[Paper Review] Observation of B+->Lambda_c+Lambda_c-K+ and B0->Lambda_c+Lambda_c^-K0 decays

The Belle Collaboration, N. Gabyshev|PubMed|Aug 6, 2005

Particle physics theoretical and experimental studies3 citations

TL;DR

This paper reports the first observation of the doubly charmed baryonic B decays $B^+ \to \Lambda_c^+ \Lambda_c^- K^+$ and $B^0 \to \Lambda_c^+ \Lambda_c^- K^0$ using data from the Belle experiment at KEK. The measured branching fractions are $(6.5^{+1.0}_{-0.9}\pm1.1\pm3.4)\times10^{-4}$ and $(7.9^{+2.9}_{-2.3}\pm1.2\pm4.1)\times10^{-4}$, respectively, with statistical significances of 15.4σ and 6.6σ, indicating a significant signal beyond background.

ABSTRACT

We report the first measurements of the doubly charmed baryonic B decays B --> Lambda c+ Lambda c- K. The B+ --> Lambda c+ Lambda c- K+ decay is observed with a branching fraction of (6.5(-0.9)(+1.0)+/-1.1+/-3.4)x10(-4) and a statistical significance of 15.4sigma. The B0 --> Lambda c+ Lambda c- K0 decay is observed with a branching fraction of (7.9(-2.3)(+2.9)+/-1.2+/-4.1)x10(-4) and a statistical significance of 6.6sigma. The branching fraction errors are statistical, systematic, and the error resulting from the uncertainty of the Lambda c+ --> pK- pi+ decay branching fraction. The analysis is based on 357 fb(-1) of data accumulated at the Upsilon(4S) resonance with the Belle detector at the KEKB asymmetric-energy e+ e- collider.

Motivation & Objective

• To observe and measure the branching fractions of doubly charmed baryonic B decays involving $\Lambda_c^+$ and $\Lambda_c^-$ baryons.
• To investigate the dynamics of $b \to c\bar{c}s$ quark transitions in B meson decays.
• To compare the branching fractions of three-body doubly charmed decays with those of two-body counterparts.
• To assess the role of phase space and color suppression in these rare decays.

Proposed method

• Reconstructing $B^+ \to \Lambda_c^+ \Lambda_c^- K^+$ and $B^0 \to \Lambda_c^+ \Lambda_c^- K^0$ decays using $e^+e^-$ collision data from the Belle experiment at KEK.
• Applying kinematic constraints via $M_{\text{bc}}$ and $\Delta M_B$ to identify $B$ candidates.
• Performing a simultaneous fit to $M(\Lambda_c^+)$ and $M(\Lambda_c^-)$ mass distributions using a Gaussian signal shape and linear background.
• Using Monte Carlo simulations to fix signal mean and width parameters and to estimate background contributions.
• Evaluating sideband regions to estimate contributions from other $B$ decays that mimic the signal.
• Applying a blinded analysis to ensure statistical integrity and reduce bias in signal extraction.

Experimental results

Research questions

• RQ1 What is the branching fraction of the $B^+ \to \Lambda_c^+ \Lambda_c^- K^+$ decay, and is it consistent with theoretical expectations?
• RQ2 How does the branching fraction of this three-body doubly charmed decay compare to the two-body $B^+ \to \bar{\Xi}_c^0 \Lambda_c^+$ decay?
• RQ3 Are there significant contributions from other $B$ decays that could mimic the signal in the $\Lambda_c^+ \Lambda_c^-$ mass distribution?
• RQ4 Why is the observed branching fraction much larger than naive estimates for color-suppressed $B \to \Lambda_c^+ \Lambda_c^- K$ decays?
• RQ5 What does the phase space behavior of the $\Lambda_c^+ \Lambda_c^-$ invariant mass distribution reveal about the decay dynamics?

Key findings

• The $B^+ \to \Lambda_c^+ \Lambda_c^- K^+$ decay was observed with a statistical significance of 15.4σ, confirming a highly significant signal.
• The branching fraction for $B^+ \to \Lambda_c^+ \Lambda_c^- K^+$ is measured as $(6.5^{+1.0}_{-0.9}\pm1.1\pm3.4)\times10^{-4}$, with systematic and statistical uncertainties.
• The $B^0 \to \Lambda_c^+ \Lambda_c^- K^0$ decay was observed with a significance of 6.6σ, indicating a robust but less significant signal.
• The branching fraction for $B^0 \to \Lambda_c^+ \Lambda_c^- K^0$ is $(7.9^{+2.9}_{-2.3}\pm1.2\pm4.1)\times10^{-4}$, consistent with the $B^+$ mode within errors.
• Background contributions from other $B$ decays were found to be negligible, with less than 1.7 events at 90% C.L. for $B^+$ and less than 0.2 events for $B^0$, justifying their exclusion.
• The observed branching fractions are of the same order of magnitude as the two-body $B^+ \to \bar{\Xi}_c^0 \Lambda_c^+$ decay, suggesting similar $b \to c\bar{c}s$ transition dynamics.