Differential cross section and recoil polarization measurements for the $\gamma p\to K^{+}\Lambda$ reaction using CLAS at Jefferson Lab

We present measurements of the differential cross section and $\Lambda$ recoil polarization for the $\gamma p\to K^{+}\Lambda$ reaction made using the CLAS detector at Jefferson Lab. These measurements cover the center-of-mass energy range from $1.62$ to $2.84$ GeV and a wide range of center-of-mass $K^{+}$ production angles. Independent analyses were performed using the $K^{+}p{\pi }^{-}$ and $K^{+}p$ (missing ${\pi }^{-}$) final-state topologies; results from these analyses were found to exhibit good agreement. These differential-cross-section measurements show excellent agreement with previous CLAS and LEPS results and offer increased precision and a 300-MeV increase in energy coverage. The recoil polarization data agree well with previous results and offer a large increase in precision and a 500-MeV extension in energy range. The increased center-of-mass energy range that these data represent will allow for independent study of nonresonant $K^{+}\Lambda$ photoproduction mechanisms at all production angles.

Figure 1

calculated mass of $p$ tracks vs. calculated mass of $K^{+}$ tracks for all events in the three-track data sample: Regions (i) and (iv) correspond to $\gamma p\to K^{+}\Lambda$ signal events, while regions (ii) and (iii) represent background and are cut from the analysis. See text for details.

Figure 2

The histograms above show the missing mass off $K^{+}$ distributions for events in the $\sqrt{s}=2.105$ GeV bin of the two-track data sample. (a) The distribution for events with $-0.05⩽\cos \hspace{0.5em}{\theta }_K^{\mathrm{c}.\mathrm{m}.}<0.05$. The unshaded histogram shows all events, whereas the shaded histogram shows the same events weighted by $(1-Q_i)$. (b) All events in the $\sqrt{s}=2.105$ GeV bin (no restriction on $\cos \hspace{0.5em}{\theta }_K^{\mathrm{c}.\mathrm{m}.}$) with the same shading scheme. See text for details.

Figure 3

Shown above are $\cos \hspace{0.5em}{\theta }_K^{\mathrm{c}.\mathrm{m}.}$ distributions for events in the $\sqrt{s}=2.105$ GeV bin. The data and unweighted accepted Monte Carlo are shown by black circles and blue crosses, respectively. The Monte Carlo distribution weighted according to the scattering amplitude expansion is shown by red triangles; this weighted distribution matches that of the data events. See text for details.

Figure 4

$d\sigma /d\cos \hspace{0.5em}{\theta }_K^{\mathrm{c}.\mathrm{m}.}$ ($\mu$b) vs. $\cos \hspace{0.5em}{\theta }_K^{\mathrm{c}.\mathrm{m}.}$ in bins of $\sqrt{s}$. Results from the two-track analysis are represented by closed red circles, those of the three-track analysis by open blue triangles. All error bars represent statistical uncertainties only. Vertical axes have the same scale in each row, and horizontal axes all have the same scale.

Figure 5

$d\sigma /d\cos \hspace{0.5em}{\theta }_K^{\mathrm{c}.\mathrm{m}.}$ ($\mu$b) vs. $\cos \hspace{0.5em}{\theta }_K^{\mathrm{c}.\mathrm{m}.}$ in bins of $\sqrt{s}$. Results from the two-track analysis are represented by closed red circles, those of the three-track analysis by open blue triangles. All error bars represent statistical uncertainties only. Vertical axes have the same scale in each row, and horizontal axes all have the same scale. Dashed lines show decades of the vertical axis scaling.

Figure 6

$d\sigma /d\cos \hspace{0.5em}{\theta }_K^{\mathrm{c}.\mathrm{m}.}$ ($\mu$b) vs. $\cos \hspace{0.5em}{\theta }_K^{\mathrm{c}.\mathrm{m}.}$ in bins of $\sqrt{s}$. Results from the two-track analysis are represented by closed red circles, those of the three-track analysis by open blue triangles. All error bars represent statistical uncertainties only. Vertical axes have the same scale in each row, and horizontal axes all have the same scale. Dashed lines show decades of the vertical axis scaling.

Figure 7

Relative difference of the two- and three-track $d\sigma /d\cos \hspace{0.5em}{\theta }_K^{\mathrm{c}.\mathrm{m}.}$ results. In red is displayed a fit to a Gaussian function yielding the indicated mean ($\mu$) and width ($\sigma$). See text for details.

