An experiment to search for the $K^-pp$ bound state was performed via the in-flight $^3$He($K^-,n)$ reaction using 5.3 $times$ $10^9$ kaons at 1 GeV/$c$ at the J-PARC hadron experimental facility. In the semi-inclusive neutron missing-mass spectrum a
t $theta_{n}^{lab}=0^circ$, no significant peak was observed in the region corresponding to $K^-pp$ binding energy larger than 80 MeV, where a bump structure has been reported in the $Lambda p$ final state in different reactions. Assuming the state to be isotropically decaying into $Lambda p$, mass-dependent upper limits on the production cross section were determined to be 30--180, 70--250, and 100--270 $mu$b/sr, for the natural widths of 20, 60, and 100 MeV, respectively, at 95% confidence level.
A new spectrometer system was designed and constructed at the secondary beam line K1.8BR in the hadron hall of J-PARC to investigate $bar K N$ interactions and $bar K$-nuclear bound systems. The spectrometer consists of a high precision beam line spe
ctrometer, a liquid $^3$He/$^4$He/D$_2$ target system, a Cylindrical Detector System that surrounds the target to detect the decay particles from the target region, and a neutron time-of-flight counter array located $sim$15 m downstream from the target position. Details of the design, construction, and performance of the detector components are described.
