Improving lepton flavour universality tests with $K_L$ decays

Abstract

Rare kaon decays provide sensitive probes of the flavour structure of the Standard Model and of possible new physics. We perform a global analysis incorporating recent experimental results and updated Standard Model predictions, including the latest measurement of $K^{+}\to {\pi }^{+}\nu \bar{\nu }$ and lepton flavour universality observables in $K^{+}\to {\pi }^{+}{\ell }^{+}{\ell }^{-}$. The fit favours a best-fit point close to the Standard Model, while a second local minimum remains phenomenologically relevant. We define benchmark scenarios associated with these two regions and investigate the prospective sensitivity of NA62 and KOTO-II to the new physics parameter space. We consider projected measurements of $K^{+}\to {\pi }^{+}\nu \bar{\nu }$ and lepton flavour universality observables in $K^{+}\to {\pi }^{+}{\ell }^{+}{\ell }^{-}$ at NA62, and of $K_L\to {\pi }^0\nu \bar{\nu }$, $K_L\to {\pi }^0{e}^{+}{e}^{-}$, and $K_L\to {\pi }^0{\mu }^{+}{\mu }^{-}$ at KOTO-II. We find that KOTO-II has significant potential to probe and discriminate between the viable new physics scenarios, with NA62 providing complementary sensitivity.

13 pages, 6 figures, 1 table