Neutron lifetime measurements with a large gravitational trap for ultracold neutrons

Abstract

Neutron lifetime is one of the most important physical constants which determines parameters of the weak interaction and predictions of primordial nucleosynthesis theory. There remains the unsolved problem of a 3.9σ discrepancy between measurements of this lifetime using neutrons in beams and those with stored neutrons (UCN). In our experiment we measure the lifetime of neutrons trapped by Earth's gravity in an open-topped vessel. Two configurations of the trap geometry are used to change the mean frequency of UCN collisions with the surfacesthis is achieved by plunging an additional surface into the trap without breaking the vacuum. The trap walls are coated with a hydrogen-less fluorine-containing polymer to reduce losses of UCN. The stability of this coating to multiple thermal cycles between 80 K and 300 K was tested. At 80 K, the probability of UCN loss due to collisions with the trap walls is just 1.5% of the probability of beta-decay. The free neutron lifetime is determined by extrapolation to an infinitely large trap with zero collision frequency. The result of these measurements is which is consistent with the conventional value of 880.2±1.0s presented by the Particle Data Group. Future prospects for this experiment are in further cooling to 10 K which will lead to an improved accuracy of measurement. In conclusion we present an analysis of currently-available data on various measurements of the neutron lifetime.

Big-Bang Nucleosynthesis after Planck

“…In the 2018 compilation of the Review of Particle Physics [72], the world average shifted slightly to τ n = 880.2 ± 1.0 s due to the updated measurement in [5] and includes a scale factor of 1.9 based on seven measurements. As noted earlier, there have been several newer measurements [6][7][8] and the world average based on all ten experiments gives τ n = 879.7±0.8…”

“…s with a scale factor of 1.9 due mainly to the high value found in [73] (887.7 ± 1.2 ± 1.9) s and the low values found in [6,7] (878.5 ± 0.7 ± 0.3 s and 877.7 ± 0.7 + 0.4/ − 0.2 s). The…”

“…Since our analysis in CFOY, there have been some new cross section measurements, notably in the 7 Be(n, p) 7 Li rate that controls the A = 7 abundance, as well as controversy over the d(p, γ) 3 He discrepancy between experiment and theory. There have also been four new measurements of the neutron lifetime [5][6][7][8], and several new measurements of the deuterium abundance in high redshift quasar absorption systems [9][10][11][12][13][14]. There remains a discrepancy between the predicted 7 Li abundance and that inferred from observations of low metallicity halo dwarf stars, further confirmed with a new observation of Li/H in an ultra-metal-poor halo star [15].…”

The CKM unitarity problem: a trace of new physics at the TeV scale?

“…8 clearly demonstrates the discrepancy. Namely, by averaging the presently available results of eight trap experiments [24][25][26][27][28][29][30][31] one obtains:…”

“…6 6 The PDG 2018 average τn = 880.2 ± 1.0 s includes the results of two beam experiments [32,33] and five trap experiments [24][25][26][27][28], with the error rescaled up by a factor χ 2 dof ≈ 2 for a loose compatibility between the data, essentially between the trap and beam experiments. Results of three recent trap experiments [29][30][31] published in 2018 were not included.…”