Neutron Oscillations Research Articles

Improved limits on n→n′ transformation from the Spallation Neutron Source

Conversions between neutrons n and dark matter candidate sterile neutrons n′ have been proposed as a mechanism for baryon number B violation. In the case that there is a small mass difference Δm between the n and the n′ states, oscillations can be induced by compensating for Δm with a magnetic field. A search for such neutron oscillations was performed at the Spallation Neutron Source by looking for anomalous neutron transmission through a strongly absorbing cadmium wafer inside of a 6.6 T magnet. The approach described here saw no regenerated neutrons above background, which provides an improved limit for neutron–sterile neutron transformations for a range of Δm between 0.1 and 1000 neV. 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.

Про можливість самопідтримної ланцюгової реакції поділу в об’єкті "Укриття" на теперішній час

Based on new modern data regarding the state of the fuel-containing masses (FCM) of the "Shelter", as well as taking into account the neutron incident of 1990 and the physical properties of the FCM of the "Shelter", the possibility of the occurrence and development of a self-sustaining chain nuclear fission reaction was considered in the FCM. It is shown that the drying of the FCM can lead to the occurrence of a single neutron burst with an amplitude comparable to the amplitude of neutron oscillations in 1990. It is also shown that the amplitude of the burst can be reduced if the rate of drying of the FCM is increased. The evaluations show that such a burst would have no impact on the environment.

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.

Neutron oscillation and baryogenesis from six dimensions

Considering a six-dimensional geometry orbifolded on ${S}^{1}/{Z}_{2}\ifmmode\times\else\texttimes\fi{}{S}^{1}/{Z}_{2}$ with quarks and leptons localized on orthogonal branes, we show that the construction admits observable $n\ensuremath{-}\overline{n}$ oscillation while naturally suppressing the proton decay rates. Consistent with other low-energy observables, the model also accommodates baryogenesis at $\mathcal{O}(10\text{ }\text{ }\mathrm{TeV})$ scale.

Experimental Search for Neutron to Mirror Neutron Oscillations as an Explanation of the Neutron Lifetime Anomaly.

An unexplained >4σ discrepancy persists between "beam" and "bottle" measurements of the neutron lifetime. A new model proposed that conversions of neutrons n into mirror neutrons n^{'}, part of a dark mirror sector, can increase the apparent neutron lifetime by 1% via a small mass splitting Δm between n and n^{'} inside the 4.6T magnetic field of the National Institute of Standards and Technology Beam Lifetime experiment. A search for neutron conversions in a 6.6T magnetic field was performed at the Spallation Neutron Source which excludes this explanation for the neutron lifetime discrepancy.

Status of the Design of an Annihilation Detector to Observe Neutron-Antineutron Conversions at the European Spallation Source

The goal of the HIBEAM/NNBAR program is to search for baryon number violation via the conversion or oscillation of neutrons into sterile neutrons and/or antineutrons at the European Spallation Source. A key experimental component of the program is the construction of an annihilation detector to directly observe the production of an antineutron following the oscillation. Design studies for the annihilation detector are presented. The predicted response of the detector models are studied using Geant4 simulations made with Monte Carlo simulations of the annihilation signal topology and cosmic ray backgrounds. Particle identification and sensitive discriminating observables, such as invariant mass and sphericity, are shown.

Low-energy dipole excitations in O20 with antisymmetrized molecular dynamics

Low-energy dipole (LED) excitations in $^{20}$O were investigated by variation after $K$-projection of deformation($\beta$)-constraint antisymmetrized molecular dynamics combined with the generator coordinate method. We obtained two LED states, namely, the $1_1^-$ state with one-proton excitation on the relatively weak deformation and the $1_2^-$ state with a parity asymmetric structure of the normal deformation. The former is characterized by a toroidal dipole (TD) mode with vortical nuclear current, whereas the latter is associated with a low-energy $E1$ mode caused by surface neutron oscillation along the prolate deformation. The TD (vortical) and $E1$ modes separately appear as the $K^\pi=1^-$ and $K^\pi=0^-$ components of the deformed states, respectively, but couple with each other in the $1_1^-$ and $1_2^-$ states of $^{20}$O because of $K$-mixing, and shape fluctuation. As a result of the mixing, TD and $E1$ transition strengths are fragmented into the $1_1^-$ and $1_2^-$ states. We also obtained the $K^\pi=0^+$, $K^\pi=0^-$, and $K^\pi=1^-$ bands with cluster structures in the energy region higher than the LED states.

