Search for the ${\Theta }^{+}$ Pentaquark in the $\gamma d\to \Lambda n{K}^{+}$ Reaction Measured with the CLAS Spectrometer

For the first time, the reaction $\gamma d\to \Lambda n{K}^{+}$ has been analyzed in order to search for the exotic pentaquark baryon ${\Theta }^{+}(1540)$. The data were taken at Jefferson Laboratory, using the Hall-$B$ tagged-photon beam of energy between 0.8 and 3.6 GeV and the CEBAF Large Acceptance Spectrometer (CLAS). No statistically significant structures were observed in the $n{K}^{+}$ invariant-mass distribution. The upper limit on the $\gamma d\to \Lambda {\Theta }^{+}$ integrated cross section has been calculated and found to be between 5 and 25 nb, depending on the production model assumed. The upper limit on the differential cross section is also reported.

Figure 1

A possible reaction mechanism for the photoproduction of $\Lambda {\Theta }^{+}$ from a deuterium nucleus.

Figure 2

Top plot: Missing mass of $\gamma d\to p{\pi }^{-}{K}^{+}X$, showing a peak at the neutron mass. The particle identification cuts for the three charged particles and the selection cut on the $\Lambda$ have been applied. Bottom plot: Invariant mass of the $p{\pi }^{-}$ system, showing a peak at the $\Lambda$ mass. The particle identification cuts for the three charged particles and the selection cut on the neutron mass have been applied. For both plots, the vertical dotted lines represent the $3\sigma$ selection cuts (with ${\sigma }_n=0.009\mathrm{GeV}/c^2$ and ${\sigma }_{\Lambda }=0.002\mathrm{GeV}/c^2$, from a Gaussian fit) applied to select the final state.

Figure 3

Missing mass of $p{K}^{+}$ before (crosses) and after (circles) applying the $\Lambda$ selection cut. The ${\Sigma }^{-}$ signal, visible before applying the $\Lambda$ cut, is eliminated when the $\Lambda n{K}^{+}$ events are selected.

Figure 4

Invariant-mass distributions of the $n{K}^{+}$ system after channel selection. Top plot: No kinematical cuts are applied. Bottom plot: The $E_{\gamma }<1.6\mathrm{GeV}$ and $p_n>0.2\mathrm{GeV}/c$ kinematical cuts are applied. No statistically significant structure is visible in the mass range around $1.54\mathrm{GeV}/c^2$, indicated by the arrows. The third-order polynomial fit used for the upper limit estimate is shown.

Figure 5

Top: Upper limit (at the 95% confidence level) on the $\gamma d\to \Lambda {\Theta }^{+}$ total cross section as a function of the $n{K}^{+}$ invariant mass. Model (a) has been used to extract the integrated acceptance. Bottom: Upper limit of the differential cross section $d\sigma /dt$ as a function of $t$, for $1.52<M(n{K}^{+})<1.56\mathrm{GeV}/c^2$. The triangles and the circles represent, respectively, the results obtained using models (a) and (c).