Geant4 simulations of NIST beam neutron lifetime experiment

A free neutron is unstable and its decay is described by the Standard Model as the transformation of a down quark into an up quark through the weak interaction. Precise measurements of the neutron lifetime test the validity of the theory of the weak interaction and provide useful information for the predictions of the theory of Big Bang nucleosynthesis of the primordial helium abundance in the universe and the number of different types of light neutrinos $N_{\nu}$. The predominant experimental methods for determination of the neutron lifetime are commonly called “beam” and “bottle” methods, and the most recent uses of each method do not agree with each other within their stated uncertainties. An improved experiment of the beam technique, which uses magnetic and electric fields to trap and guide the decay protons of a beam of cold neutrons to a detector, is in progress at the National Institute of Standards and Technology, Gaithersburg, MD with a precision goal of 0.1%. This study uses both the Geant4 simulation toolkit and the ROOT analysis framework to examine the effects of certain experimental conditions on the precision, in order to understand and reduce systematic uncertainties, especially those related to proton backscattering from the detector.