Measurement of BR(Λb0 --> Λc+ π- π+ π-)/BR(Λb0 --> Λc+ π-)

Authors: P. Azzurri, P. Barria, M.A. Ciocci, S. Donati, P. Maestro, E. Vataga

Introduction

Due to the high b-quark mass, weak decays of baryons containing a b quark are a good testing ground of some approximations in quantum chromodynamics (QCD) calculations, such as heavy-quark effective theory (HQET). Alternatively, when using such calculations, the Λb0 may provide a determination of the Cabibbo-Kobayashi-Maskawa (CKM) couplings with systematic uncertainties different from the determinations from the decays of B mesons. While the B mesons are well studied, less is known about the Λb0 baryon. Only nine decay modes of the Λb0 have been observed so far, with the sum of their measured branching fractions of the order of only 0.1 and with large uncertainties on the measurements [1]. While theoretical predictions are available for the Λb0 --> Λc+ π- branching fraction, no prediction is currently available for the Λb0 --> Λc+ π- π+ π- decay mode.

In this analysis, we studied the Λb0 --> Λc+ π- π+ π- decay and reconstructed with high yields the charm resonant decay modes:

Λb0 -->Λc(2595)+ π- --> Λc+ π- π+ π-

Λb0 -->Λc(2625)+ π- -->Λc+ π- π+ π-

Λb0 -->Σc(2455)++ π- π- -->Λc+ π- π+ π-

Λb0 -->Σc(2455)0 π+ π- -->Λc+ π- π+ π-

We measured the ratio BR(Λb0 --> Λc+ π- π+ π-)/ BR(Λb0 --> Λc+ π-) and the contribution of each resonant decay modes to the inclusive Λb0 --> Λc+ π- π+ π- branching fraction.

The details of the analysis (blessed on July 21st 2011) are reported in arXiv:1112.3334 (accepted by Phys. Rev. D).

Analysis overview

The analysis uses 2.4 fb-1 of data collected by the CDF trigger on impact parameters (two-track-trigger). The Λc+ is reconstructed in the p K- π+ decay mode and three tracks, assumed to be pions, are added to reconstruct the Λb0 --> Λc+ 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 ].

The Λb0 -->Λc(2595)+ π- and Λb0 -->Λc(2625)+ π- candidates are identified by searching for resonant signals in the mass difference M(Λc+ π- π+) - M(Λc+) distribution, reconstructed in a ±3 sigma mass window on the Λb0 signal.

The Λb0 -->Σc(2455)++ π- π- candidates are identified by searching for a resonant signal in the mass difference M(Λc+ π+) - M(Λc+) distribution, reconstructed in a ±3 sigma mass window on the Λb0 signal.

The Λb0 -->Σc(2455)0 π+ π- candidates are identified by searching for a resonant signal in the mass difference M(Λc+ π-) - M(Λc+) distribution, reconstructed in a ±3 sigma mass window on the Λb0 signal.

The Λb0 -->Λc+ π- signal used as a normalisation for the relative branching fraction measurement is reconstructed in the same sample of 2.4 fb-1 of data collected by two-track-trigger and by applying the same optimised selection [ Optimised Selection]. Table [ Yields ] reports the measured yields of the reconstructed signals and their significance.

The relative Branching Fractions of the resonant decay modes are measured using the yields of each decay mode estimated by fitting the data and the relative efficiencies estimated with Monte Carlo.

As a cross-check of our analysis, we have also measured the relative branching fraction BR( B0 --> D- π+ π- π+) /BR( B0 --> D- π+) where the B0 candidates have been reconstructed in the same set of data used for Λb0 .

Final results

  • We quote the relative branching fractions of the Λb0 --> Λc+ π- π+ π- resonant decay modes to the Λb0 --> Λc+ π- decay mode: [ Relative BF ]

    The estimated contribution of each source of systematic error to the overall systematic uncertainty is detailed in the tables [ Systematics I ] [ Systematics II ].

  • We also quote the absolute branching fractions of the Λb0 --> Λc+ π- π+ π- decay modes, computed by multiplying the measured relative branching fractions by the current world average BR(Λb0 --> Λ+c π-)=(8.8 ±3.2) × 10-3: [ Absolute BF ]

  • We made a cross-check of the analysis reported in [2,3] and measured the following relative branching fractions of the Λb0 --> Λc+ π- π+ π- resonant decay modes:

    The estimated contribution of each source of systematic error to the overall systematic uncertainty is detailed in the tables [ Systematics III ] [ Systematics IV ].

  • As a cross-check of our analysis, we also measured:

  • List of Figures

    References

    1. K. Nakamura et al. (Particle Data Group), J. Phys. G 37, 075021 (2010).
    2. P. Azzurri et al, "Measurement of resonant contributions of the Λb0 --> Λc+ π- π+ π- decay", CDF Internal Note 9854
    3. P. Azzurri et al, "Measurement of resonant contributions of the Λb0 --> Λc+ π- π+ π- decay", CDF Public Note 10001
    4. R. Aaij et al. (LHCb Collaboration), Phys. Rev. D 84, 092001 (2011)
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