Determination of the ratio of $b$-quark fragmentation fractions $f_s/f_d$ in $pp$ collisions at $\sqrt{s}=7$ TeV with the ATLAS detector

Abstract: With an integrated luminosity of 2.47 fb$<sup>{-1}$ recorded by the ATLAS experiment at the LHC, the exclusive decays $B_{s}^{{0}\rightarrow} J/\psi\phi$ and $B_{d}^{{0}\rightarrow} J/\psi K<sup>{*0}$ of $B$ mesons produced in $pp$ collisions at $\sqrt{s}=7$ TeV are used to determine the ratio of fragmentation fractions $f_s/f_d$. From the observed $B_{s}^{{0}\rightarrow} J/\psi\phi$ and $B_{d}^{{0}\rightarrow} J/\psi K<sup>{*0}$ yields, the quantity $$\frac{f_s}{f_d}\frac{\mathcal{B}(B_{s}^{0} \rightarrow J/\psi \phi)}{\mathcal{B}(B_{d}^{0} \rightarrow J/\psi K<sup>{*0})}$$ is measured to be 0.199 $\pm$ 0.004(stat) $\pm$ 0.008(sys). Using a recent theory prediction for $$\frac{\mathcal{B}(B_{s}^{{0}\rightarrow} J/\psi\phi)}{\mathcal{B}(B_{d}^{{0}\rightarrow} J/\psi K<sup>{*0})}$$ yields $\frac{f_s}{f_d} =$ 0.240 $\pm$ 0.004(stat) $\pm$ 0.010(sys) $\pm$ 0.017(th). This result is based on a new approach that provides a significant improvement of the world average.