Measurement of Bs → Ds(*)+ Ds(*)- Branching Ratios

Primary Authors: Michael Feindt, Dominik Horn, Michal Kreps, Thomas Kuhr

Introduction

In case of a finite decay width difference between the mass eigenstates of the Bs system, ΔΓs = Γ(Bs,L) - Γ(Bs,H), the experimental sensitivity to CP violation is improved and can even be measured in analyses without flavor tagging. The dominant contribution to ΔΓs is believed to come from the Bs → Ds(*)+ Ds(*)- decay, which is predominantly CP-even and saturates ΔΓs under certain theoretical assumptions, resulting in the relation 2 BR(Bs → Ds(*)+ Ds(*)-) ≈ ΔΓs / (Γs + ΔΓs/2).

In a data sample corresponding to an integrated luminosity of 6.8 fb-1 collected by the displaced track trigger we reconstruct about 750 Bs → Ds(*)+ Ds(*)- decays and measure the Bs production rate times Bs → Ds(*)+ Ds(*)- branching ratios relative to the normalization mode B0 → Ds+ D-:

  fDs(*)Ds(*) = fs/fd BR(Bs → Ds(*)+ Ds(*)-) / BR(Ds+ D-)

The relative branching fractions are determined in a simultaneous mass fit to two signal and two normalization samples. The measured fDs(*)Ds(*) values depend on the yields observed on the four samples and the relative efficiency of signal and normalization modes. An accurate determination of the relative efficiencies is achieved by taking into account, for the first time, the Dalitz structure of the Ds+ decay.

The analysis is described in detail in Public Note 10721.

Results

We measure the following relative branching ratios

fDsDs = 0.183 ± 0.021 ± 0.017
fDs*Ds = 0.424 ± 0.046 ± 0.035
fDs*Ds* = 0.654 ± 0.072 ± 0.065
fDs(*)Ds(*) = 1.261 ± 0.095 ± 0.112,

where the uncertainties are statistical and systematic. Using measured values of production and branching fractions, the following absolute branching fractions are derived

BR(Bs → Ds+ Ds-) = (0.49 ± 0.06 ± 0.05 ± 0.08) %,
BR(Bs → Ds Ds) = (1.13 ± 0.12 ± 0.09 ± 0.19) %,
BR(Bs → Ds*+ Ds*-) = (1.75 ± 0.19 ± 0.17 ± 0.29) %,
BR(Bs → Ds(*)+ Ds(*)-) = (3.38 ± 0.25 ± 0.30 ± 0.56) %,

where the uncertainties are statistical, systematic, and due to the normalization. Assuming the equation for ΔΓs given above to hold this would translate into a decay width difference contribution of the Bs → Ds(*)+ Ds(*)- modes of

ΔΓs / Γs = (6.99 ± 0.54 ± 0.64 ± 1.20)%.

List of Figures