Angular distributions of B → Kll decays

Abstract

We model-independently analyze the angular distributions of Kl decays, l = e,μ, for low dilepton mass using QCD factorization. Besides the decay rate, we study the forward-backward asymmetry A lFB and a further observable, F lH, which gives rise to a flat term in the angular distribution. We find that in the Standard Model F lHm l2, hence vanishing F eH and F μH of around 2% (exact value depends on cuts) with a very small theoretical uncertainty of a few percent. We also give predictions for R K, the ratio of Kμ to Ke decay rates. We analytically show using large recoil symmetry relations that in the Standard Model R K equals one up to lepton mass corrections of the order 10 -4 including α s and subleading 1/E power corrections. The New Physics reach of the observables from the Kl angular analysis is explored together with R K and the s l and X sl branching ratios for both l = e and l = μ. We find substantial room for signals from (pseudo-) scalar and tensor interactions beyond the Standard Model. Experimental investigations of the Kμ angular distributions are suitable for the LHC environment and high luminosity B factories, where also studies of the electron modes are promising. © SISSA 2007.