Neutron Lifetime Measurements Research Articles

τSPECT: a spin-flip loaded magnetic ultracold neutron trap for a determination of the neutron lifetime

The confinement of ultracold neutrons (UCNs) in three-dimensional magnetic field gradient or magneto-gravitational traps allows for a measurement of the free neutron lifetime τ n with superior control over loss channels related to UCNs interacting with material surfaces. The most precise measurement τ n has been achieved using a magneto-gravitational trap, in which UCN are prevented from escaping at the top of their trap by gravity. More compact horizontal confinement geometries with variable energy acceptance ranges can be obtained by using steep magnetic field gradients in all spatial directions, generated by combinations of either permanent or (variable) superconducting magnets. In this paper, we present the first successful implementation of a pulsed spin-flip based loading scheme to fill a three-dimensional magnetic trap with externally produced UCN. The measurements with the τSPECT experiment were performed at the pulsed UCN source of the research reactor TRIGA Mainz. The extracted neutron storage time constant of τ = 859(16) s is compatible with the most precise determinations of τ n . We report on detailed, but statistically limited, investigations of major systematic effects influencing the neutron storage time. The statistical limitations are mitigated by the relocation of the experiment to a stronger UCN source.

Feasibility of a strong focusing synchrotron for cold neutron beams

We study the feasibility of storing cold neutron beams in a strong focusing synchrotron. We also propose an alternating-current (ac) dipole magnet to be used as a neutron acceleration device and a pulsed quadrupole as a neutron beam kicker for injection and extraction. The ac acceleration device can provide adiabatic capture, acceleration, or deceleration of neutron beams. It seems feasible to design a high-quality neutron synchrotron for many future neutron beam applications, such as neutron life time measurements, monoenergetic neutron scattering experiments, and pencil neutron beam applications. This synchrotron can also be used as an injector for a neutron accumulator ring. Published by the American Physical Society 2024

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

The neutron’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 β-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 τn*=880(+158/−78)stat.(+230/−114)sys.s (68.3% C.I.). 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.

The Neutron Mean Life and Big Bang Nucleosynthesis

We explore the effect of neutron lifetime and its uncertainty on standard big bang nucleosynthesis (BBN). BBN describes the cosmic production of the light nuclides, 1H, D, 3H+3He, 4He, and 7Li+7Be, in the first minutes of cosmic time. The neutron mean life τn has two roles in modern BBN calculations: (1) it normalizes the matrix element for weak n↔p interconversions, and (2) it sets the rate of free neutron decay after the weak interactions freeze-out. We review the history of the interplay between τn measurements and BBN, and present a study of the sensitivity of the light element abundances to the modern neutron lifetime measurements. We find that τn uncertainties dominate the predicted 4He error budget, but these theory errors remain smaller than the uncertainties in 4He observations, even with the dispersion in recent neutron lifetime measurements. For the other light element predictions, τn contributes negligibly to their error budget. Turning the problem around, we combine present BBN and cosmic microwave background (CMB) determinations of the cosmic baryon density to predict a “cosmologically preferred” mean life of τn(BBN+CMB)=870±16s, which is consistent with experimental mean life determinations. We show that if future astronomical and cosmological helium observations can reach an uncertainty of σobs(Yp)=0.001 in the 4He mass fraction Yp, this could begin to discriminate between the mean life determinations.

Neutron Lifetime Anomaly and Mirror Matter Theory

This paper reviews the puzzles in modern neutron lifetime measurements and related unitarity issues in the CKM matrix. It is not a comprehensive and unbiased compilation of all historic data and studies, but rather a focus on compelling evidence leading to new physics. In particular, the largely overlooked nuances of different techniques applied in material and magnetic trap experiments are clarified. Further detailed analysis shows that the “beam” approach of neutron lifetime measurements is likely to give the “true” β-decay lifetime, while discrepancies in “bottle” measurements indicate new physics at play. The most feasible solution to these puzzles is a newly proposed ordinary-mirror neutron (n−n′) oscillation model under the framework of mirror matter theory. This phenomenological model is reviewed and introduced, and its explanations of the neutron lifetime anomaly and possible non-unitarity of the CKM matrix are presented. Most importantly, various new experimental proposals, especially lifetime measurements with small/narrow magnetic traps or under super-strong magnetic fields, are discussed in order to test the surprisingly large anomalous signals that are uniquely predicted by this new n−n′ oscillation model.

