Search for ${\rm\Theta}(1540)^ +$ in the exclusive proton-induced reaction ${\rm p + {C(N)\to \Theta^ + \bar{K}^0 + {C}(N)}}$ at the energy of 70 GeV

Abstract.

A search for narrow $\Theta(1540)^ +$, a candidate for pentaquark baryon with positive strangeness, has been performed in an exclusive proton-induced reaction $p + \mathrm{C}(N)\to \Theta^ + \bar{K}^0 + \mathrm{C}(N)$ on carbon nuclei or quasifree nucleons at E _beam = 70 GeV ($\sqrt{s} = 11.5$ GeV) studying nK ⁺ , pK _S ⁰ and pK _L ⁰ decay channels of $\Theta(1540)^ +$ in four different final states of the $\Theta^ + \bar{K}^0$ system. In order to assess the quality of the identification of the final states with neutron or K ⁰ _L , we reconstructed $\Lambda(1520)\to nK^0_S$ and $\phi\to K^0_LK^0_S$ decays in the calibration reactions $p + \mathrm{C}(N)\to \Lambda(1520)K^ + + \mathrm{C}(N)$ and $p + \mathrm{C}(N)\to p\phi + \mathrm{C}(N)$. We found no evidence for narrow pentaquark peak in any of the studied final states and decay channels. Assuming that the production characteristics of the $\Theta^ + \bar{K^0}$ system are not drastically different from those of the $\Lambda(1520)K^ +$ and $p\phi$ systems, we established upper limits on the cross-section ratios $\sigma(\Theta^ + \bar{K}^0) / \sigma(\Lambda(1520)K^ + ) <$ 0.02 and $\sigma(\Theta^ + \bar{K}^0) / \sigma(p\phi ) <$ 0.15 at 90% CL and a preliminary upper limit for the forward hemisphere cross-section $\sigma(\Theta^ + \bar{K}^0) <$ 30 nb/nucleon.