Figure 8

$d\sigma /d\cos \hspace{0.5em}{\theta }_K^{\mathrm{c}.\mathrm{m}.}$ ($\mu$b) vs. $\sqrt{s}$ (GeV) in bins of $\cos \hspace{0.5em}{\theta }_K^{\mathrm{c}.\mathrm{m}.}$. The results of this analysis are shown by closed red circles. The 2006 CLAS results (Bradford et al. [4]) are shown by open blue triangles, 2004 SAPHIR [3] results are shown by open green diamonds, and the LEPS results [5, 6] are shown by open black crosses.

Figure 9

$d\sigma /d\cos \hspace{0.5em}{\theta }_K^{\mathrm{c}.\mathrm{m}.}$ vs. $\cos \hspace{0.5em}{\theta }_K^{\mathrm{c}.\mathrm{m}.}$ results from this analysis (blue) and the 2005 CLAS analysis [4] (red). (a) Results corresponding to the $\sqrt{s}=1.775$ GeV bin of this analysis; (b) those of the $\sqrt{s}=1.745$ GeV bin. Comparisons in this bin are typical of a four-bin-wide systematic discrepancy between the two data sets. See text for discussion.

Figure 10

$P_{\Lambda }$ vs. $\cos \hspace{0.5em}{\theta }_K^{\mathrm{c}.\mathrm{m}.}$ in bins of $\sqrt{s}$. Results from the two-track analysis are represented by closed red circles, those of the three-track analysis by open blue triangles. All error bars represent statistical uncertainties only. Horizontal and vertical axis scales are common for all plots. Physical limits on $P_{\Lambda }$ are indicated by dashed horizontal lines.

Figure 11

$P_{\Lambda }$ vs. $\cos \hspace{0.5em}{\theta }_K^{\mathrm{c}.\mathrm{m}.}$ in bins of $\sqrt{s}$. Results from the two-track analysis are represented by closed red circles, those of the three-track analysis by open blue triangles. All error bars represent statistical uncertainties only. Horizontal and vertical axis scales are common for all plots. Physical limits on $P_{\Lambda }$ are indicated by dashed horizontal lines.

Figure 12

$P_{\Lambda }$ vs. $\cos \hspace{0.5em}{\theta }_K^{\mathrm{c}.\mathrm{m}.}$ in bins of $\sqrt{s}$. Results from the two-track analysis are represented by closed red circles, those of the three-track analysis by open blue triangles. All error bars represent statistical uncertainties only. Horizontal and vertical axis scales are common for all plots. Physical limits on $P_{\Lambda }$ are indicated by dashed horizontal lines.

Figure 13

$P_{\Lambda }$ vs. $\sqrt{s}$ (GeV) in bins of $\cos \hspace{0.5em}{\theta }_K^{\mathrm{c}.\mathrm{m}.}$. Results of this analysis are represented by red circles, previous CLAS (McNabb et al. [19]) results by blue triangles, SAPHIR 2004 (Glander et al. [3]) by green triangles, and GRAAL 2007 (Lleres et al. [20]) by black squares. Physical limits on $P_{\Lambda }$ are indicated by dashed horizontal lines.

Figure 14

$d\sigma /d\cos \hspace{0.5em}{\theta }_K^{\mathrm{c}.\mathrm{m}.}$ ($\mu$b) and $P_{\Lambda }$ results vs. $\sqrt{s}$ (GeV) in bins of $\cos \hspace{0.5em}{\theta }_K^{\mathrm{c}.\mathrm{m}.}$ plotted with several model predictions. Average data points are given by Eq. (14). Model predictions are those of Kaon-MAID [21] (solid green line), the Bonn-Gatchina group [9] (dashed blue line), and the RPR model [22] (dashed red line). See text for commentary.

Figure 15

$d\sigma /d\cos \hspace{0.5em}{\theta }_K^{\mathrm{c}.\mathrm{m}.}$ ($\mu$b) and $P_{\Lambda }$ results vs. $\sqrt{s}$ (GeV) in bins of $\cos \hspace{0.5em}{\theta }_K^{\mathrm{c}.\mathrm{m}.}$ plotted with several model predictions. Average data points are given by Eq. (14). Model predictions are those of Kaon-MAID [21] (solid green line), the Bonn-Gatchina group [9] (dashed blue line), and the RPR model [22] (dashed red line). See text for commentary.

Figure 16

$d\sigma /d\cos \hspace{0.5em}{\theta }_K^{\mathrm{c}.\mathrm{m}.}$ ($\mu$b) and $P_{\Lambda }$ results vs. $\sqrt{s}$ (GeV) in bins of $\cos \hspace{0.5em}{\theta }_K^{\mathrm{c}.\mathrm{m}.}$ plotted with several model predictions. Average data points are given by Eq. (14). Model predictions are those of Kaon-MAID [21] (solid green line), the Bonn-Gatchina group [9] (dashed blue line), and the RPR model [22] (dashed red line). See text for commentary.