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

TL;DR

The E36 experiment at J-PARC aims to precisely measure the $K_{e2}/K_{2}$ decay ratio to test lepton universality and search for deviations indicating new physics beyond the Standard Model.

Contribution

This paper describes the design, setup, and initial status of the E36 experiment, a novel high-precision measurement of the kaon decay ratio for new physics searches.

Findings

Experimental apparatus successfully commissioned at J-PARC.

Data collection of $K^+$ decay products completed.

Analysis of the decay ratio underway.

Abstract

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. 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 $4\pi$) and 3 different particle identification systems. Details of the E36 experimental apparatus, as well as the status of the data analysis will be presented.