Measurement of the lepton forward-backward asymmetry in B →xs+-decays with a sum of exclusive modes

We report the first measurement of the lepton forward-backward asymmetry AFB as a function of the squared four-momentum of the dilepton system, q2, for the electroweak penguin process B→Xs+- with a sum of exclusive final states, where is an electron or a muon and Xs is a hadronic recoil system with an s quark. The results are based on a data sample containing 772×106 BB¯ pairs recorded at the (4S) resonance with the Belle detector at the KEKB e+e- collider. AFB for the inclusive B→Xs+- is extrapolated from the sum of 10 exclusive Xs states whose invariant mass is less than 2 GeV/c2. For q2>10.2 GeV2/c2, AFB<0 is excluded at the 2.3σ level, where σ is the standard deviation. For q2<4.3 GeV2/c2, the result is within 1.8σ of the standard model theoretical expectation.