Free Neutron Decay Research Articles

The Neutron Lifetime Discrepancy and Its Implications for Cosmology and Dark Matter

Free neutron decay is the prototype for nuclear beta decay and other semileptonic weak particle decays. It provides important insights into the symmetries of the weak nuclear force. Neutron decay is important for understanding the formation and abundance of light elements in the early universe. The two main experimental approaches for measuring the neutron lifetime, the beam method and the ultracold neutron storage method, have produced results that currently differ by 9.8 ± 2.0 s. While this discrepancy probably has an experimental origin, a more exciting prospect is that it may be explained by new physics, with possible connections to dark matter. The experimental status of the neutron lifetime is briefly reviewed, with an emphasis on its implications for cosmology, astrophysics, and dark matter.

Reanalysis of the β-ν[over ¯]_{e} Angular Correlation Measurement from the aSPECT Experiment with New Constraints on Fierz Interference.

On the basis of revisions of some of the systematic errors, we reanalyzed the electron-antineutrino angular correlation (a coefficient) in free neutron decay inferred from the recoil energy spectrum of the protons which are detected in 4π by the aSPECT spectrometer. With a=-0.104 02(82) the new value differs only marginally from the one published in 2020. The experiment also has sensitivity to b, the Fierz interference term. From a correlated (b,a) fit to the proton recoil spectrum, we derive a limit of b=-0.0098(193) which translates into a somewhat improved 90% confidence interval region of -0.041≤b≤0.022 on this hypothetical term. Tighter constraints on b can be set from a combined [shown as superscript (c)] analysis of the PERKEO III (β asymmetry) and aSPECT measurement which suggests a finite value of b with b^{(c)}=-0.0181±0.0065 deviating by 2.82σ from the standard model.

Accelerated electron thermometer: observation of 1D Planck radiation

Abstract We report on the observation of thermal photons from an accelerated electron via examination of radiative beta decay of free neutrons measured by the RDK II collaboration. The emitted photon spectrum is shown to corroborate a thermal distribution consistent with the dynamical Casimir effect. Supported by a robust chi-squared statistic, we find the photons reside in a 1D Planck spectrum with a temperature predicted by the moving mirror model. Subject Indices: B50 (Electromagnetic processes and properties), D29 (Nuclear decays and radioactivities (including fission)), and E76 (Quantum field theory on curved space)

Jets from Neutron-Star Merger Remnants and Massive Blue Kilonovae.

We perform three-dimensional general-relativistic magnetohydrodynamic simulations with weak interactions of binary neutron-star (BNS) mergers resulting in a long-lived remnant neutron star, with properties typical of galactic BNS and consistent with those inferred for the first observed BNS merger GW170817. We demonstrate self-consistently that within ≲30 ms postmerger magnetized (σ∼5-10) incipient jets emerge with asymptotic Lorentz factor Γ∼5-10, which successfully break out from the merger debris within ≲20 ms. A fast (v≲0.6c), magnetized (σ∼0.1) wind surrounds the jet core and generates a UV/blue kilonova precursor on timescales of hours, similar to the precursor signal due to free neutron decay in fast dynamical ejecta. Postmerger ejecta are quickly dominated by magnetohydrodynamically driven outflows from an accretion disk. We demonstrate that, within only 50ms postmerger, ≳2×10^{-2}M_{⊙} of lanthanide-free, quasispherical ejecta with velocities ∼0.1-0.2c is launched, yielding a kilonova signal consistent with GW170817 on timescales of ≲5 d.

Quark mass difference effects in hadronic Fermi matrix elements from first principles

It was recently estimated that the strong isospin-symmetry breaking (ISB) corrections to the Fermi matrix element in free neutron decay could be of the order 10−4, one order of magnitude larger than the naïve estimate based on the Behrends-Sirlin-Ademollo-Gatto theorem. To investigate this claim, we derive a general expression of the leading ISB correction to hadronic Fermi matrix elements, which takes the form of a four-point correlation function in lattice gauge theory and is straightforward to compute from first principles. Our formalism paves the way for the first determination of such correction in the neutron sector with fully-controlled theory uncertainties.

