Evidence for a Kaon-Bound State $K^{-}pp$ Produced in $K^{-}$ Absorption Reactions at Rest

We have searched for a deeply bound kaonic state by using the FINUDA spectrometer installed at the $e^{+}{e}^{-}$ collider $\mathrm{DA}\Phi \mathrm{NE}$. Almost monochromatic $K^{-}$’s produced through the decay of $\varphi (1020)$ mesons are used to observe $K^{-}$ absorption reactions stopped on very thin nuclear targets. Taking this unique advantage, we have succeeded to detect a kaon-bound state $K^{-}pp$ through its two-body decay into a $\Lambda$ hyperon and a proton. The binding energy and the decay width are determined from the invariant-mass distribution as ${115}_{-5}^{+6}(\mathrm{stat}{)}_{-4}^{+3}(\mathrm{syst})\mathrm{MeV}$ and ${67}_{-11}^{+14}(\mathrm{stat}{)}_{-3}^{+2}(\mathrm{syst})\mathrm{MeV}$, respectively.

Figure 1

Global views (top, complete apparatus; bottom, interaction/target region) of the FINUDA apparatus.

Figure 2

(a) Invariant-mass distribution of a proton and a ${\pi }^{-}$ for all the events in which these two particles are observed, fitted by a single Gaussian together with a linear background in the invariant-mass range of $1100–1130\mathrm{MeV}/c^2$. (b) Opening angle distribution between a $\Lambda$ and a proton: solid line, ${}^6\mathrm{Li}$, ${}^7\mathrm{Li}$, and ${}^{12}\mathrm{C}$; dashed line, ${}^{27}\mathrm{Al}$ and ${}^{51}\mathrm{V}$. The shaded area ($\cos {\theta }^{\mathrm{Lab}}<-0.8$) is selected as the back-to-back event.

Figure 3

Invariant mass of a $\Lambda$ and a proton in back-to-back correlation ($\cos {\theta }^{\mathrm{Lab}}<-0.8$) from light targets before the acceptance correction. The inset shows the result after the acceptance correction for the events which have two protons with well-defined good tracks. Only the bins between 2.22 and $2.33\mathrm{GeV}/c^2$ are used for the fitting.