  Using data collected with the LHCb detector in proton-proton collisions at a
centre-of-mass energy of 7 TeV, the hadronic decay Bs -> J/psi f0(980) is
observed. This CP eigenstate mode could be used to measure mixing-induced CP
violation in the B_s system. Using a fit to the pi+ pi- mass spectrum with
interfering resonances gives R_{f0/phi} = [Gamma(Bs -> J/psi f0, f0 -> pi+
pi-)]/[Gamma(Bs -> J/psi phi, phi -> K+K-)] =
0.252^{+0.046+0.027}_{-0.032-0.033}, where the uncertainties are statistical
and systematic, respectively.
