Interference Effect between $\varphi$ and $\mathrm{\Lambda }(1520)$ Production Channels in the $\gamma p\to K^{+}{K}^{-}p$ Reaction near Threshold

The $\varphi \text{−}\mathrm{\Lambda }(1520)$ interference effect in the $\gamma p\to K^{+}{K}^{-}p$ reaction has been measured for the first time in the energy range from 1.673 to 2.173 GeV. The relative phases between $\varphi$ and $\mathrm{\Lambda }(1520)$ production amplitudes were obtained in the kinematic region where the two resonances overlap. The measurement results support strong constructive interference when $K^{+}{K}^{-}$ pairs are observed at forward angles but destructive interference for proton emission at forward angles. Furthermore, the observed interference effect does not account for the $\sqrt{s}=2.1\mathrm{GeV}$ bump structure in forward differential cross sections for $\varphi$ photoproduction. This fact suggests possible exotic structures such as a hidden-strangeness pentaquark state, a new Pomeron exchange, or rescattering processes via other hyperon states.

Figure 1

Top: Scatter plots of the invariant mass of the $K^{+}{K}^{-}$ system versus that of the $K^{-}p$ system for forward $K^{-}{K}^{+}$ events (a) before and (b) after the kinematic fit. Middle: The same as the top but for forward $K^{-}p$ events (c) before and (d) after the kinematic fit. Bottom: The same as the top but for forward $K^{+}p$ events (e) before and (f) after the kinematic fit. Each scatter plot shows the projections onto each invariant mass axis.

Figure 2

The invariant mass spectra for (top) $K^{+}{K}^{-}$ and (bottom) $K^{-}p$ systems are displayed as closed circles for forward $K^{+}{K}^{-}$ events in the energy region from 1.973 to 2.073 GeV, respectively. The best-fit line shapes for $\varphi$ are overlaid with dotted lines, while those for $\mathrm{\Lambda }(1520)$ are represented as dot-dashed lines. Dashed lines represent the contributions of nonresonant $K^{+}{K}^{-}p$ production.

Figure 3

Forward differential cross sections for (left) $\varphi$ and (right) $\mathrm{\Lambda }(1520)$ photoproduction. Open squares represent cross-section results obtained without considering interference, while solid circles represent new measurement results.

Figure 4

Difference between event yields in the interference region and the sum of $\varphi$ and $\mathrm{\Lambda }(1520)$ events for forward $K^{+}{K}^{-}$ events, in four 0.1-GeV-wide energy regions from 1.773 to 2.173 GeV. The best-fit results for the relative phase are overlaid with solid curves, while dashed lines are from theoretical estimates assuming maximum constructive $\varphi \text{−}\mathrm{\Lambda }(1520)$ interference with $\psi =\pi /2$.

Figure 5

(Left) Phase angles for $K^{+}{K}^{-}$ (circles), $K^{-}p$ (squares), and $K^{+}p$ (triangles) events. (Right) Integrated yields (when $K^{+}{K}^{-}$ pairs are detected at forward angles) in the interference region (circles) compared to the predicted levels for the maximum and minimum bounds (solid lines). The dashed line indicates the predicted levels for no $\varphi \text{−}\mathrm{\Lambda }(1520)$ interference.