MFV approach to robust estimate of neutron lifetime

Aiming at evaluating the lifetime of the neutron, we introduce a novel statistical method to analyse the updated compilation of precise measurements including the 2022 dataset of Particle Data Group (PDG). Based on the minimization for the information loss principle, unlike the median statistics method, we apply the most frequent value (MFV) procedure to estimate the neutron lifetime, irrespective of the Gaussian or non-Gaussian distributions. Providing a more robust way, the calculated result of the MFV is tau _n=881.16^{+2.25}_{-2.35} s with statistical bootstrap errors, while the result of median statistics is tau _n=881.5^{+5.5}_{-3} s according to the binomial distribution. Using the different central estimates, we also construct the error distributions of neutron lifetime measurements and find the non-Gaussianity, which is still meaningful.

Precise measurement of neutron lifetime

Precise measurement of neutron lifetime

Superfluid helium film may greatly increase the storage time of ultracold neutrons in material traps

We propose a method to increase both the neutron storage time and the precision of its lifetime measurements by at least tenfold. The storage of ultracold neutrons (UCN) in material traps now provides the most accurate measurements of neutron lifetime and is used in many other experiments. The precision of these measurements is limited by the interaction of UCN with the trap walls. We show that covering trap walls with liquid helium may strongly decrease the UCN losses from material traps. $^4$He does not absorb neutrons at all. Superfluid He covers the trap walls as a thin film, $\sim 10$ nm thick, due to the van der Waals attraction. However, this He film on a flat wall is too thin to protect the UCN from their absorption inside a trap material. By combining the van der Waals attraction with capillary effects we show that surface roughness may increase the thickness of this film much beyond the neutron penetration depth $\sim 33$nm. Using liquid He for UCN storage requires low temperature $T<0.5$ K to avoid neutron interaction with He vapor, while the neutron losses because of the interaction with surface waves are small and can be accounted for using their linear temperature dependence.

Improved Neutron Lifetime Measurement with UCNτ.

We report an improved measurement of the free neutron lifetime τ_{n} using the UCNτ apparatus at the Los Alamos Neutron Science Center. We count a total of approximately 38×10^{6} surviving ultracold neutrons (UCNs) after storing in UCNτ's magnetogravitational trap over two data acquisition campaigns in 2017 and 2018. We extract τ_{n} from three blinded, independent analyses by both pairing long and short storage time runs to find a set of replicate τ_{n} measurements and by performing a global likelihood fit to all data while self-consistently incorporating the β-decay lifetime. Both techniques achieve consistent results and find a value τ_{n}=877.75±0.28_{stat}+0.22/-0.16_{syst} s. With this sensitivity, neutron lifetime experiments now directly address the impact of recent refinements in our understanding of the standard model for neutron decay.

Measurement of the free neutron lifetime using the neutron spectrometer on NASA's Lunar Prospector mission

We use data from the Lunar Prospector Neutron Spectrometer to make the second space-based measurement of the free neutron lifetime finding $\tau_n=887 \pm 14_\text{stat}{\:^{+7}_{-3\:\text{syst}}}$ s, which is within 1$\sigma$ of the accepted value. This measurement expands the range of planetary bodies where the neutron lifetime has been quantified from space, and by extending the modeling to account for non-uniform elemental composition, we mitigated a significant source of systematic uncertainty on the previous space-based lifetime measurement. This modeling moves space-based neutron lifetime measurement towards the ultimate goal of reducing the magnitude of the systematics on a future space-measurement to the level of those seen in laboratory-based experiments.

