Indication of a Deeply Bound and Compact $K^{-}pp$ State Formed in the $pp\to p\Lambda K^{+}$ Reaction at 2.85 GeV

We have analyzed data of the DISTO experiment on the exclusive $pp\to p\Lambda K^{+}$ reaction at 2.85 GeV to search for a strongly bound compact $K^{-}pp(\equiv X)$ state to be formed in the $pp\to K^{+}+X$ reaction. The observed spectra of the $K^{+}$ missing mass and the $p\Lambda$ invariant-mass with high transverse momenta of $p$ and $K^{+}$ revealed a broad distinct peak of $26\mathrm{\text{−}}\sigma$ confidence with a mass $M_X=2267\pm 3(\mathrm{stat})\pm 5(\mathrm{syst})\mathrm{MeV}/c^2$ and a width ${\Gamma }_X=118\pm 8(\mathrm{stat})\pm 10(\mathrm{syst})\mathrm{MeV}$. The enormously large cross section indicates formation of a compact $K^{-}pp$ with a large binding energy of $B_K=103\mathrm{MeV}$, which can be a possible gateway toward cold and dense kaonic nuclear matter.

Figure 1

(Left) A $\Delta M(p{K}^{+})$ spectrum of raw data after the kinematically constrained refit, acceptance uncorrected; the small shaded histogram includes events after $p$ and $K^{+}$ cuts applied to obtain final spectra. (Right) An acceptance-corrected Dalitz plot of the exclusive $pp\to p\Lambda K^{+}$ reaction products at 2.85 GeV.

Figure 2

Momentum distributions, $P$, versus $\cos \theta$ in c.m. of (a) $p$ and (b) $K^{+}$. Each block consists of UNC (uncorrected), SIM (simulated), and DEV (deviation) data. The horizontal dotted lines define $-0.6<\cos {\theta }_{\mathrm{cm}}(p)<0.6$ and $-0.2<\cos {\theta }_{\mathrm{cm}}(K)<0.4$, to be used for proton-angle and $K^{+}$-angle cuts.

Figure 3

(a) Events with LAP cut [$|\cos {\theta }_{\mathrm{cm}}(p)|<0.6$] and (b) events with SAP cut [$|\cos {\theta }_{\mathrm{cm}}(p)|>0.6$]. Each frame consists of (upper) DEV spectra of 2-dimensional $M(p\Lambda {)}^2$ and $\Delta M(K{)}^2$ vs $\cos {\theta }_{\mathrm{cm}}(K)$ and (lower) DEV spectra of 1-dimensional $M(p\Lambda {)}^2$ and $\Delta M(K{)}^2$.

Figure 4

(a) Observed DEV spectra of $\Delta M(K^{+})$ of events with LAP emission [$|\cos {\theta }_{\mathrm{cm}}(p)|<0.6$] and (b) with SAP emission [$|\cos {\theta }_{\mathrm{cm}}(p)|>0.6$]. Both selected with large-angle $K^{+}$ emission [$-0.2<\cos {\theta }_{\mathrm{cm}}(K^{+})<0.4$].