Observation of a <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:mover><mml:mi>K</mml:mi><mml:mo>¯</mml:mo></mml:mover><mml:mrow><mml:mi>N</mml:mi><mml:mi>N</mml:mi></mml:mrow></mml:mrow></mml:math> bound state in the <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:mmultiscripts><mml:mi>He</mml:mi><mml:mprescripts/><mml:none/><mml:mn>3</mml:mn></mml:mmultiscripts><mml:mo>(</mml:mo><mml:msup><mml:mi>K</mml:mi><mml:mo>−</mml:mo></mml:msup><mml:mo>,</mml:mo><mml:mi mathvariant="normal">Λ</mml:mi><mml:mi>p</mml:mi><mml:mo>)</mml:mo><mml:mi>n</mml:mi></mml:mrow></mml:math> reaction
T. Yamaga, S. Ajimura, H. Asano, G. Beer, H. Bhang, M. Bragadireanu, P. Buehler, L. Busso, M. Cargnelli, S. Choi, C. Curceanu, S. Enomoto, Hiroyuki Fujioka, Yuya Fujiwara, Tomokazu Fukuda, C. Guaraldo, T. Hashimoto, R. Hayano, T. Hiraiwa, M. Iio, M. Iliescu, Kentaro Inoue, Y. Ishiguro, Takashi Ishikawa, S. Ishimoto, K. Itahashi, M. Iwai, M. Iwasaki, Koki Kanno, K. Kato, Yuko Kato, Shingo Kawasaki, P. Kienle, H. Kou, Y. G., J. Márton, Y. Matsuda, Y. Mizoi, O. Morra, T. Nagae, H. Noumi, H. Ohnìshì, S. Okada, H. Outa, K. Piscicchia, Y. Sada, A. Sakaguchi, F. Sakuma, M. Sato, A. Scordo, M. Sekimoto, H. Shi, K. Shirotori, D. Sirghi, F. Sirghi, S. Suzuki, T. Suzuki, K. Tanida, H. Tatsuno, Makoto Tokuda, D. Tomono, A. Toyoda, K. Tsukada, O. Vázquez Doce, E. Widmann, T. Yamazaki, H. Yim, Q. Zhang, J. Zmeskal
Abstract
We have performed an exclusive measurement of the ${K}^{\ensuremath{-}}+\phantom{\rule{0.16em}{0ex}}^{3}\mathrm{He}\ensuremath{\rightarrow}\mathrm{\ensuremath{\Lambda}}pn$ reaction at an incident kaon momentum of $1\phantom{\rule{4pt}{0ex}}\mathrm{GeV}/c$. In the $\mathrm{\ensuremath{\Lambda}}p$ invariant mass spectrum, a clear peak was observed below the mass threshold of $\overline{K}+N+N$, as a signal of the kaonic nuclear bound state, $\overline{K}NN$. The binding energy, decay width, and $S$-wave Gaussian reaction form factor of this state were observed to be ${B}_{K}=42\ifmmode\pm\else\textpm\fi{}3{(\mathrm{stat}.)}_{\ensuremath{-}4}^{+3}(\mathrm{syst}.)\phantom{\rule{4pt}{0ex}}\mathrm{MeV}, {\mathrm{\ensuremath{\Gamma}}}_{K}=100\ifmmode\pm\else\textpm\fi{}7{(\mathrm{stat}.)}_{\ensuremath{-}9}^{+19}(\mathrm{syst}.)\phantom{\rule{4pt}{0ex}}\mathrm{MeV}$, and ${Q}_{K}=383\ifmmode\pm\else\textpm\fi{}11{(\mathrm{stat}.)}_{\ensuremath{-}1}^{+4}(\mathrm{syst}.)\phantom{\rule{4pt}{0ex}}\mathrm{MeV}/c$, respectively. The total production cross section of $\overline{K}NN$, determined by its $\mathrm{\ensuremath{\Lambda}}p$ decay mode, was ${\ensuremath{\sigma}}_{K}^{\mathrm{tot}}B{R}_{\mathrm{\ensuremath{\Lambda}}p}=9.3\ifmmode\pm\else\textpm\fi{}0.8{(\mathrm{stat}.)}_{\ensuremath{-}1.0}^{+1.4}(\mathrm{syst}.)\phantom{\rule{4pt}{0ex}}\ensuremath{\mu}\mathrm{b}$. We estimated the branching ratio of the $\overline{K}NN$ state to the $\mathrm{\ensuremath{\Lambda}}p$ and ${\mathrm{\ensuremath{\Sigma}}}^{0}p$ decay modes as $B{R}_{\mathrm{\ensuremath{\Lambda}}p}/B{R}_{{\mathrm{\ensuremath{\Sigma}}}^{0}p}\ensuremath{\sim}1.7$, by assuming that the physical processes leading to the $\mathrm{\ensuremath{\Sigma}}NN$ final states are analogous to those of $\mathrm{\ensuremath{\Lambda}}pn$.