Analysis and correction of the measurement of the neutron lifetime

Abstract

224New results of neutron lifetime measurements have recently appeared [1][2][3], which noticeably (by 5-6 s) differ from a world average value of 885.7(0.8) s [4]. Since a noticeable contribution to the world average value came from the value τ β = 885.4 ± 0.9 stat ± 0.4 syst s presented in our work [5], we additionally analyzed this result. Moreover, this result was critically analyzed in [6,7].The method of measurement of τ β in [5] was based on the storage of ultracold neutrons in two vessels with liquid fluoropolymer coating and different frequencies of collisions of ultracold neutrons with the walls (Fig. 1). In geometries 1 and 2, neutrons were stored inside vessel 7 and in the slit space between vessels 7 and 8, respectively. The vacuum housing of the vessel was surrounded by 24 thermal neutron counters 10, which detected ultracold neutrons that inelastically scattered from the vessel walls.In the process of storage of ultracold neutrons, the total probabilities of their leakage from the vessels were determined:Here, , , , and are the numbers of ultracold neutrons detected when leaking from the vessels, T (1) and T (2) are the durations of the storage intervals (superscripts 1 and 2 mark geometries and subscripts i and f refer to the initial and final times of storage, respectively), λ β = 1/τ β , and the probabilitiesof loss of ultracold neutrons interacting with the vessel walls are given by the expressions Here, ε (1) and ε (2) are the efficiencies of the detection of ultracold neutrons, J (1) and J (2) are the numbers of neutrons detected by external detector 10 in the stor age interval, and are the efficiencies of the detection of inelastically scattered ultracold neutrons, and σ ie and σ c are the probabilities of the inelastic scat tering and capture of ultracold neutrons interacting with the walls of the vessel. If ε (1) = ε (2) and = , the ratio of the loss probabilities is ξ = = ; therefore, λ β = .A more accurate relation for ξ associated with the use of a wide spectrum of ultracold neutrons was given in [5,8].The statistical error of the result λ β was determined in terms of the statistical errors of the measurement of six quantities , , , , J (1) , and J (2) tak ing into account the data on the time dependence of the count rate of detectors of thermal neutrons j (1) (t) and j (2) (t) in the storage intervals. The systematic errorCorrections have been introduced into the result τ β = 885.4 ± 0.9 stat ± 0.4 syst s of our measurements of the neu tron lifetime. The corrected value is τ β = 881.6 ± 0.8 stat ± 1.9 syst s.

Precision experiments with cold and ultra-cold neutrons

“…Observables in free neutron decay are abundant: besides the lifetime τ n [1][2][3][4], (older references can be found in [5]), angular correlations involving the neutron spin as well as momenta and spins of the emitted particles are characterized by individual coefficients, which can be related to the underlying coupling strengths of the weak interaction, including yet unobserved ones. Examples are the electron-antineutrino correlation coefficient a [6][7][8], the beta asymmetry parameter A [9][10][11][12][13], the neutrino asymmetry parameter B [14,15] (reconstructed from proton and electron momenta), the proton asymmetry parameter C [16], the triple correlation coefficient D [17,18], the Fierz interference term b, and various correlation coefficients involving the electron spin [19,20].…”

Comparison of ultracold neutron sources for fundamental physics measurements

“…This was for a long time the only source with sufficient UCNs available. At the beginning experiments concentrated on measurements of the free neutron lifetime (τ n ) [5][6][7][8] and on the search for a permanent electric dipole moment of the neutron (nEDM) [9][10][11][12][13]. In addition to the fundamental particle aspect, τ n is also an important quantity in understanding the primordial nucleo-synthesis and contributes presently one of the larger uncertainties to its description [14].…”

Пространственное И Энергетическое Формирование Нейтронных Пучков С Помощью Гибких Фторполимерных Нейтроноводов

“…В работах [1][2] было обнаружено исключительно высокое пропускание ультрахолодных нейтронов (УХН) полихлорвиниловыми трубками промышленного про-изводства. Полихлорвинил (ПВХ) представляет собой высокомолекулярный полимер (СН 2 −CHСl) n с высоким молекулярным весом молекул в пределах от 30 000 до 100 000 amu.…”