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First Observation of Heavy Baryons Σ_b and Σ_b^*

We report an observation of new bottom baryons produced in proton-antiproton collisions at the Tevatron. Using 1.1 fb^{-1} of data collected by the CDF II detector, we observe four Λ_b^0π^{\pm} resonances in the fully reconstructed decay mode Λ_b^0 \to Λ_c^+ π^-, where Λ_c^+ \to p K^- π^+. We interpret these states as the Σ_b^{(*)\pm} baryons and measure the following masses: m_{Σ_b^+} = 5807.8^{+2.0}_{-2.2}(stat.) \pm 1.7(syst.) MeV/c^2, m_{Σ_b^-} = 5815.2 \pm 1.0(stat.) \pm 1.7(syst.) MeV/c^2, and m(Σ_b^*) - m(Σ_b) = 21.2^{+2.0}_{-1.9}(stat.) ^{+0.4}_{-0.3}(syst.) MeV/c^2.