Measurement of B_s → D_s^(*)+ D_s^(*)- Branching Ratios

Introduction

In case of a finite decay width difference between the mass eigenstates of the B_s system, ΔΓ_s = Γ(B_s,L) - Γ(B_s,H), the experimental sensitivity to CP violation is improved and can even be measured in analyses without flavor tagging. The dominant contribution to ΔΓ_s is believed to come from the B_s → D_s^(*)+ D_s^(*)- decay, which is predominantly CP-even and saturates ΔΓ_s under certain theoretical assumptions, resulting in the relation 2 BR(B_s → D_s^(*)+ D_s^(*)-) ≈ ΔΓ_s / (Γ_s + ΔΓ_s/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 B_s → D_s^(*)+ D_s^(*)- decays and measure the B_s production rate times B_s → D_s^(*)+ D_s^(*)- branching ratios relative to the normalization mode B⁰ → D_s⁺ D^-:

The relative branching fractions are determined in a simultaneous mass fit to two signal and two normalization samples. The measured f_Ds(*)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 D_s⁺ decay.

The analysis is described in detail in Public Note 10721.

Results

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

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

ΔΓ_s / Γ_s = (6.99 ± 0.54 ± 0.64 ± 1.20)%.

List of Figures

• Invariant mass distribution of B_s candidates: (eps) (png)
• Invariant mass distribution of B⁰ candidates: (eps) (png)
• Network output distribution for B_s and B⁰ candidates with D_s⁺ → φπ⁺: (eps) (png)
• Network output distribution for B_s and B⁰ candidates with D_s⁺ → K^*0K⁺: (eps) (png)
• Purity vs. network output for B_s and B⁰ candidates with D_s⁺ → φπ⁺: (eps) (png)
• Purity vs. network output for B_s and B⁰ candidates with D_s⁺ → K^*0K⁺: (eps) (png)