Primary authors:Olga Norniella, Benjamin Carls, Kevin Pitts

We report the first observation of B_s &rarr J/&psi K^* and B_s &rarr J/&psi K_S decays in a sample corresponding to 5.9 fb^-1 collected by a dedicated low p_T di-muon trigger. A cut based optimization was carried out for the B_s &rarr J/&psi K^* analysis, while a neural network was used for the observation of B_s &rarr J/&psi K_S. 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 f_sBR(B_s &rarr J/&psi K^*)/f_dBR(B⁰ &rarr J/&psi K^*) and f_sBR(B_s &rarr J/&psi K_S)/f_dBR(B⁰ &rarr J/&psi K_S), absolute branching fractions were determined using known values of f_s/f_d 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 B_s decays. This analysis is part of a broader CDF program aimed at a systematic exploration of the physics of B_s mesons. B_s &rarr J/&psi K_S is a CP eigenstate and has never been observed. Measurement of its lifetime directly probes &tau_Bs(heavy). Additionally, large samples of B_s &rarr J/&psi K_S can be used to extract the angle &gamma of the unitary triangle (R. Fleischer, Eur. Phys. J. C10:299-306,1999). B_s &rarr J/&psi K^* is yet another unobserved mode which contains an admixture of CP final states. An angular analysis of a significantly large enough sample of B_s &rarr J/&psi K^* can be carried out to extract sin(2 &beta_s) as a compliment to B_s &rarr J/&psi &Phi.

Procedure

The analysis initially sought out the reconstruction of B⁰ &rarr J/&psi K^* and B⁰ &rarr J/&psi K_S from a sample of di-muons with p_T > 1.5 GeV/c (J/&psi &rarr &mu⁺&mu^&minus ). Using the reconstructed B⁰ &rarr J/&psi K^* and B⁰ &rarr J/&psi K_S samples, a series of cuts were applied to optimize selection of B_s &rarr J/&psi K^* and B_s &rarr J/&psi K_S. After signal optimization, a binned likelihood fit was applied to the J/&psi K_S or J/&psi K &pi mass distributions to extract the ratio of candidates of B_s &rarr J/&psi K^*_(S) to B⁰ &rarr J/&psi K^*_(S). In both channels, the B_s 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 B_s &rarr J/&psi K^* sample, a two gaussian template, normalized from a separate measurement of B_s &rarr J/&psi &Phi, was applied to account for B_s &rarr J/&psi &Phi contributions. After application of a relative acceptance factor determined from simulation, the quantity f_sBR(B_s &rarr J/&psi K^*_(S))/f_dBR(B⁰ &rarr J/&psi 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 B_s 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 B_s &rarr J/&psi K^*.

Significance

In comparing the signal hypotheses to the null hypothesis, we obtain a p-value for B_s &rarr J/&psi K^* of 8.9 × 10^{&minus 16} or 8 σ. Alternatively, for B_s &rarr J/&psi K_S, we determined a p-value of 3.9 × 10^{&minus 13} or 7.2 σ.

Results

For B_s &rarr J/&psi K^* we measure:

• B⁰ &rarr J/&psi K^* yield: 9530 ± 110, B_s &rarr J/&psi K^* yield: 151 ± 25
• f_sBR(B_s &rarr J/&psi K^*)/f_dBR(B⁰ &rarr J/&psi K^*) = 0.0168 ± 0.0024 (stat.) ± 0.0068 (syst.)

• BR(B_s &rarr J/&psi K^*)/BR(B⁰ &rarr J/&psi K^*) = 0.062 ± 0.009 (stat.) ± 0.025 (syst.) ± 0.008 (frag.)

• BR(B_s &rarr J/&psi K^*) = (8.3 ± 1.2 (stat.) ± 3.3 (syst.) ± 1.0 (frag.) ± 0.4(PDG)) × 10 ^{&minus 5}

• B⁰ &rarr J/&psi K_S yield: 5954 ± 79, B_s &rarr J/&psi K_S yield: 64 ± 14
• f_sBR(B_s &rarr J/&psi K_S)/f_dBR(B⁰ &rarr J/&psi K_S) = 0.0109 ± 0.0019 (stat.) ± 0.0011 (syst.)

• BR(B_s &rarr J/&psi K_S)/BR(B⁰ &rarr J/&psi K_S) = 0.0405 ± 0.0070 (stat.) ± 0.0041 (syst.) ± 0.0050 (frag.)

• BR(B_s &rarr J/&psi K⁰) = (3.53 ± 0.61 (stat.) ± 0.35 (syst.) ± 0.43 (frag.) ± 0.13 (PDG)) × 10 ^{&minus 5}

• Figure 1: B_s &rarr J/&psi K^* mass distribution, fit, and residuals (.ps) (.gif)
• Figure 2: Log likelihood scan of N(B_s &rarr J/&psi K^*)/N(B⁰ &rarr J/&psi K^*) (.ps) (.gif)
• Figure 3: Sideband subtracted B proper decay length of B_s &rarr J/&psi K^* signal region (.ps) (.gif)
• Figure 4: Optimization of efficiency/(1.5 + &radic B) as a function of K p_T for B_s &rarr J/&psi K^* (.ps) (.gif)
• Figure 5: Optimization of efficiency/(1.5 + &radic B) as a function of &pi p_T for B_s &rarr J/&psi K^* (.ps) (.gif)
• Figure 6: Optimization of efficiency/(1.5 + &radic B) as a function of B L_xy for B_s &rarr J/&psi K^* (.ps) (.gif)
• Figure 7: Optimization of efficiency/(1.5 + &radic B) as a function of Prob(B) for B_s &rarr J/&psi K^* (.ps) (.gif)
• Figure 8: Mass distribution and fit with layered contributions for B_s &rarr J/&psi K^* (.ps) (.gif)
• Figure 9: Zoomed in Mass distribution and fit with layered contributions for B_s &rarr J/&psi K^* (.ps) (.gif)
• Figure 10: B_s &rarr J/&psi K_S mass distribution, fit, and residuals (.ps) (.gif)
• Figure 11: Log likelihood scan of N(B_s &rarr J/&psi K_S)/N(B⁰ &rarr J/&psi K_S) (.ps) (.gif)
• Figure 12: K_S mass distribution for B_s &rarr J/&psi K_S (.ps) (.gif)
• Figure 13: The J/&psi K_S invariant mass distribution without the neural network cut (.ps) (.gif)
• Figure 14: The J/&psi K_S invariant mass distribution with the neural network cut (.ps) (.gif)
• Figure 15: The J/&psi K_S invariant mass distribution with different neural network cuts (.ps) (.gif)
• Figure 16: The optimization of efficiency/(1.5 + &radic B) for B_s &rarr J/&psi K_S (.ps) (.gif)
• Figure 17: The B_s &rarr J/&psi K_S signal and background regions for the neural network (.ps) (.gif)
• Figure 18: Neural network response for B_s &rarr J/&psi K_S (.ps) (.gif)
• Figure 19: Mass distribution and fit with layered contributions for B_s &rarr J/&psi K_S (.ps) (.gif)
• Figure 20: Zoomed in Mass distribution and fit with layered contributions for B_s &rarr J/&psi K_S (.ps) (.gif)