Search for a Stable Six-Quark State at BABAR

Recent investigations have suggested that the six-quark combination $uuddss$ could be a deeply bound state ($S$) that has eluded detection so far, and a potential dark matter candidate. We report the first search for a stable, doubly strange six-quark state in $\mathrm{\Upsilon }\to S\overline{\mathrm{\Lambda }}\overline{\mathrm{\Lambda }}$ decays based on a sample of $90\times 10^6\mathrm{\Upsilon }(2S)$ and $110\times 10^6\mathrm{\Upsilon }(3S)$ decays collected by the BABAR experiment. No signal is observed, and 90% confidence level limits on the combined $\mathrm{\Upsilon }(2S,3S)\to S\overline{\mathrm{\Lambda }}\overline{\mathrm{\Lambda }}$ branching fraction in the range $(1.2–1.4)\times 10^{-7}$ are derived for $m_S<2.05\mathrm{GeV}$. These bounds set stringent limits on the existence of such exotic particles.

Figure 1

The distribution of the extra neutral energy, $E_{\text{extra}}$, before performing the kinematic fit for the combined $\mathrm{\Upsilon }(2S)$ and $\mathrm{\Upsilon }(3S)$ datasets, together with various background estimates (stacked histograms) and signal MC predictions (solid line). The requirement on $E_{\text{extra}}$ is indicated by a dashed line. The signal MC prediction is normalized to a branching fraction $\mathcal{B}(\mathrm{\Upsilon }\to S\overline{\mathrm{\Lambda }}\overline{\mathrm{\Lambda }})=5\times 10^{-7}$. The background MC normalization is described in the text.

Figure 2

The distribution of the $p\pi$ invariant mass, $m(p\pi )$, before performing the kinematic fit for the combined $\mathrm{\Upsilon }(2S)$ and $\mathrm{\Upsilon }(3S)$ datasets, together with various background estimates (stacked histograms). Two entries per event are plotted.

Figure 3

The distribution of the recoil mass squared against the $\mathrm{\Lambda }\mathrm{\Lambda }$ system, $m_{\text{rec}}^2$, after performing the kinematic fit for the combined $\mathrm{\Upsilon }(2S)$ and $\mathrm{\Upsilon }(3S)$ datasets, together with various background estimates (stacked histograms) and a signal example for (a) the $E_{\text{extra}}<0.5\mathrm{GeV}$ signal region and (b) the $E_{\text{extra}}>0.5\mathrm{GeV}$ sideband data sample.

Figure 4

The 90% C.L. upper limits on the $\mathrm{\Upsilon }(2S,3S)\to S\overline{\mathrm{\Lambda }}\overline{\mathrm{\Lambda }}$ branching fraction for the $\mathrm{\Upsilon }(2S)$ and $\mathrm{\Upsilon }(3S)$ datasets, as well as the combined sample assuming the same partial width.