B⁺ → K⁺ℓ⁺ℓ^- to probe physics beyond standard model

In the standard model calculations where the decay can only occur by tree level diagram, R_k reduces to unity. However, various experiments at different energy scale have reported different results for R_k. The discrepancies in the experimental observations and standard model predictions can be used to test physics beyond standard model (SM) where excitations of more energetic particles is possible