Search for explanation of the neutron lifetime anomaly

All results of the neutron lifetime measurements performed in the last 20 years with the ultracold neutron storage method are in a good agreement. These results are also consistent with the latest most accurate measurements of the neutron decay asymmetry within the Standard Model. However, there is a significant discrepancy at the $3.6\ensuremath{\sigma}$ (1% of the decay probability) level between the averaged result of the storage method experiments and the most accurate measurements performed with the beam method. This article addresses the possible causes of the neutron lifetime discrepancy. We focused on finding the spectrum of possible systematic corrections in the beam experiment. Four separate sources of the systematic errors that had not been properly addressed in articles dedicated to the beam technique were considered. Two of those sources are related to the motion of protons in an electromagnetic field and the elastic scattering by the residual gas. These problems are associated with the geometrical configuration of the setup and the proton detector size. The Monte Carlo simulation shows that these effects are negligible and do not affect measurement results. The third error concerns proton loss in the dead layer of the detector. It is shown that the correction for the dead layer requires a more detailed analysis than that given in the papers describing the beam measurement method. The fourth source of the systematic error is the charge exchange process on the residual gas. The influence of the residual gas was neglected in the beam method experiments. We present arguments showing that careful analysis of this issue is necessary since the proposed proton losses correction decreases the measured lifetime, bringing it closer to the storage method results. For the problem of proton charge exchange on a residual gas, the spectrum of possible corrections is investigated. It is shown that for an accurate calculation of the correction, it is necessary to directly measure the concentration and composition of the residual gas inside the proton trap. The analysis reveals that even the presence of only ${\mathrm{H}}_{2}$ molecules inside the proton trap can lead to the significant correction, which is the most probable explanation of the neutron anomaly.

Measurement of γ rays from 6LiF tile as an inner wall of a neutron-decay detector

A neutron lifetime measurement conducted at the Japan Proton Accelerator Research Complex (J-PARC) is counting the number of electrons from neutron decays with a time projection chamber (TPC). The γ rays produced in the TPC cause irreducible background events. To achieve the precise measurement, the inner walls of the TPC consist of 6Li-enriched lithium-fluoride (6LiF) tiles to suppress the amount of γ rays. In order to estimate the amount of γ rays from the 6LiF tile, prompt gamma ray analysis (PGA) measurements were performed using germanium detectors. We reconstructed the measured γ-ray energy spectrum using a Monte Carlo simulation with the stripping method. Comparing the measured spectrum with a simulated one, the number of γ rays emitted from the6LiF tile was (2.3+0.7−0.3) × 10−4 per incident neutron. This is 1.4+0.5−0.2 times the value assumed for a mole fraction of the 6LiF tile. We concluded that the amount of γ rays produced from the 6LiF tile is not more twice the originally assumed value.

Neutron lifetime measurement with pulsed cold neutrons

Abstract The neutron lifetime has been measured by comparing the decay rate with the reaction rate of $^3$He nuclei of a pulsed neutron beam from the spallation neutron source at the Japan Proton Accelerator Research Complex (J-PARC). The decay rate and the reaction rate were determined by simultaneously detecting electrons from the neutron decay and protons from the $^3$He(n,p)$^3$H reaction using a gas chamber, the working gas of which contains diluted $^3$He. The measured neutron lifetime was $898\,\pm\,10\,_{\rm stat}\,^{+15}_{-18}\,_{\rm sys}\,$s.

Space-based measurements of neutron lifetime: Approaches to resolving the neutron lifetime anomaly

Free neutrons have a measured lifetime of 880 s, but disagreement between existing laboratory measurements of ∼10 s have persisted over many years. This uncertainty has implications for multiple physics disciplines, including standard-model particle physics and Big-Bang nucleosynthesis. Space-based neutron lifetime measurements have been shown to be feasible using existing data taken at Venus and the Moon, although the uncertainties for these measurements of tens of seconds prevent addressing the current lifetime discrepancy. We investigate the implementation of a dedicated space-based experiment that could provide a competitive and independent lifetime measurement. We considered a variety of scenarios, including measurements made from orbit about the Earth, Moon, and Venus, as well as on the surface of the Moon. For a standard-sized neutron detector, a measurement with three-second statistical precision can be obtained from Venus orbit in less than a day; a one-second statistical precision can be obtained from Venus orbit in less than a week. Similarly precise measurements in Earth orbit and on the lunar surface can be acquired in less than 40 days (three-second precision) and ∼300 days (one-second precision). Systematic uncertainties that affect a space-based neutron lifetime measurement are investigated, and the feasibility of developing such an experiment is discussed.