A new approach to search for free neutron-antineutron oscillations using coherent neutron propagation in gas

Coherent forward neutron propagation in gas is discussed as a new approach to search for neutron-antineutron oscillations (n−n¯), which violate both B and B−L conservation. We show that one can increase the probability of neutron - antineutron transitions in the presence of a nonzero external magnetic field to essentially free neutron oscillation probability by tuning the density of an appropriate mixture of gases so that the neutron optical potential of the gas cancels that from the magnetic field.

Neutron oscillations for solving neutron lifetime and dark matter puzzles

A model of n−n′ (neutron-mirror neutron) oscillations is proposed under the framework of the mirror matter theory with slightly broken mirror symmetry. It resolves the neutron lifetime discrepancy, i.e., the 1% difference in neutron lifetime between measurements from “beam” and “bottle” experiments. In consideration of the early universe evolution, the n−n′ mass difference is determined to be about 2×10−6 eV/c2 with the n−n′ mixing strength of about 2×10−5. The picture of how the mirror-to-ordinary matter density ratio is evolved in the early universe into the observed dark-to-baryon matter density ratio of about 5.4 is presented. Reanalysis of previous data and new experiments that can be carried out under current technology are discussed and recommended to test this proposed model. Other consequences of the model on astrophysics and possible oscillations of other neutral particles are discussed as well.

Annihilation signatures of neutron dark decay models in neutron oscillation and proton decay searches

We point out that two models that reconcile the neutron lifetime anomaly via dark decays of the neutron, also predict dark matter-neutron ( overline{chi} − n) annihilation that may be observable in neutron-antineutron oscillation and proton decay searches at Super-Kamiokande, Hyper-Kamiokande and DUNE. We study signatures of overline{chi} n → γπ0 (or multi-π0) and overline{chi} n → ϕγπ0 (or ϕ+multi-π0), where ϕ is an almost massless boson in one of the two models.

New search for mirror neutron regeneration

The possibility of relatively fast neutron oscillations into a mirror neutron state is not excluded experimentally when a mirror magnetic field is considered. Direct searches for the disappearance of neutrons into mirror neutrons in a controlled magnetic field have previously been performed using ultracold neutrons, with some anomalous results reported. We describe a technique using cold neutrons to perform a disappearance and regeneration search, which would allow us to unambiguously identify a possible oscillation signal. An experiment using the existing General Purpose-Small Angle Neutron Scattering instrument at the High Flux Isotope Reactor at Oak Ridge National Laboratory will have the sensitivity to fully explore the parameter space of prior ultracold neutron searches and confirm or refute previous claims of observation. This instrument can also conclusively test the validity of recently suggested oscillation-based explanations for the neutron lifetime anomaly.

New experimental limits on neutron \u2013 mirror neutron oscillations in the presence of mirror magnetic field

Present experimental and astrophysical limits do not exclude that the neutron (n) oscillation into mirror neutron (n'), a sterile state exactly degenerate in mass with the neutron, can be a very fast process, in fact faster than the neutron decay itself, in which case it would have very interesting implications in cosmology and astrophysics. This process is sensitive to the magnetic field. Namely, if the mirror magnetic field mathbf {B}' exists at the Earth, n{-}n' oscillation probability can be suppressed or resonantly amplified by the applied magnetic field mathbf {B}, depending on its strength and on the angle beta between mathbf {B} and mathbf {B}'. We present the results of ultra-cold neutron storage measurements aiming to check the anomalies observed in previous experiments which could be a signal for n{-}n' oscillation in the presence of mirror magnetic field B'sim 0.1 G. From the analysis of the experimental data new lower limits on n{-}n' oscillation time as a function of B' were obtained, assuming that the mirror magnetic field is constant in time: tau _{nn'} > 17 s (95 % C.L.) for any B' between 0.08 and 0.17 G, and tau _{nn'}/sqrt{cos beta } > 27 s (95 % C.L.) for any B' in the interval (0.06div 0.25) G.

