Measurement of the neutron lifetime by counting trapped protons in a cold neutron beam

A measurement of the neutron lifetime ${\ensuremath{\tau}}_{n}$ performed by the absolute counting of in-beam neutrons and their decay protons has been completed. Protons confined in a quasi-Penning trap were accelerated onto a silicon detector held at a high potential and counted with nearly unit efficiency. The neutrons were counted by a device with an efficiency inversely proportional to neutron velocity, which cancels the dwell time of the neutron beam in the trap. The result is ${\ensuremath{\tau}}_{n}=(886.3\ifmmode\pm\else\textpm\fi{}1.2[\mathrm{stat}]\ifmmode\pm\else\textpm\fi{}3.2[\mathrm{sys}])\phantom{\rule{0.3em}{0ex}}s$, which is the most precise measurement of the lifetime using an in-beam method. The systematic uncertainty is dominated by neutron counting, in particular, the mass of the deposit and the $^{6}\mathrm{Li}$$(n,t)$ cross section. The measurement technique and apparatus, data analysis, and investigation of systematic uncertainties are discussed in detail.