A Novel Technique of Extracting UCN Decay Lifetime from Storage Chamber Measurements Dominated by Scattering Losses

Abstract

The neutron&rsquo;s lifetime is a critical parameter in the standard model. Its measurements, particularly measurements using both beamline and ultracold neutron storage techniques, have revealed significant tension. In this work, we review the status of the tension between various measurements, especially in light of the insights provided by the <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mi>&beta;</mi></semantics></math></inline-formula>-decay correlation measurements. We revisit the lifetime measurement in a material storage chamber, dominated by losses from scattering off the walls of the storage chamber. The neutron energy spectra and associated uncertainties were, for the first time, well-characterized using storage data alone. Such models have applications in the extraction of the mean time between wall bounces, which is a key parameter for neutron storage disappearance experiments in search of neutron oscillation. A comparison between the loss model and the number of neutrons stored in a single chamber allowed us to extract a neutron lifetime of <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><msubsup><mi>&tau;</mi><mi>n</mi><mo>*</mo></msubsup><mo>=</mo><mn>880</mn><mspace width="3.33333pt"></mspace><msub><mrow><mo>(</mo><mrow><mo>+</mo><mn>158</mn></mrow><mo>/</mo><mrow><mo>&minus;</mo><mn>78</mn></mrow><mo>)</mo></mrow><mrow><mi>stat</mi><mo>.</mo></mrow></msub><mspace width="3.33333pt"></mspace><msub><mrow><mo>(</mo><mo>+</mo><mn>230</mn><mo>/</mo><mo>&minus;</mo><mn>114</mn><mo>)</mo></mrow><mrow><mi>sys</mi><mo>.</mo></mrow></msub><mspace width="3.33333pt"></mspace><mi mathvariant="normal">s</mi><mo>&nbsp;</mo><mrow><mo>(</mo><mn>68.3</mn><mo>%</mo><mo>&nbsp;</mo><mi mathvariant="normal">C</mi><mo>.</mo><mi mathvariant="normal">I</mi><mo>.</mo><mo>)</mo></mrow></mrow></semantics></math></inline-formula>. Though the uncertainty of this lifetime is not competitive with currently available measurements, the highlight of this work is that we precisely identified the systematic sources of uncertainty that contribute to the neutron lifetime measurements in material storage bottles, namely from the uncertainty in the energy spectra, as well as from the storage chamber surface parameters of the Fermi potential and loss per bounce. In doing so, we highlight the underestimation of the uncertainties in the previous Monte Carlo simulations of experiments using the technique of ultracold neutron storage in material bottles.