High-resolution search for the ${\Theta }^{+}$ pentaquark via a pion-induced reaction at J-PARC

The pentaquark ${\Theta }^{+}$ has been searched for via the ${\pi }^{-}p\to K^{-}X$ reaction with beam momenta of 1.92 and 2.01 GeV/$c$ at J-PARC. A missing mass resolution of 2 MeV (FWHM) was achieved but no sharp peak structure was observed. The upper limits on the production cross section averaged over the scattering angle from $2^{\circ }$ to ${15}^{\circ }$ in the laboratory frame were found to be less than 0.28 $\mu \mathrm{b}/\mathrm{sr}$ at the 90% confidence level for both the 1.92- and 2.01-GeV/$c$ data. The systematic uncertainty of the upper limits was controlled within 10%. Constraints on the ${\Theta }^{+}$ decay width were also evaluated with a theoretical calculation using an effective Lagrangian. The present result implies that the width should be less than 0.36 and 1.9 MeV for the spin-parity of $1/2^{+}$ and $1/2^{-}$, respectively.

Figure 1

Schematic view of the experimental setup in 2012.

Figure 2

Squared mass distribution for scattered particles in the ${\Theta }^{+}$ search data in 2012. It is shown in a momentum range of 0.9–1.1 GeV/$c$. The vertex cut and the scattering angle selection of ${2–15}^{\circ }$ have been applied. The dashed lines indicate the kaon selection gate.

Figure 3

Vertex distribution along the $z$ axis (beam direction). The open histogram shows the ${\mathrm{LH}}_2$ target data. The red hatched histogram shows the empty target data normalized by the beam flux. The subtraction of these histograms represents a net contribution from ${\mathrm{LH}}_2$. $({\pi }^{-},{\pi }^{-})$ events with scattering angles from $2^{\circ }$ to ${15}^{\circ }$ are selected in both histograms. The blue dashed lines indicate the vertex cut positions.

Figure 4

Missing mass spectrum of the ${\pi }^{-}p\to K^{+}X$ reaction at 1.46 GeV/$c$ after applying the mass scale calibration. The ordinate represents the differential cross section averaged over 2–${15}^{\circ }$ in the laboratory frame. The ${\Sigma }^{-}$ peak is identified with a resolution of 2.21 $\pm$ 0.05(stat.) $\pm$ 0.1(syst.) MeV in FWHM. The quoted errors in the figure are statistical.

Figure 5

Angular distribution for the ${\pi }^{+}p\to K^{+}{\Sigma }^{+}$ reaction at 1.38 GeV/$c$. The black solid circles and the red open circles are the present E19 data in 2012 and 2010, respectively. The blue crosses are the data from Candlin et al. [37], converted from the center-of-mass frame to the laboratory one. The scattering angle is defined as the one between the outgoing kaon and the incoming pion. The quoted errors are statistical only.

Figure 6

Missing mass spectrum of the ${\pi }^{-}p\to K^{-}X$ reaction at 2.01 GeV/$c$. The experimental data are indicated by black points with statistical errors. The red histogram represents the total background shape obtained by a MC simulation including three processes: nonresonant $\overline{K}KN$ (blue dashed), $\varphi$-intermediated $K^{-}{K}^{+}n$ (green dotted) and $\Lambda \left(1520\right)$-intermediated $K^{-}{K}^{0}p$ (magenta dashed-dotted). The scale of the nonresonant components is normalized to the experimental data.

Figure 7

Missing mass spectrum of the ${\pi }^{-}p\to K^{-}X$ reaction at 2.01 GeV/$c$. The ordinate represents the differential cross section averaged over 2–${15}^{\circ }$ in the laboratory frame. The quoted errors are statistical.

Figure 8

(Top) Missing mass spectrum of the ${\pi }^{-}p\to K^{-}X$ reaction at 2.01 GeV/$c$ in the mass region of the ${\Theta }^{+}$ search. The quoted errors of the data are statistical. A fitting result at a mass of 1537 MeV/$c^2$ is also shown. The fitting function (red solid) is a second-order polynomial (green dashed) with a Gaussian peak (blue dotted) whose width is fixed by the experimental resolution of 2.13 MeV. (Bottom) Allowed signal yields for each mass. The error bars denote the statistical uncertainty. The red line indicates the upper limit at the 90% confidence level.

Figure 9

90% C.L. upper limit on the ${\Theta }^{+}$ decay width for the spin-parity of $1/2^{+}$ (top) and $1/2^{-}$ (bottom) cases. Each line indicates the different theoretical treatments: pseudoscalar (PS) and pseudovector (PV) couplings, and static ($F_s$) and covariant ($F_c$) types of form factors. The DIANA result [13] is also indicated by an open square (magenta).