Observation of the Decay Λ_{b}^{0}→Λ_{c}^{+}τ^{-}ν[over ¯]_{τ}

Area of Science:

• Particle Physics
• High-Energy Physics
• B-Physics

Background:

• Semileptonic b-baryon decays are crucial for testing the Standard Model of particle physics.
• Previous studies have focused on decays involving lighter leptons, leaving the tau lepton channel less explored.
• The LHCb experiment at the Large Hadron Collider (LHC) provides a unique environment for studying such rare decays.

Purpose of the Study:

• To report the first observation of the semileptonic decay Lambda_b^0 -> Lambda_c^+ tau^- nu.
• To measure the branching fraction and ratios of this decay mode.
• To compare the results with Standard Model predictions and probe for potential new physics.

Main Methods:

• Analysis of a data sample corresponding to 3 fb^-1 of integrated luminosity collected by LHCb at 7 and 8 TeV.
• Reconstruction of the tau^- lepton through its hadronic decay channel (three charged pions).
• Measurement of branching fractions and ratios using a normalization channel (Lambda_b^0 -> Lambda_c^+ pi^- pi^+ pi^-).

Main Results:

• First observation of Lambda_b^0 -> Lambda_c^+ tau^- nu with a significance of 6.1 sigma.
• Measured branching fraction B(Lambda_b^0 -> Lambda_c^+ tau^- nu) = (1.50 +/- 0.16 +/- 0.25 +/- 0.23)%.
• Derived ratio of semileptonic branching fractions R(Lambda_c^+) = 0.242 +/- 0.026 +/- 0.040 +/- 0.059, consistent with the Standard Model.

Conclusions:

• The observation of this decay provides a new probe of b-baryon physics.
• The measured branching fractions and ratios are in agreement with the Standard Model predictions.
• This result contributes to the precise testing of the Standard Model in the heavy-quark sector.