Search for beyond standard model physics in free neutron decay

Applying a Mott polarimetry for measurement of the transverse polarization components of electrons from free neutron decay as well as proton momentum reconstruction using the combination of the time of flight method and the kinematical constrains of this three body decay, one gets access to eleven correlation coefficients of the neutron β-decay. Successful measurement of some of these coefficients would allow for an unique access to exotic scalar and tensor couplings of weak interactions and obtaining new constraints on their imaginary part, known with much worse accuracy. Results of the performance studies of some key experimental components of the prototype setup performed during the test run in 2021 at ILL PF1B neutron beam line are presented.

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.

Production of Very Light Elements and Strontium in the Early Ejecta of Neutron Star Mergers

Abstract We study the production of very light elements (Z < 20) in the dynamical and spiral-wave wind ejecta of binary neutron star mergers by combining detailed nucleosynthesis calculations with the outcome of numerical relativity merger simulations. All our models are targeted to GW170817 and include neutrino radiation. We explore different finite-temperature, composition-dependent nuclear equations of state, and binary mass ratios, and find that hydrogen and helium are the most abundant light elements. For both elements, the decay of free neutrons is the driving nuclear reaction. In particular, ∼0.5–2 × 10−6 M ⊙ of hydrogen are produced in the fast expanding tail of the dynamical ejecta, while ∼1.5–11 × 10−6 M ⊙ of helium are synthesized in the bulk of the dynamical ejecta, usually in association with heavy r-process elements. By computing synthetic spectra, we find that the possibility of detecting hydrogen and helium features in kilonova spectra is very unlikely for fiducial masses and luminosities, even when including nonlocal thermodynamic equilibrium effects. The latter could be crucial to observe helium lines a few days after merger for faint kilonovae or for luminous kilonovae ejecting large masses of helium. Finally, we compute the amount of strontium synthesized in the dynamical and spiral-wave wind ejecta, and find that it is consistent with (or even larger than, in the case of a long-lived remnant) the one required to explain early spectral features in the kilonova of GW170817.

Precise Measurements of the Decay of Free Neutrons

The impact of new and highly precise neutron β decay data is reviewed. We focus on recent results from neutron lifetime, β asymmetry, and electron–neutrino correlation experiments. From these results, weak interaction parameters are extracted with unprecedented precision, which is possible also because of progress in effective field theory and lattice QCD. Limits on New Physics beyond the Standard Model derived from neutron decay data are sharper than those derived from high-energy experiments, except for processes involving right-handed neutrinos.

Space-based measurement of the neutron lifetime using data from the neutron spectrometer on NASA's MESSENGER mission

We establish the feasibility of measuring the neutron lifetime via an alternative, space-based class of methods, which use neutrons generated by galactic cosmic ray spallation of planets' surfaces and atmospheres. Free neutrons decay via the weak interaction with a mean lifetime of around 880 s. This lifetime constrains the unitarity of the CKM matrix and is a key parameter for studies of Big-Bang nucleosynthesis. However, current laboratory measurements, using two independent approaches, differ by over 4$\sigma$. Using data acquired in 2007 and 2008 during flybys of Venus and Mercury by NASA's MESSENGER spacecraft, which was not designed to make this measurement, we estimate the neutron lifetime to be $780\pm60_\textrm{stat}\pm70_\textrm{syst}$ s, thereby demonstrating the viability of this new approach.

Improved determination of the β−ν¯e angular correlation coefficient a in free neutron decay with the aSPECT spectrometer

This work presents the until now most precise measurement of the angular correlation coefficient $a$ between the momenta of the electron and the electron-antineutrino in free neutron decay. The coefficient $a$ is inferred from a detailed analysis of the integral proton recoil spectrum measured with the $a$SPECT setup, a MAC-E-Filter type spectrometer with $4\ensuremath{\pi}$ acceptance. The result can be used to determine the axial-vector coupling constant ${g}_{A}$ in the weak interaction and shows a tantalizing disagreement with recent measurements of ${g}_{A}$ via the $\ensuremath{\beta}$-decay asymmetry in neutron decay.

