First observation and measurement of the relative Branching Fractions of charm resonant decay modes in the Λ_b⁰ --> Λ_c⁺ π^- π⁺ π^- decay

Primary authors: P. Azzurri, P. Barria. M. A. Ciocci, S. Donati. E. Vataga

Analysis blessed by the B Group on October 29, 2009. Analysis details can be found in: Public note

Introduction

This page documents the first observation of the Λ_b⁰ --> Λ_c⁺ π^- π⁺ π^- decay mode and the first observation and first measurement of the relative Branching Fractions of the charm resonant decay modes

Λ_b⁰ -->Λ(2595)_c⁺ π^- --> Λ_c⁺ π^- π⁺ π^-

Λ_b⁰ -->Λ(2625)_c⁺ π^- -->Λ_c⁺ π^- π⁺ π^-

Λ_b⁰ -->Σ(2455)_c⁺⁺ π^- π^- -->Λ_c⁺ π^- π⁺ π^-

Λ_b⁰ -->Σ(2455)_c⁰ π⁺ π^- -->Λ_c⁺ π^- π⁺ π^-

This analysis was motivated by the recent observation of the similar resonant structure in the semileptonic decay mode Λ_b⁰ --> Λ_c⁺ π^- π⁺ l^- ν ([1]).

The measurements of the Branching Fractions can be compared to the theoretical predictions to test the heavy quark effective theory (HQET, [2]).

Analysis overview

The analysis uses 2.4 fb^-1 of data collected by the CDF trigger on impact parameters (two-track-trigger). The Λ_c⁺ is reconstructed in the p K^- π⁺ decay mode and three tracks, assumed to be pions, are added to reconstruct the Λ_b⁰ --> Λ_c⁺ π^- π⁺ π^- decay mode. The offline selection is optimized by maximising the figure of merit S/(S+B)^1/2 where S is the signal yield and B is the background [ Optimised Selection ]. A clean signal is observed in the mass difference m(Λ_c⁺ π^- π⁺ π^-) - m(Λ_c⁺).

The Λ_b⁰ -->Λ(2595)_c⁺ π^- and Λ_b⁰ -->Λ(2625)_c⁺ π^- candidates are identified by searching for resonant signals in the mass difference m(Λ_c⁺ π^- π⁺) - m(Λ_c⁺) distribution, reconstructed in a +-3 sigma mass window on the Λ_b⁰ signal.

The Λ_b⁰ -->Σ(2455)_c⁺⁺ π^- π^- candidates are identified by searching for a resonant signal in the mass difference m(Λ_c⁺ π⁺) - m(Λ_c⁺) distribution, reconstructed in a +-3 sigma mass window on the Λ_b⁰ signal.

The Λ_b⁰ -->Σ(2455)_c⁰ π⁺ π^- candidates are identified by searching for a resonant signal in the mass difference m(Λ_c⁺ π^-) - m(Λ_c⁺) distribution, reconstructed in a +-3 sigma mass window on the Λ_b⁰ signal.

Final results.

List of Figures.

• Mass distributions
• • m(Λ_b⁰)-m(Λ_c⁺) distribution made for Λ_b⁰ --> Λ_c⁺ π^- π⁺ π^- candidates selected by the optimised selection [ eps ]
  • m(Λ_c⁺ π⁺ π ^-)-m(Λ_c⁺) distribution made in a +- 3 sigma mass window on the Λ_b⁰ [ eps ]
  • m(Λ_b⁰)-m(Λ_c⁺) distribution made for candidates in the Λ(2595)_c⁺ and Λ(2595)_c⁺ mass window. [ eps ]
  • m(Λ_b⁰)-m(Λ_c⁺) distribution made for candidates in the Λ(2595)_c⁺ mass window. [ eps ]
  • m(Λ_b⁰)-m(Λ_c⁺) distribution made for candidates in the Λ(2625)_c⁺ mass window. [ eps ]
  • m(Λ_c⁺ π⁺)-m(Λ_c⁺) distribution made in a +- 3 sigma mass window on the Λ_b⁰ [ eps ]
  • m(Λ_b⁰)-m(Λ_c⁺) distribution made for candidates in the Σ(2455)_c⁺⁺ mass window. [ eps ]
  • m(Λ_c⁺ π^-)-m(Λ_c⁺) distribution made in a +- 3 sigma mass window on the Λ_b⁰ [ eps ]
  • m(Λ_b⁰)-m(Λ_c⁺) distribution made for candidates in the Σ(2455)_c⁰ mass window. [ eps ]
  • m(Λ_b⁰)-m(Λ_c⁺) distribution made for Λ_b⁰ --> Λ_c⁺ π^- π⁺ π^- candidates selected with the veto on the charm resonances[ eps ]

References.

1. T. Aaltonen et al, The CDF Collaboration, "First measurement of the ratio of Branching Fractions BR(Λ_b⁰ --> Λ_c⁺ μ^- ν)/ BR(Λ_b⁰ --> Λ_c⁺ π^-)", Phys. Rev. D79, 032001 (2009).
2. A. V. Manohar and M. B. Wise, Cambr. Monogr. Part. Phys. Nucl. Phys. Cosmol. 10, 1 (2000); S. Godfrey and N. Isgur, Phys. Rev. D32, 189 (1985);