Precise measurement of the Ke2/Kμ2 branching ratio and search for new physics beyond the Standard Model

Abstract

The E36 experiment recently conducted at J-PARC by the TREK Collaboration will provide a precise mesurement of the decay ratio RK = Γ(K+→e+υe)/Γ(K+→μ+υμ) with the aim of testing lepton universality, a basic assumption of the Standard Model (SM). The SM prediction for the decay ratio RK is highly precise (δRK/RK = 4 × 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 × 106K+ per spill. The K+ decay products were then momentum analyzed by a 12-sector superconducting toroidal spectrometer and charged particles were tracked with high precision by Multi-Wire Proportional Chambers (MWPC) combined with a photon calorimeter with a large solid angle (75% of 4π) and 3 different particle identification systems. Details of the E36 experimental apparatus, as well as the status of the data analysis will be presented.