Improved limits on Fierz interference using asymmetry measurements from the Ultracold Neutron Asymmetry (UCNA) experiment

The Ultracold Neutron Asymmetry (UCNA) experiment was designed to measure the β-decay asymmetry parameter, A0, for free neutron decay. In the experiment, polarized ultracold neutrons are transported into a decay trap, and their β-decay electrons are detected with ≈4π acceptance into two detector packages which provide position and energy reconstruction. The experiment also has sensitivity to bn, the Fierz interference term in the neutron β-decay rate. In this work, we determine bn from the energy dependence of A0 using the data taken during the UCNA 2011–2013 run. In addition, we present the same type of analysis using the earlier 2010 A dataset. Motivated by improved statistics and comparable systematic errors compared to the 2010 data-taking run, we present a new bn measurement using the weighted average of our asymmetry dataset fits, to obtain bn=0.066±0.041stat±0.024syst which corresponds to a limit of −0.012<bn<0.144 at the 90% confidence level.Received 17 November 2019Accepted 25 February 2020DOI:https://doi.org/10.1103/PhysRevC.101.035503©2020 American Physical SocietyPhysics Subject Headings (PhySH)Research AreasBeta decayElectroweak interactions in nuclear physicsNuclear tests of fundamental interactionsNuclear Physics

Electromagnetic signals from the decay of free neutrons in the first hours of neutron star mergers

ABSTRACT The first hours following a neutron star merger are considered to provide several ultraviolet (UV)/optical/near-infrared signals: β-decay emission from free neutrons, radioactive decay of shocked heavy elements in the cocoon and cocoon’s cooling emission. Here, we consider two additional emission sources: β-decay of free neutrons in the cocoon and synchrotron by the β-decay electrons. We present three-dimensional relativistic hydrodynamic simulations of jets that propagate in a multi-layer ejecta from the merger and calculate semi-analytically the resulting light curves. We find that the free neutrons emission at high latitudes is enhanced by the cocoon by a factor of a few to power a wide (≲60°) and brief (∼1 h) UV signal that can reach an absolute magnitude of ≳−15, comparable with the cooling emission. If the ejected neutron matter mass is $M_{\rm n} \gtrsim 10^{-4}\, {\rm M_{\odot }}$, the synchrotron emission may yield a long (∼8 h) quasi-isotropic UV/optical signal with an absolute magnitude between −12 and −15, depending on the magnetic field. Such a high mass of a mildly relativistic component may partly obscure the cocoon’s shocked r-process elements, thereby attenuating its radioactive decay emission. Future observations on these time-scales, including null detections, may place constraints on the ejected neutron matter mass and shed light on the ejecta and jet-cocoon characteristics.

The Application of Neural Networks to Detecting Decay Events of a Neutron

Abstract Neutron decay time is an important parameter in physics, for which science still does not have a single definitive answer. Free neutrons decay into a pro-ton, electron, and antineutrino; however, different experiments give different results for the time a neutron takes to decay. The bottle method results in a neutron half-life of 14.64 minutes while the beam method results in a neutron decay time of 14.8 minutes. The third method, the standard model, results in a prediction of around 14.75 minutes. The experiment measured neutron decay using a cloud chamber and neutrons produced by a thermonuclear fusion using a fusor. In this case however given the lack of current application research, having a human classify events in the cloud chamber isn’t effective, and where the application of Neural Networks and the application of cognition systems can be applied to make the task more accurate. In this paper, a method to directly measure neutron decay time based on the timing of cloud chamber events is presented, instead of calculating it from half-life results as other experiments have done this presented the details of training machine learning models for the application in neural networks and analysis using the cognitive system, Independent Core Observer Model (ICOM) to analyze results.

Measurement of the Weak Axial-Vector Coupling Constant in the Decay of Free Neutrons Using a Pulsed Cold Neutron Beam.

