$, and (c) and (d) $m\left({\pi }^0{\pi }^0\right)$, obtained for the experimental $\gamma p\to 3{\pi }^{0}p$ events (crosses) and for the MC simulation of $\gamma p\to K^0{\Sigma }^{+}\to 3{\pi }^{0}p$ (solid line) and $\gamma p\to 2{\pi }^0{\Delta }^{+}\left(1232\right)\to 3{\pi }^{0}p$ (dashed line). All distribution are obtained for events above the $\gamma p\to K^0{\Sigma }^{+}$ threshold. The $m\left({\pi }^{0}p\right)$ and $m\left({\pi }^0{\pi }^0\right)$ distributions are obtained for $m\left(3{\pi }^0\right)>0.6$ GeV/$c^2$. The events in (d) are selected by testing the $\gamma p\to {\pi }^0{\pi }^0{\Sigma }^{+}\to 3{\pi }^{0}p$ hypothesis.

Figure 2

The $m\left({\pi }^0{\pi }^0\right)$ distributions for (a) the MC simulation of $\gamma p\to K^0{\Sigma }^{+}\to 3{\pi }^{0}p$ with a Gaussian fit, (b) the MC simulation of $\gamma p\to 2{\pi }^0{\Delta }^{+}\left(1232\right)\to 3{\pi }^{0}p$ with a fit of a polynomial of order five, and (c) the experimental data with a fit of the sum of a Gaussian and a polynomial of order five.

Figure 3

$\gamma p\to K^0{\Sigma }^{+}$ differential cross sections of this work (solid squares) compared to the predictions of various models: BG2011-2 (solid line), BG2011-1 (dashed line) [20], RPR-3 (dotted line) [19], RPR-A (dash-dotted line) [25], and chiral-unitary approach (long-dash-dotted line with gray error band) [24].

Figure 4

$\gamma p\to K^0{\Sigma }^{+}$ differential cross sections from the additional set of this work (solid squares), compared to the previous measurements: SAPHIR (open circles, 100-MeV bins) [16], CB-ELSA 2008 (open squares, 100-MeV bins) [13], CB-ELSA 2010 (open triangles, 100-MeV bins) [14], and CLAS (open diamonds, 200-MeV bins, with CLAS data at 1450 MeV being compared to the results of this work at an energy of 1425 MeV) [17].

Figure 5

$\gamma p\to K^0{\Sigma }^{+}$ total cross sections of this work (solid squares) compared to (a) previous measurements by SAPHIR (open circles) [16], CLAS (solid triangles) [17] normalized to the full $\cos {\theta }_{K^0}$ range, CB-ELSA 2008 (open squares) [13], and CB-ELSA 2010 (open triangles) [14]; (b) predictions by BG2011-2 (solid line), BG2011-1 (dashed line) [20], RPR-3 (dotted line) [19], RPR-A (dash-dotted line) [25], and the chiral-unitary approach (long-dash-dotted line with gray error band) [24]; (c) the $\gamma p\to K^{+}{\Sigma }^0$ total cross sections by SAPHIR 2004 (open circles; multiplied by a factor of 0.3) [7], CLAS 2006 (open triangles, $\times 0.3$) [4], BG2011-2 (solid line, $\times 0.28$), BG2011-1 (dashed line, $\times 0.28$), and RPR-3 (dotted line, $\times 0.18$).

Figure 6

Results of this work (solid squares) for recoil polarization of ${\Sigma }^{+}$ compared to predictions from the models BG2011-2 (solid line), BG2011-1 (dashed line), RPR-3 (dotted line), and RPR-A (dash-dotted line) and from the chiral-unitary approach (long-dash-dotted line with gray error band).

Figure 7

${\Sigma }^{+}$ recoil-polarization results from the additional set of this work (solid squares) compared to the previous measurements: SAPHIR (open circles, 250-MeV bins), CB-ELSA 2008 (open squares, 100-MeV bins), and CB-ELSA 2010 (open triangles, 150-MeV bins).

Figure 8

$\gamma p\to K^0{\Sigma }^{+}$ differential cross sections from this work (solid squares) compared to the predictions based on new fits of models: BG2011-2 (solid line), BG2011-1 (dashed line), and RPR-2007 (dotted line).

Figure 9

$\gamma p\to K^0{\Sigma }^{+}$ total cross sections from this work (solid squares) compared to the predictions based on new fits of models: BG2011-2 (solid line), BG2011-1 (dashed line), and RPR-2007 (dotted line).

Figure 10

Results of this work (solid squares) for recoil polarization of ${\Sigma }^{+}$ compared to predictions based on new fits of models: BG2011-2 (solid line), BG2011-1 (dashed line), and RPR-2007 (dotted line).