Monte Carlo simulation for ultracold neutron experiments searching for neutron–mirror neutron oscillation

Neutron oscillation into mirror neutron, a sterile state exactly degenerate in mass with the neutron, could be a very rapid process, even faster than the neutron decay itself. It can be observed by comparing the neutron lose rates in an ultracold neutron trapping experiment for different experimental magnetic fields. We developed a Monte Carlo code that simulates many of the features of this kind of experiment with nonuniform magnetic fields. The aim of the simulation is to provide all necessary tools, needed for analyzing experimental results for neutron traps with different geometry and different configurations of magnetic fields. This work contains technical details on the Monte Carlo simulation used for the analysis in Ref. 46 not presented in it.

Quantum field theory of particle oscillations: Neutron-antineutron conversion

We formulate the quantum field theory description of neutron-antineutron oscillations in the framework of canonical quantization, in analogy with the Bardeen--Cooper--Schrieffer (BCS) theory and the Nambu--Jona-Lasinio model. The physical vacuum of the theory is a condensate of pairs of {\it would-be neutrons and antineutrons} in the absence of the baryon-number violating interaction. The quantization procedure defines uniquely the mixing of massive Bogoliubov quasiparticle states which represent the neutron. In spite of not being mass eigenstates, neutron and antineutron states are defined on the physical vacuum and the oscillation formulated in asymptotic states. The exchange of baryonic number with the vacuum condensate engenders what may be observed as neutron-antineutron oscillation. The convergence between the present canonical approach and the Lagrangian/path integral approach to neutron oscillations is shown by the calculation of the anomalous (baryon-number violating) propagators. The quantization procedure proposed here can be extended to neutrino oscillations and, in general, to any particle oscillations.

Constraints on mirror models of dark matter from observable neutron-mirror neutron oscillation

The process of neutron-mirror neutron oscillation, motivated by symmetric mirror dark matter models, is governed by two parameters: n−n′ mixing parameter δ and n−n′ mass splitting Δ. For neutron mirror neutron oscillation to be observable, the splitting between their masses Δ must be small and current experiments lead to δ≤2×10−27 GeV and Δ≤10−24 GeV. We show that in mirror universe models where this process is observable, this small mass splitting constrains the way that one must implement asymmetric inflation to satisfy the limits of Big Bang Nucleosynthesis on the number of effective light degrees of freedom. In particular we find that if asymmetric inflation is implemented by inflaton decay to color or electroweak charged particles, the oscillation is unobservable. Also if one uses SM singlet fields for this purpose, they must be weakly coupled to the SM fields.

Baryogenesis from oscillations of charmed or beautiful baryons

We propose a model for CP violating oscillations of neutral, heavy-flavored baryons into antibaryons at rates which are within a few orders of magnitude of their lifetimes. The flavor structure of the baryon violation suppresses neutron oscillations and baryon number violating nuclear decays to experimentally allowed rates. We also propose a scenario for producing such baryons in the early Universe via the out-of-equilibrium decays of a neutral particle, after hadronization but before nucleosynthesis. We find parameters where CP violating baryon oscillations at a temperature of a few MeV could result in the observed asymmetry between baryons and antibaryons. Furthermore, part of the relevant parameter space for baryogenesis is potentially testable at Belle II via decays of heavy flavor baryons into an exotic neutral fermion. The model introduces four new particles: three light Majorana fermions and a colored scalar. The lightest of these fermions is typically long lived on collider timescales and may be produced in decays of bottom and possibly charmed hadrons.