We present a precision measurement of the axial-vector coupling constant g_{A} in the decay of polarized free neutrons. For the first time, a pulsed cold neutron beam was used for this purpose. By this method, leading sources of systematic uncertainty are suppressed. From the electron spectra we obtain λ=g_{A}/g_{V}=-1.27641(45)_{stat}(33)_{sys}, which confirms recent measurements with improved precision. This corresponds to a value of the parity violating beta asymmetry parameter of A_{0}=-0.11985(17)_{stat}(12)_{sys}. We discuss implications on the Cabibbo-Kobayashi-Maskawa matrix element V_{ud} and derive a limit on left-handed tensor interaction.

Neutron disappearance inside the nucleus

We consider the possibility that a neutron may disappear inside the nucleus, which will demonstrate the existence of baryon violating ΔB = 1 interactions. It has recently been proposed that such a process may have an effect on the free neutron decay life time. We evaluate the widths for and , with χ being a light dark matter particle emitted by a loosely bound neutron in various light nuclei. We find that, assuming a mass mχ close to 938 MeV, the obtained width for in 11Be is much larger than the observed beta decay width. This suggests a severe limit on the possible decay channel of for free neutron.

ACORN: Measuring the electron-antineutrino correlation in neutron beta decay

The aCORN experiment uses a novel asymmetry method to measure the electron-antineutrino correlation (a-coefficient) in free neutron decay that does not require precision proton spectroscopy. aCORN completed two physics runs at the NIST Center for Neutron Research. The first run on the NG-6 beam line obtained the result a = 0.1090 +/- 0.0030 (stat) +/- 0.0028 (sys), the most precise to date. The second run on the new NG-C high flux beam line promises an improvement in precision to ¡ 2%. In addition we show that an improved measurement of the neutrino asymmetry (B-coefficient) can be made using the aCORN apparatus on a highly polarized neutron beam.

Viscous-dynamical Ejecta from Binary Neutron Star Mergers

Abstract General-relativistic simulations of binary neutron star (NS) mergers with viscosity reveal a new outflow mechanism operating in unequal mass binaries on dynamical timescales and enabled by turbulent viscosity. These “viscous-dynamical” ejecta are launched during the merger due to the thermalization of mass exchange streams between the secondary and the primary NS. They are characterized by asymptotic velocities extending up to ∼0.8c, and have masses that depend on the efficiency of the viscous mechanism. Depending on the unknown strength of the effective viscosity arising from magnetohydrodynamic instabilities operating during the merger, the overall mass of the dynamical ejecta could be enhanced by a factor of a few and the mass of the fast tail of the ejecta, having asymptotic velocities ≥0.6c, by up to four orders of magnitude. The radioactive decay of the expanding viscous-dynamical ejecta could produce bright kilonova transients with signatures of free neutron decay in the first hour, and enhanced near-infrared flux on a timescale of a few days. The synchrotron remnant produced by the interaction between the ejecta and the interstellar medium could also be significantly enhanced by viscosity. Such a remnant could be detected in the case of GW170817 as a rebrightening of the radio signal in the next months to years.

Measurements of the Neutron Lifetime

Free neutron decay is a fundamental process in particle and nuclear physics. It is the prototype for nuclear beta decay and other semileptonic weak particle decays. Neutron decay played a key role in the formation of light elements in the early universe. The precise value of the neutron mean lifetime, about 15 min, has been the subject of many experiments over the past 70 years. The two main experimental methods, the beam method and the ultracold neutron storage method, give average values of the neutron lifetime that currently differ by 8.7 s (4 standard deviations), a serious discrepancy. The physics of neutron decay, implications of the neutron lifetime, previous and recent experimental measurements, and prospects for the future are reviewed.

In situ collection of dust grains falling from Saturn’s rings into its atmosphere

Saturn's main rings are composed of >95% water ice, and the nature of the remaining few percent has remained unclear. The Cassini spacecraft's traversals between Saturn and its innermost D ring allowed its cosmic dust analyzer (CDA) to collect material released from the main rings and to characterize the ring material infall into Saturn. We report the direct in situ detection of material from Saturn's dense rings by the CDA impact mass spectrometer. Most detected grains are a few tens of nanometers in size and dynamically associated with the previously inferred "ring rain." Silicate and water-ice grains were identified, in proportions that vary with latitude. Silicate grains constitute up to 30% of infalling grains, a higher percentage than the bulk silicate content of the rings.