Traces of the ${\Theta }^{+}$ Pentaquark in $K^{+}$-Nucleus Dynamics

Long-standing anomalies in $K^{+}$-nucleus integral cross sections could be resolved by extending the impulse-approximation $t\rho$ optical-potential framework to incorporate $K^{+}$ absorption on pairs of nucleons. Substantially improved fits to the data at $p_{\mathrm{l}\mathrm{a}\mathrm{b}}\sim 500–700\mathrm{M}\mathrm{e}\mathrm{V}/c$ are obtained. An upper bound on the absorption cross section per nucleon is derived, ${\sigma }_{\mathrm{a}\mathrm{b}\mathrm{s}}^{(K^{+})}/A\sim 3.5\mathrm{m}\mathrm{b}$. We conjecture that the underlying microscopic absorption process is $K^{+}nN\to {\Theta }^{+}N$, where ${\Theta }^{+}(1540)$ is the newly discovered exotic $Y=2$, $I=0$, $Z=1$ pentaquark baryon, and estimate that $\sigma (K^{+}d\to {\Theta }^{+}p)$ is a fraction of millibarn. Comments are made on ${\Theta }^{+}$ production reactions on nuclei.

Figure 1

Data and calculations for $K^{+}$ reaction cross sections per nucleon (${\sigma }_R/A$) at $p_{\mathrm{l}\mathrm{a}\mathrm{b}}=488\mathrm{M}\mathrm{e}\mathrm{V}/c$ are shown in the upper part. Calculated $K^{+}$ absorption cross sections per nucleon (${\sigma }_{\mathrm{a}\mathrm{b}\mathrm{s}}/A$) are shown in the lower part; see text.