Search for the deeply bound $K^{-}pp$ state from the semi-inclusive forward-neutron spectrum in the in-flight $K^{-}$ reaction on helium-3

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 at ${\theta }_n^{lab}=0^{\circ }$, 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 $\mathrm{\Lambda }p$ final state in different reactions. Assuming the state to be isotropically decaying into $\mathrm{\Lambda }p$, 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.