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The Θ(+) pentaquark baryon was searched for via the π(-)p→K(-)X reaction with a missing mass resolution of 1.4 MeV/c(2) (FWHM) at the Japan Proton Accelerator Research Complex (J-PARC). π(-) meson beams were incident on the liquid hydrogen target with a beam momentum of 1.92 GeV/c. No peak structure corresponding to the Θ(+) mass was observed. The upper limit of the production cross section averaged over the scattering angle of 2° to 15° in the laboratory frame is obtained to be 0.26 μb/sr in the mass region of 1.51-1.55 GeV/c(2). The upper limit of the Θ(+) decay width is obtained to be 0.72 and 3.1 MeV for J(Θ)(P)=1/2(+) and J(Θ)(P)=1/2(-), respectively, using the effective Lagrangian approach.

The forward inclusive pion double charge exchange reaction, 16 O(π − , π + )X, at T0 = 0.50 and 0.75 GeV has been studied in the kinematic region where an additional pion production is forbidden by energy-momentum conservation. The experiment was performed with the SKS spectrometer at KEK PS. The measured ratio of double charge exchange cross-section for these energies dσ(0.50 GeV)/dΩ / dσ(0.75 GeV)/dΩ = 1.7 ± 0.2, disagrees with the value of 7.2 predicted within the conventional sequential single charge exchange mechanism. Possible reasons for the disagreement are discussed in connection with the Glauber inelastic rescatterings.PACS numbers: 13.75. Gx, 25.10.+s, 25.80.Gn The pion double charge exchange reaction (DCX) is a good testing ground for probing nucleon-nucleon correlations in a nucleus due to its two nucleon nature. In the past this reaction was extensively studied at LAMPF energies T 0 ≤ 0.50 GeV [1] and it was shown that the DCX can be described reasonably well in the framework of sequential single charge exchange (SSCX) models. In these models, pion DCX is explained by two successive single charge exchanges with a real neutral pion in an intermediate state ( Fig. 1(a), H 0 = π 0 ). The SSCX mechanism predicts rapid decrease of the forward DCX cross sections for pion energies above 0.5 GeV. For this reason high-energy DCX was suggested (see, e.g. Ref. [2,3]) as a probe of the short-range nucleon-nucleon correlations in a nucleus and new mechanisms of pion propagation in the nuclear medium. Pion DCX above 0.5 GeV can be studied experimentally only in the narrow kinematic region of outgoing pion momenta near the high energy end point where pro- * Anna.Krutenkova@itep.ru duction of additional pions is forbidden by the energymomentum conservation. This requires both a high intensity pion beam and a good spectrometer system in the 1 GeV/c region. We studied the processusing superconducting kaon spectrometer (SKS) [4] at KEK 12-GeV PS sharing the apparatus of the E438 experiment which measured the (π − ,K + ) reaction [5]. In this paper we give a brief description of the experimental setup, data taking and analysis procedures which are discussed in detail in Ref. [6].Negative pions of kinetic energy T 0 = 0.50 and 0.75 GeV with the beam flux of (1 -2) ×10 6 pions per spill were delivered to the target at the K6 beamline. The beam spectrometer consisted of a QQDQQ magnet system, four sets of the drift chambers with 24 layers of sense wire planes in total used for the momentum reconstruction and three trigger counters: a freon-gasCerenkov counter (GC) and two sets of segmented scintillation counters BH1 and BH2. Beam particle, identified by the beam spectrometer, interacted in the 5-cm long H 2 O target. Positive particles emitted in the forward direction (the reaction angle was θ ≤ 15 o ) in the processwere measured with the SKS spectrometer consisting of a superconducting dipole magnet, four sets of the drift chambers with 22 layers of sense wire planes in total and several trigger counters: T OF wall ...