Exploring scalar contributions with K+ → π+ℓ+ℓ−

Abstract

The rare kaon decay K+→π+ℓ+ℓ− offers insights into Standard Model (SM) physics and beyond. Driven by vector form factor in the SM, it can also probe non-standard contributions. In this letter we study the scalar contribution, fS. Using differential decay width and Forward-Backward Asymmetry we propose a simultaneous fit to vector and scalar contributions which is necessary for a consistent analysis. Novel bounds on |fS| are presented for the first time through a reinterpretation of the E865, NA48/2, and NA62 experimental results. The analysis results in the most precise bound fS<7.9×10−6 at 90% confidence level.