Angular Distributions of B -> K ll Decays
Abstract
We model-independently analyze the angular distributions of B -> K ll decays, l = e, mu, for low dilepton mass using QCD factorization. Besides the decay rate, we study the forward-backward asymmetry AFBl and a further observable, FHl, which gives rise to a flat term in the angular distribution. We find that in the Standard Model FHl ~ ml2, hence vanishing FHe and FHmu of around 2% (exact value depends on cuts) with a very small theoretical uncertainty of a few percent. We also give predictions for RK, the ratio of B -> K mu mu to B -> K ee decay rates. We analytically show using large recoil symmetry relations that in the Standard Model RK equals one up to lepton mass corrections of the order 10(-4). The New Physics reach of the observables from the B -> K ll angular analysis is explored together with RK and the Bs -> ll and B -> Xs ll branching ratios for both l = e and l = mu. We find substantial room for signals from (pseudo-) scalar and tensor interactions beyond the Standard Model. Experimental investigations of the B -> K mu mu angular distributions are suitable for the LHC environment and high luminosity B factories, where also studies of the electron modes are promising.