Measurement of the mass of the Λb baryon

In a data sample of four million hadronic Z decays collected with the ALEPH detector at LEP, four Λ_b baryon candidates are exclusively reconstructed in the Λ_b → Λ_c⁺π⁻ channel, with the Λ_c⁺ decaying into pK⁻π⁺, , or Λπ⁺π⁺π⁻. The probability of the observed signal to be due to a background fluctuation is estimated to be 4.2 × 10⁻⁴. The mass of the Λ_b is measured to be 5614±21 (stat.) ± 4 (syst.) MeV/c².