Observation of the ${\Omega }_b^{-}$ baryon and measurement of the properties of the ${\Xi }_b^{-}$ and ${\Omega }_b^{-}$ baryons

We report the observation of the bottom, doubly-strange baryon ${\Omega }_b^{-}$ through the decay chain ${\Omega }_b^{-}\to J/\psi {\Omega }^{-}$, where $J/\psi \to {\mu }^{+}{\mu }^{-}$, ${\Omega }^{-}\to \Lambda K^{-}$, and $\Lambda \to p{\pi }^{-}$, using $4.2{\mathrm{fb}}^{-1}$ of data from $p\overline{p}$ collisions at $\sqrt{s}=1.96\mathrm{TeV}$, and recorded with the Collider Detector at Fermilab. A signal is observed whose probability of arising from a background fluctuation is $4.0\times {10}^{-8}$, or 5.5 Gaussian standard deviations. The ${\Omega }_b^{-}$ mass is measured to be $6054.4\pm 6.8(\mathrm{stat})\pm 0.9(\mathrm{syst})\mathrm{MeV}/c^2$. The lifetime of the ${\Omega }_b^{-}$ baryon is measured to be ${1.13}_{-0.40}^{+0.53}(\mathrm{stat})\pm 0.02(\mathrm{syst})\mathrm{ps}$. In addition, for the ${\Xi }_b^{-}$ baryon we measure a mass of $5790.9\pm 2.6(\mathrm{stat})\pm 0.8(\mathrm{syst})\mathrm{MeV}/c^2$ and a lifetime of ${1.56}_{-0.25}^{+0.27}(\mathrm{stat})\pm 0.02(\mathrm{syst})\mathrm{ps}$. Under the assumption that the ${\Xi }_b^{-}$ and ${\Omega }_b^{-}$ are produced with similar kinematic distributions to the ${\Lambda }_b^0$ baryon, we find $\frac{\sigma ({\Xi }_b^{-})\mathcal{B}({\Xi }_b^{-}\to J/\psi {\Xi }^{-})}{\sigma ({\Lambda }_b^0)\mathcal{B}({\Lambda }_b^0\to J/\psi \Lambda )}={0.167}_{-0.025}^{+0.037}(\mathrm{stat})\pm 0.012(\mathrm{syst})$ and $\frac{\sigma ({\Omega }_b^{-})\mathcal{B}({\Omega }_b^{-}\to J/\psi {\Omega }^{-})}{\sigma ({\Lambda }_b^0)\mathcal{B}({\Lambda }_b^0\to J/\psi \Lambda )}={0.045}_{-0.012}^{+0.017}(\mathrm{stat})\pm 0.004(\mathrm{syst})$ for baryons produced with transverse momentum in the range of $6–20\mathrm{GeV}/c$.

Figure 1

(a) The ${\mu }^{+}{\mu }^{-}$ mass distribution obtained in an integrated luminosity of $4.2{\mathrm{fb}}^{-1}$. The mass range used for the $J/\psi$ sample is indicated by the shaded area. (b) The $p{\pi }^{-}$ mass distribution obtained in events containing $J/\psi$ candidates. The mass range used for the $\Lambda$ sample is indicated by the shaded area.

Figure 2

An illustration (not to scale) of the ${\Omega }_b^{-}\to J/\psi {\Omega }^{-}$, $J/\psi \to {\mu }^{+}{\mu }^{-}$, ${\Omega }^{-}\to \Lambda K^{-}$, and $\Lambda \to p{\pi }^{-}$ final states as seen in the view transverse to the beam direction. Five charged tracks are used to identify three decay vertices. The final fit of these track trajectories constrains the decay hadrons ($J/\psi$, ${\Omega }^{-}$, and $\Lambda$) to their nominal masses and the helix of the ${\Omega }^{-}$ to originate from the $J/\psi$ decay vertex. The trajectory of the $K^{-}$ is projected back, indicated by a dotted curve, to illustrate how an alternative, incorrect intersection with the $\Lambda$ trajectory could exist. A comparison of the fit quality of the two $\Lambda K^{-}$ intersections is used to choose a preferred solution.

Figure 3

The invariant mass distributions of (a) $\Lambda {\pi }^{-}$ combinations and (b) $\Lambda K^{-}$ combinations in events containing $J/\psi$ candidates. Shaded areas indicate the mass ranges used for ${\Xi }^{-}$ and ${\Omega }^{-}$ candidates. The dashed histograms in each distribution correspond to $\Lambda {\pi }^{+}$ (a) and $\Lambda K^{+}$ (b) combinations. Additional shading in (b) correspond to sideband regions discussed in Sec. 4.

