Angular distributions of → Kl decays

We model-independently analyze the angular distributions of → Kldecays, l = e,μ, for low dilepton mass using QCD factorization. Besides the decay rate, we study the forward-backward asymmetry A^l_FB and a further observable, F^l_H, which gives rise to a flat term in the angular distribution. We find that in the Standard Model F^l_H∝m_l², hence vanishing F^e_H and F^μ_H 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 → Kēe 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_slbranching 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.