A new neutron lifetime experiment with cold neutron beam decay in superfluid helium-4

The puzzle remains in the large discrepancy between neutron lifetime measured by the two distinct experimental approaches—counts of beta decays in a neutron beam and storage of ultracold neutrons in a potential trap, namely, the beam method versus the bottle method. In this paper, we propose a new experiment to measure the neutron lifetime in a cold neutron (CN) beam with a sensitivity goal of 0.1% or sub-1 s. The neutron beta decays will be counted in a superfluid helium-4 scintillation detector at 0.5 K, and the neutron flux will be simultaneously monitored by the helium-3 captures in the same volume. The CN beam must be of wavelength λ > 16.5 Å to eliminate scattering with superfluid helium. A new precise measurement of neutron lifetime with the beam method of unique inherent systematic effects will greatly advance in resolving the puzzle.

Accurate Measurement of the Beta-Asymmetry in Neutron Decay Rules out Dark Decay Mode

The question of the nature of dark matter is one of the major challenges of elementary particle physics. Not surprisingly, the recent suggestion of a dark decay channel as a solution to persisting discrepancies in neutron lifetime measurements has initiated substantial research activity. We discuss the accurate measurement of the parity violating β-asymmetry using Perkeo III. The result is about five times more precise than the current world average and resolves a long-standing discrepancy. Based on this, we largely rule out the dark decay mode interpretation. We derive a new world average of the weak axial coupling and obtain a competitive value of the first element of the quark mixing matrix Vud from neutron decay.

Neutron’s dark secret

The existing discrepancy between neutron lifetime measurements in bottle and beam experiments has been interpreted as a sign of the neutron decaying to dark particles. We summarize the current status of this proposal, including a discussion of particle physics models involving such a portal between the Standard Model and a baryonic dark sector. We also review further theoretical developments around this idea and elaborate on the prospects for verifying the neutron dark decay hypothesis in current and upcoming experiments.

The Effect of Neutron Decaying to Dark Matter on Properties of Neutron Stars with Hyperons

It has been reported that there is a discrepancy of 4σ in the neutron lifetime measurement with two methods, i.e., the trap method (measuring the number of neutrons remaining in the various time intervals) and beam method (measuring the number of protons formed from regulated neutrons currents). Fornal and Grinstein (2019) proposed that the possibility neutrons decay into the dark matter can explain this discrepancy. In this study, the author asses the consequence of the proposal by studying the impact on neutron stars with Hyperon. Based on our numerical calculation we found that dark matter may be present in the core of a neutron star because it only appears at very high densities, and the population is negligible with a maximum population of about 0.1%

A meta-analysis of neutron lifetime measurements

Abstract We calculate the median as well as weighted mean central estimates for the neutron lifetime from a subset of measurements compiled in the 2019 update of the Particle Data Group (PDG). We then reconstruct the error distributions for the residuals using three different central estimates and then check for consistency with a Gaussian distribution. We find that although the error distributions using the weighted mean as well as median estimate are consistent with a Gaussian distribution, the Student’s $t$ and Cauchy distribution provide a better fit. This median statistic estimate of the neutron lifetime from these measurements is given by $881.5 \pm 0.47$ seconds. This can be used as an alternate estimate of the neutron lifetime. We also note that the discrepancy between beam and bottle-based measurements using median statistics of the neutron lifetime persists with a significance between 4 $\sigma$ and 8 $\sigma$, depending on which combination of measurements is used.

Neutron lifetime measurement with the big gravitational trap for ultracold neutrons. Current state and future prospects.

A new measurement of the neutron lifetime, carried out with the aid of a large gravitational spectrometer made in Petersburg Institute of Nuclear Physics (PNPI) is presented: τn = 881.5 ± 0.7 ± 0.6 s. In our experiment the measurement of neutron lifetime is carried out using the method of storing ultracold neutrons in a material trap with gravitational barrier. Further improvement of the obtained result can be achieved at the helium temperatures. Here we present our neutron lifetime result, modified installation scheme and the first results of cryogenic tests are discussed.