Figure 4

The invariant mass distributions of (a) $\Lambda {\pi }^{-}$ combinations and (b) $\Lambda K^{-}$ combinations in events containing $J/\psi$ candidates. These combinations require silicon information to be used on the hyperon track and the impact distance with respect to the primary vertex must not exceed 3 times its measurement resolution. Shaded areas indicate the mass ranges used for ${\Xi }^{-}$ and ${\Omega }^{-}$ candidates. The dashed histograms in each distribution correspond to $\Lambda {\pi }^{+}$ and $\Lambda K^{+}$ combinations.

Figure 5

(a) The mass distribution of all $J/\psi {\Omega }^{-}$ combinations. (b) The $J/\psi {\Omega }^{-}$ mass distribution for candidates with $ct>0$. This requirement removes half of the combinations due to prompt production. (c) The $J/\psi {\Omega }^{-}$ mass distribution for candidates with $ct>100\mu \mathrm{m}$. This requirement removes nearly all combinations directly produced in the $p\overline{p}$ collision.

Figure 6

(a) The invariant mass distributions of $J/\psi \Lambda K^{-}$ combinations for candidates with $\Lambda K^{-}$ in the ${\Omega }^{-}$ sidebands. (b) The invariant mass distributions of $J/\psi \Lambda K^{+}$ combinations for candidates with $\Lambda K^{+}$ in the ${\Omega }^{-}$ signal range. All other selection requirements are as in Fig. 5.

Figure 7

(a) The invariant mass distribution of $\Lambda K^{-}$ combinations for candidates with $J/\psi \Lambda K^{-}$ masses in the range $6.0–6.1\mathrm{GeV}/c^2$. (b) The invariant mass distribution of $J/\psi \Lambda K^{-}$ combinations for candidates with $\Lambda K^{-}$ masses within $6\mathrm{MeV}/c^2$ of the ${\Omega }^{-}$ mass. (c) The invariant mass distributions of $J/\psi \Lambda K^{-}$ combinations for candidates with $\Lambda K^{-}$ masses within $4\mathrm{MeV}/c^2$ of the ${\Omega }^{-}$ mass. All other selection requirements are as in Fig. 5.

Figure 8

(a,b) The invariant mass distribution of $J/\psi {\Omega }^{-}$ combinations for candidates where the transverse flight requirement of the ${\Omega }^{-}$ is greater than 0.5 and 2.0 cm. (c) The invariant mass distribution of $J/\psi {\Omega }^{-}$ combinations for candidates with at least one SVX II measurement on the ${\Omega }^{-}$ track. All other selection requirements are as in Fig. 5.

Figure 9

The invariant mass distributions of $J/\psi {\Omega }^{-}$ combinations for candidates with three alternative requirements for the transverse momentum of the $K^{-}$. (a) $p_T(K^{-})>0.8\mathrm{GeV}/c$. (b) $p_T(K^{-})>1.2\mathrm{GeV}/c$. (c) $p_T(K^{-})>1.4\mathrm{GeV}/c$. All other selection requirements are as in Fig. 5.

Figure 10

The invariant mass distributions of (a) $J/\psi K^{*}(892{)}^0$, (b) $J/\psi K_s^0$, and (c) $J/\psi \Lambda$ combinations for candidates with $ct>100\mu \mathrm{m}$. The projections of the unbinned mass fits are indicated by the dashed histograms.

Figure 11

The invariant mass distributions of (a) $J/\psi {\Xi }^{-}$ and (b) $J/\psi {\Omega }^{-}$ combinations for candidates with $ct>100\mu \mathrm{m}$. The projections of the unbinned mass fit are indicated by the dashed histograms.

Figure 12

The solid histograms represent the number of (a) $B^0\to J/\psi K^{*}(892{)}^0$, (b) $B^0\to J/\psi K_s^0$, and (c) ${\Lambda }_b^0\to J/\psi \Lambda$ candidates found in each $ct$ bin. The dashed histogram is the fit value. Yields and fit values are normalized to candidates per cm, and the bin edges are indicated. The highest and lowest bins are not bounded, but are truncated here for display purposes.

Figure 13

The solid histograms represent the number of (a) ${\Xi }_b^{-}\to J/\psi {\Xi }^{-}$ and (b) ${\Omega }_b^{-}\to J/\psi {\Omega }^{-}$ candidates found in each $ct$ bin. The dashed histogram is the fit value. Yields and fit values are normalized to candidates per cm, and the bin edges are indicated. The highest and lowest bins are not bounded, but are truncated here for display purposes.