Search for lepton number violating decays B ⁺→π ^-μ ⁺μ ⁺ and B ⁺→K ^-μ ⁺μ ⁺

A search is performed for the lepton number violating decay B ⁺→h ^-μ ⁺μ ⁺, where h ^- represents a K ^- or a π ^-, using an integrated luminosity of 36pb ^-1 of data collected with the LHCb detector. The decay is forbidden in the standard model but allowed in models with a Majorana neutrino. No signal is observed in either channel and limits of B(B ⁺→K ^-μ ⁺μ ⁺)<5. 4×10 ^-8 and B(B ⁺→π ^-μ ⁺μ ⁺)<5.8×10 ^-8 are set at the 95% confidence level. These improve the previous best limits by factors of 40 and 30, respectively.