Study of the $B_c^{+}\to J/\psi D_s^{+}$ and $B_c^{+}\to J/\psi D_s^{\ast +}$ decays with the ATLAS detector

Abstract

The decays $B_c^{+}\to J/\psi D_s^{+}$ and $B_c^{+}\to J/\psi D_s^{\ast +}$ are studied with the ATLAS detector at the LHC using a dataset corresponding to integrated luminosities of 4.9 fb${}^{-1}$ and 20.6 fb${}^{-1}$ of $pp$ collisions collected at centre-of-mass energies $\sqrt{s}=7$ TeV and 8 TeV, respectively. Signal candidates are identified through $J/\psi \to {\mu }^{+}{\mu }^{-}$ and $D_s^{(\ast )+}\to \varphi {\pi }^{+}(\gamma /{\pi }^0)$ decays. With a two-dimensional likelihood fit involving the $B_c^{+}$ reconstructed invariant mass and an angle between the ${\mu }^{+}$ and $D_s^{+}$ candidate momenta in the muon pair rest frame, the yields of $B_c^{+}\to J/\psi D_s^{+}$ and $B_c^{+}\to J/\psi D_s^{\ast +}$, and the transverse polarisation fraction in $B_c^{+}\to J/\psi D_s^{\ast +}$ decay are measured. The transverse polarisation fraction is determined to be ${\Gamma }_{\pm \pm }(B_c^{+}\to J/\psi D_s^{\ast +})/\Gamma (B_c^{+}\to J/\psi D_s^{\ast +})=0.38\pm 0.23\pm 0.07$, and the derived ratio of the branching fractions of the two modes is ${\mathcal{B}}_{B_c^{+}\to J/\psi D_s^{\ast +}}/{\mathcal{B}}_{B_c^{+}\to J/\psi D_s^{+}}=2.8{}_{-0.8}^{+1.2}\pm 0.3$, where the first error is statistical and the second is systematic. Finally, a sample of $B_c^{+}\to J/\psi {\pi }^{+}$ decays is used to derive the ratios of branching fractions ${\mathcal{B}}_{B_c^{+}\to J/\psi D_s^{+}}/{\mathcal{B}}_{B_c^{+}\to J/\psi {\pi }^{+}}=3.8\pm 1.1\pm 0.4\pm 0.2$ and ${\mathcal{B}}_{B_c^{+}\to J/\psi D_s^{\ast +}}/{\mathcal{B}}_{B_c^{+}\to J/\psi {\pi }^{+}}=10.4\pm 3.1\pm 1.5\pm 0.6$, where the third error corresponds to the uncertainty of the branching fraction of $D_s^{+}\to \varphi (K^{+}{K}^{-}){\pi }^{+}$ decay. The available theoretical predictions are generally consistent with the measurement.

Comments: 17 pages plus author list + cover pages (33 pages total), 7 figures, 5 tables, submitted to Eur. Phys. J. C. All figures including auxiliary figures are available at https://atlas.web.cern.ch/Atlas/GROUPS/PHYSICS/PAPERS/BPHY-2014-04/

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