Precise measurement of the $K_{e2}/K_{{\mu}2}$ branching ratio and search for new physics beyond the Standard Model

Understand

The E36 experiment recently conducted at J-PARC by the TREK Collaboration will provide a precise mesurement of the decay ratio $ R_K = \Gamma(K^+ \rightarrow e^+\nu_e) / \Gamma(K^+ \rightarrow \mu^+\nu_{\mu}) $ with the aim of testing lepton universality, a basic assumption of the Standard Model (SM).

• The SM prediction for the decay ratio $R_K$ is highly precise ($\delta R_K / R_K = 4 \times 10^{-4}$) and any observed deviation from this prediction would clearly indicate the existence of new physics.
• The E36 experimental apparatus, designed to offer small systematic errors, was installed at the J-PARC K1.1BR kaon beam line in the fall of 2014, fully commissioned in the spring of 2015, and the production data were collected in the fall of 2015.
• A scintillating fiber target was used to stop a beam of $1.2 \times 10^6 K^+$ per spill.