Observation of new suppressed Bs decays and measurement of their branching ratios


Primary authors:Olga Norniella, Benjamin Carls, Kevin Pitts

Introduction

We report the first observation of Bs →J/ψ K* and Bs →J/ψ KS decays in a sample corresponding to 5.9 fb-1 collected by a dedicated low pT di-muon trigger. A cut based optimization was carried out for the Bs →J/ψ K* analysis, while a neural network was used for the observation of Bs →J/ψ KS. In addition to the observation of the new decay modes, their decay rates were measured using B0 decays in the same final states as a reference. From the measured quantities fsBR(Bs →J/ψ K*)/fdBR(B0 →J/ψ K*) and fsBR(Bs →J/ψ KS)/fdBR(B0 →J/ψ KS), absolute branching fractions were determined using known values of fs/fd and the branching fractions of the reference B0 decays as inputs.

The analysis is described in detail in Public Note 10240.

Motivation

While the phenomenology of B+ and B0 decays has been extensively studied at the B factory experiments, much less is known about Bs decays. This analysis is part of a broader CDF program aimed at a systematic exploration of the physics of Bs mesons. Bs →J/ψ KS is a CP eigenstate and has never been observed. Measurement of its lifetime directly probes τBs(heavy). Additionally, large samples of Bs →J/ψ KS can be used to extract the angle γ of the unitary triangle (R. Fleischer, Eur. Phys. J. C10:299-306,1999). Bs →J/ψ K* is yet another unobserved mode which contains an admixture of CP final states. An angular analysis of a significantly large enough sample of Bs →J/ψ K* can be carried out to extract sin(2 βs) as a compliment to Bs →J/ψ φ.

Procedure

The analysis initially sought out the reconstruction of B0 →J/ψ K* and B0 →J/ψ KS from a sample of di-muons with pT > 1.5 GeV/c (J/ψ →μ+μ ). Using the reconstructed B0 →J/ψ K* and B0 →J/ψ KS samples, a series of cuts were applied to optimize selection of Bs →J/ψ K* and Bs →J/ψ KS. After signal optimization, a binned likelihood fit was applied to the J/ψ KS or J/ψ K π mass distributions to extract the ratio of candidates of Bs →J/ψ K*(S) to B0 →J/ψ K*(S). In both channels, the Bs and B0 were modeled by three gaussian templates constructed from MC samples. Combinatorial backgrounds were modeled using an exponential. Partially reconstructed B decays were modeled by ARGUS functions convoluted with a gaussian. In the Bs →J/ψ K* sample, a two gaussian template, normalized from a separate measurement of Bs →J/ψ φ, was applied to account for Bs →J/ψ φ contributions. After application of a relative acceptance factor determined from simulation, the quantity fsBR(Bs →J/ψ K*(S))/fdBR(B0 →J/ψ K*(S)) was measured.

Systematic uncertainties are classified as either fit-related uncertainties or uncertainties on the relative acceptance. Fit uncertainties included combinatorial background modeling and modeling of the Bs and B0 signals. For the required relative acceptances, there were uncertainties in the modeling of the B hadron transverse momentum spectrum, B hadron lifetime, and polarization for Bs →J/ψ K*.

Significance

In comparing the signal hypotheses to the null hypothesis, we obtain a p-value for Bs →J/ψ K* of 8.9 × 10−16 or 8 σ. Alternatively, for Bs →J/ψ KS, we determined a p-value of 3.9 × 10−13 or 7.2 σ.

Results

For Bs →J/ψ K* we measure:

After application of the CDF value of fs/fd = 0.269 ± 0.033 (determined from Phys. Rev. D77, 072003 (2008) and the new PDG value for BR(Ds →φ π)), the ratio of branching ratios is determined to be:

Using the PDG value of BR(B0 →J/ψ K*) = (1.33 ± 0.06) × 10−3, we arrive at the absolute branching ratio:



Alternatively, for Bs →J/ψ KS we measure:

After application of the CDF value of fs/fd = 0.269 ± 0.033, the ratio of branching ratios is determined to be:

Using the PDG value of BR(B0 →J/ψ KS) = (8.71 ± 0.32) × 10−3, we arrive at the absolute branching ratio:


Figures

The figures crucial to the analyses appear below: