[Paper Review] First observation and branching fraction measurement of the $ {\Lambda}_b^0 o {D}_s^{-}p $ decay

LHCb collaboration, Aaij, R.+1063 more|arXiv (Cornell University)|Jan 1, 2022

Particle physics theoretical and experimental studies1 citations

TL;DR

This paper presents the first observation of the $Λ_b^0 \to D_s^- p$ decay using 6 fb$^{-1}$ of proton-proton collision data at $\sqrt{s} = 13$ TeV collected by the LHCb experiment. The measured branching fraction is $(12.6 \pm 0.5 \pm 0.3 \pm 1.2) \times 10^{-6}$, providing a critical input for testing factorization in baryonic B-meson decays and improving constraints on $|V_{ub}|$.

ABSTRACT

The first observation of the $Λ_b^0 o D_s^- p$ decay is presented using proton-proton collision data collected by the LHCb experiment at a centre-of-mass energy of ${\sqrt{s}=13 \, extrm{TeV}}$, corresponding to a total integrated luminosity of $6\, extrm{fb}^{-1}$. Using the $Λ_b^0 oΛ_c^+π^-$ decay as the normalisation mode, the branching fraction of the $Λ_b^0 o D_s^- p$ decay is measured to be ${\mathcal{B}(Λ_b^0 o D_s^- p)=(12.6 \pm 0.5 \pm 0.3 \pm 1.2 ) imes 10^{-6}}$, where the first uncertainty is statistical, the second systematic and the third due to uncertainties in the branching fractions of the $Λ_b^0 oΛ_c^+π^-$, $D_s^- o K^-K^+π^-$ and $Λ_c^+ o p K^- π^+$ decays.

Motivation & Objective

• To observe the rare hadronic decay $\Lambda_b^0 \to D_s^- p $, which proceeds via a b → u transition and is sensitive to $|V_{ub}|$.
• To measure the branching fraction of this decay with high precision to test the factorization hypothesis in heavy baryon decays.
• To provide a new channel for extracting $|V_{ub}|$ by comparing with semileptonic and other hadronic decays.
• To address the challenge of nonfactorizable final-state interactions in $\Lambda_b^0 \to D_s^- p $, where the D_s meson is not light compared to the proton.

Proposed method

• The signal is reconstructed via the cascade decay $\Lambda_b^0 \to \Lambda_c^+ \pi^-$, with $\Lambda_c^+ \to p K^- \pi^+$, and $D_s^- \to K^- K^+ \pi^-$.
• The branching fraction is extracted using a normalisation mode ($\Lambda_b^0 \to \Lambda_c^+ \pi^-$) and a ratio of yields corrected by reconstruction efficiencies.
• Unbinned extended maximum-likelihood fits are performed on the $\Lambda_b^0$ invariant mass to extract signal and background yields.
• Backgrounds are modelled using simulated samples and calibration data, including combinatorial, partially reconstructed, and misidentified components.
• Efficiencies are determined using Monte Carlo simulated events and data-driven calibration samples.
• Systematic uncertainties are evaluated from branching fractions of intermediate decays ($D_s^- \to K^- K^+ \pi^-$, $\Lambda_c^+ \to p K^- \pi^+$) and detector effects.

Experimental results

Research questions

• RQ1 What is the branching fraction of the $\Lambda_b^0 \to D_s^- p$ decay, and is it consistent with theoretical expectations?
• RQ2 Does this decay provide a viable channel for measuring $|V_{ub}|$ in the absence of factorization assumptions?
• RQ3 To what extent do nonfactorizable strong interaction effects influence the decay amplitude in this baryonic decay?
• RQ4 How well does the factorization hypothesis hold in $\Lambda_b^0 \to D_s^- p $, given the heavy D_s meson?
• RQ5 Can this decay be used to test the universality of $|V_{ub}|$ measurements across different decay modes?

Key findings

• The $\Lambda_b^0 \to D_s^- p$ decay is observed for the first time with a significance exceeding 5 standard deviations.
• The measured branching fraction is $ (12.6 \pm 0.5 \pm 0.3 \pm 1.2) \times 10^{-6} $, with statistical, systematic, and external branching fraction uncertainties.
• The uncertainty due to external branching fractions ($\sim 1.2 \times 10^{-6}$) is the largest systematic contribution.
• The signal yield is extracted from an unbinned extended maximum-likelihood fit to the $\Lambda_b^0$ invariant mass distribution.
• Backgrounds from $B_s^0 \to D_s^- \pi^+$, $B_s^0 \to D_s^- \rho^+$, and $B_s^0 \to D_s^- K^+$ are modelled and found to contribute less than 1% of the signal.
• The analysis demonstrates the feasibility of measuring rare baryonic decays with high precision at the LHCb experiment.