Study of exotic hadrons, which cannot be interpreted as ordinary threequark baryons or quark-antiquark mesons, would offer us a good opportunity to investigate low-energy quark dyanmics. The pentaquark Θ + has a strangeness quantum number S = +1 with its minimal quark configuration of uudds. The distinct features of the Θ + are a light mass of about 1540 MeV and a narrow width of a few MeV or less. If such an exotic pentaquark exits, it is quite interesting from a viewpoint of the hadron structure. Many experiments searched for the Θ + so far, but the experimental situation was still controversial. For further investigation, a high-statistics and high-resolution experiment has been required. The differential cross section of the π − p → K − X reaction was measured in forward scattering angles to search for the Θ + pentaquark with the missing-mass technique. The experiment was performed in 2012 using a beam momentum of 2.01 GeV/c at the K1.8 beam line in the J-PARC hadron facility. A liquid hydrogen target with a thickness of 0.85 g/cm 2 was exposed to 8.1 × 10 10 π − beams with a typical intensity of 1.7 × 10 6 per 2.2-second spill. We constructed a high-resolution spectrometer system. The beam pions were measured with the beam spectrometer with a momentum resolution of 1 × 10 −3 (FWHM). The outgoing kaons were identified with the SKS spectrometer with a momentum resolution of 2 × 10 −3 (FWHM). We measured the missing mass of the π − p → K − X reaction at forward scattering angles of 2-15 • in the laboratory frame. The missing-mass resolution for the Θ + was estimated to be 2.13 ± 0.15 MeV (FWHM). Using the Σ ± production data and the beam-through data, the absolute scale of the missing mass for the Θ + production data was calibrated with an uncertainty of 1.4 MeV/c 2. Thus, it is demonstrated that we are able to observe a sharp missing-mass peak and determine the mass and, possibly, width with a good precision, if
