Measurement of the branching fractions of semileptonic K⁺ decays at the NA62 experiment

Abstract

The NA62 experiment based at CERN aims to make a precision measurement of the ultra-rare kaon decay, K⁺→π⁺νν. To allow the safe operation of the NA62 CEDAR with hydrogen, several hardware and software modifications to the detector were completed. An assessment of the contribution of the current radiator gas, nitrogen, towards downstream detector rates was made, finding it to be comparable to that of beam related effects.The ratios amongst the branching fractions of the semileptonic kaon decays, K⁺→π⁰μ⁺ν_μ (K_μ3), K⁺→π⁰e⁺ν_e (K_e3), and the hadronic decay K⁺→π⁺π⁰ (K_2π) were measured to be:
BF(K_μ3)/BF(K_2π) = 0.1610(2)_stat(10)_sys 
BF(K_e3)/BF(K_2π) = 0.2431(3)_stat(9)_sys
BF(K_μ3)/BF(K_e3) = 0.6623(11)_stat(36)_sys
The precision of the results is limited by systematic effects, however the low statistical errors of the measurement demonstrate the successful application of the Barlow-Beeston method to NA62 data and simulation, and its potential for making precision measurements. With further undertaking, the systematic effects can be driven down, allowing the branching fractions measurement to reach potentially world-beating precision.

Date of Award	24 Mar 2020
Original language	English
Awarding Institution	University of Bristol
Supervisor	Helen F Heath (Supervisor)