Observation of the decay $B^0_d \to J/\psi (\mu^+\mu^-) K^0_S (\pi^+\pi^-) $ with the ATLAS experiment at the LHC

The production of $B_d^0$ mesons is observed with the ATLAS experiment at the LHC through their decays to $B_d^0 \to J/\psi(\mu^+\mu^-) K^0_S(\pi^+\pi^-)$. The decays are reconstructed in 40 pb$^{-1}$ of data taken in 2010 and the mass distribution obtained is fitted using an unbinned maximum likelihood fit. The mass fit yields $616 \pm 30(\rm stat.)$ signal events. The fitted $B_d^0$ mass value, $M = 5279.7 \pm 1.0(\rm stat.)$ MeV, is consistent with the world average value, $M_{B^0} = 5279.50 \pm 0.30$ MeV.