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

A search for narrow Theta(1540)^+, a candidate for pentaquark baryon with positive strangeness, has been performed in an exclusive proton-induced reaction p+C(N) \to Theta^+ \bar{K}^0 + C(N) on carbon nuclei or quasifree nucleons at E_{beam}=70 GeV (sqrt{s} = 11.5 GeV) studying nK^+, pK_S and pK_L 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_L we reconstructed Lambda(1520)\to nK_S and phi\to K_LK_S decays in the calibration reactions p+C(N)\to Lambda(1520)K^+ + C(N) and p+C(N)\to p\phi + 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.