Toroidal, compressive, and E1 properties of low-energy dipole modes in Be10

We studied dipole excitations in $^{10}$Be based on an extended version of the antisymmetrized molecular dynamics, which can describe 1p-1h excitations and large amplitude cluster modes. Toroidal and compressive dipole operators are found to be good proves to separate the low-energy and high-energy parts of the isoscalar dipole excitations, respectively. Two low-energy $1^-$ states, the toroidal dominant $1^-_1$ state at $E\sim 8$ MeV and the $E1$ dominant $1^-_2$ state at $E\sim 16$ MeV, were obtained. By analysis of transition current densities, the $1^-_1$ states is understood as a toroidal dipole mode with exotic toroidal neutron flow caused by rotation of a deformed $^6\textrm{He}$ cluster, whereas the $1^-_2$ state is regarded as a neutron-skin oscillation mode, which are characterized by surface neutron flow with inner isoscalar flow caused by the surface neutron oscillation against the $2\alpha$ core.

Gauged B-L number and neutron\u2013antineutron oscillation: long-range forces mediated by baryophotons

Transformation of a neutron to an antineutron n rightarrow {tilde{n}} has not yet been experimentally observed. In principle, it can occur with free neutrons in the vacuum or with neutrons bound inside the nuclei. In a nuclear medium the neutron and the antineutron have different potentials and for that reason n–{tilde{n}} conversion in nuclei is heavily suppressed. This transformation can also be suppressed for free neutrons in the presence of an environmental vector field that distinguishes the neutron from the antineutron. We consider the case of a gauge field coupled to the B-L charge of the particles (B-L photon), and we show that discovery of n–{tilde{n}} oscillation in experiment will lead to few order of magnitudes stronger limits on its coupling constant than present limits from the tests of the equivalence principle. If n–{tilde{n}} oscillation will be discovered via nuclear instability, but not in free neutron oscillations at a corresponding level, this would indicate the presence of such environmental fifth forces. In the latter case the B-L potential can be measurable by varying the external magnetic field for achieving the resonance conditions for n–{tilde{n}} conversion. As for neutron–mirror neutron oscillation, such potentials should have no effect once the fifth forces are associated to a common quantum number (B-L)-(B'-L') shared by the ordinary and mirror particles.

Neutron-antineutron oscillations beyond the quasifree limit

Prompted by plans to conduct a new neutron oscillation experiment at the European Spallation Source (ESS), we consider issues associated with the magnetic field that must be present, some of which are potentially exacerbated by the significantly larger length $l$ contemplated for the neutron propagation region. To this end, we introduce a stochastic model of the residual magnetic field within the propagation region which draws on features of magnetic profiles measured during the last free neutron oscillation experiment [conducted at the Institut Laue-Langevin (ILL) in the 1990's]. We average over both fluctuations in the magnetic field sampled by neutrons, and representative spectra of neutron speeds. We find that deviations from the quasi-free result for the antineutron probability do not depend quadratically on $l$ (as a naive perturbative estimate would suggest) but increase only linearly with $l$. As regards the large spikes in the magnetic field which can be expected at, for example, joints in the magnetic shielding of the propagation region (despite compensating currents and magnetic idealization of the shield), we demonstrate that their effect scales as $l/D^{3/2}$, where $D$ is the diameter of the cylindrical magnetic shielding. Our arguments suggest that, provided the dimensions of the propagation region are such that the ratio $l/D^{3/2}$ does not exceed the value pertinent to the ILL experiment, and these spikes occur close to either end of the propagation region, they can be neglected. We also establish that any large magnetic field encountered after the propagation region is exited will not diminish the probability for antineutron detection. For the range of values of $l$ of most interest to the ESS experiment, it should suffice to improve on the level of magnetic suppression achieved at the ILL by a factor of two.