PT Journal AU LHCb Collaboration (Aaij, Rea Martinez-Vidal, F Oyanguren, A Ruiz Valls, P Sanchez Mayordomo, C TI First observations of the rare decays B (+) -> K (+)pi (+)pi (-)mu(+)mu (-) and B (+)-> phi K (+)mu(+)mu (-) SO Journal of High Energy Physics JI J. High Energy Phys. PY 2014 BP 064 EP 18pp VL 10 IS 10 DI 10.1007/JHEP10(2014)064 LA English DE Rare decay; Hadron-Hadron Scattering; B physics; Flavor physics AB First observations of the rare decays B (+) -> K (+)pi (+) pi (-) μ(+) μ(-) and B (+)-> phi K+ mu(+)mu(-) are presented using data corresponding to an integrated luminosity of 3.0 fb(-1), collected by the LHCb experiment at centre-of-mass energies of 7 and 8 TeV. The branching fractions of the decays are B(B (+) -> K (+)pi (+) pi (-) μ(+) μ(-) ) = (4.36 (-0.27) (+0.29) (stat) +/- 0.21 (syst) +/- (norm)) x 10(-7), B(B (+)-> phi K+ mu(+)mu(-)) = (0.82 (+0.19)(-0.17) (stat) (+0.10)(-0.04) (syst) +/- 0.27 (norm)) x 10(-7) where the uncertainties are statistical, systematic, and due to the uncertainty on the branching fractions of the normalisation modes. A measurement of the differential branching fraction in bins of the invariant mass squared of the dimuon system is also presented for the decay B (+) -> K (+)pi (+) pi (-) μ(+) μ(-) ER