Measurement of the $\overrightarrow{\gamma }p\to K^{+}\Lambda$ reaction at backward angles

Cross sections for the $\gamma p\to K^{+}\Lambda$ reaction were measured at backward angles using linearly polarized photons in the range 1.50 to 2.37 GeV. In addition, the beam asymmetry for this reaction was measured for the first time at backward angles. The Λ was detected at forward angles in the LEPS spectrometer via its decay to $p{\pi }^{-}$ and the $K^{+}$ was inferred using the technique of missing mass. These measurements, corresponding to kaons at far backward angles in the center-of-mass frame, complement similar CLAS data at other angles. Comparison with theoretical models shows that the reactions in these kinematics provide further opportunities to investigate the reaction mechanisms of hadron dynamics.

Figure 1

Diagram of the $u$-channel for the reaction $\gamma p\to K^{+}\Lambda$ that contributes at backward kaon angles.

Figure 2

Invariant mass of the $p{\pi }^{-}$ events, showing a peak for the Λ riding on top of a smooth background: (a) raw data with a reconstructed vertex from the target, (b) with an additional requirement that the missing mass is a kaon.

Figure 3

Missing mass of the $\gamma p\to \Lambda X$ reaction for the angular bin $0.90<\cos {\theta }_{\mathrm{CM}}<1.0$, where ${\theta }_{\mathrm{CM}}$ is the center-of-mass angle of the Λ. Each figure is for the photon energy shown by the label.

Figure 4

Differential cross sections as a function of the Mandelstam variable $u$ for the given photon beam energy. The curves are calculations of the contribution from the diagram in Fig. 1 only, using exchange of $\Lambda ,{\Sigma }^0$, and ${\Sigma }^{*}$ hyperons.

Figure 5

Differential cross sections at the angle shown as a function of photon beam energy. Calculations in the model of Ref. [3] are shown without (dashed) and with (hatched region) coupled channels.

Figure 6

Beam asymmetry at the angles shown as a function of photon beam energy. The points have been averaged over the range of photon energy shown. Calculations in the model of Ref. [3] with coupled channels (hatched region) are shown for comparison.