arXiv subjects

Leendert Hayen

Publications and source records attributed to Leendert Hayen.

At least 19 recordsLinked to original sources

arXiv2026

The form factor expansion in the precision $\beta$ decay era

Precision tests of the Standard Model using $\beta$ decay have always relied on a careful choice of transition to minimize residual nuclear structure uncertainties. Following breakthroughs in nucleon-level radiative corrections in the last decade, however, corrections due to nuclear structure are once more a limiting factor in several scenarios. Progress in ab initio nuclear theory provides a path forward, but common recoil-order approximations in traditional formalisms often go unnoticed. Here, we critically examine their origin and address recently resolved and identify open questions.

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arXiv2025

Characterization of Low-energy Ionization Signals in Silicon Detectors for the Nab Experiment

The Nab (Neutron a b) experiment is designed to measure the beta-antineutrino angular correlation in free neutron $\beta$ decay with an ultimate precision goal of 0.1%, providing input for tests of Cabibbo-Kobayashi-Maskawa (CKM) matrix unitarity. This measurement is performed via detection of electrons and protons in delayed coincidence using custom large-area segmented silicon detectors. We present the characterization of one such detector system to establish the proton energy and timing response, using a dedicated proton accelerator. The detected proton peak was studied for 25 keV, 30 keV, and 35 keV incident protons on a set of detector segments and multiple cooling cycles over a one year period. Ionization losses were consistent with models of the detector dead layer with thicknesses less than 100 nm. The detected proton peak was stable within the uncertainty from energy calibration (0.25 keV). The rise times of detector pulses from $^{109}$Cd and $^{113}$Sn conversion electron sources were used to extract the impurity density profile and establish a precise model for the detector timing response. The observed impurity density profile varied from $(2 \pm 2) \times 10^9$ cm$^{-3}$ at the center to $(26 \pm 2) \times 10^9$ cm$^{-3}$ at the edge. This impurity density profile was then used to characterize systematic effects in proton time-of-flight measurements due to detector pulse-shape effects; the resultant proton timing systematic uncertainties were below 0.3 ns, which is sufficient for the Nab experiment.

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arXiv2025

First Full Dalitz Plot Measurement in Neutron $\beta$-Decay using the Nab Spectrometer and Implications for New Physics

Precision measurements of observables in neutron $\beta$-decay are used to test the Standard Model description of the weak interaction and search for evidence of new physics. The Nab experiment at the Fundamental Neutron Physics Beamline at the Spallation Neutron Source was constructed to measure correlations in neutron decay by utilizing an asymmetric spectrometer and novel detection system to accurately reconstruct the proton momentum and electron energy for each $\beta$-decay. This work describes the detection of neutron $\beta$-decay products in the Nab spectrometer and presents the first full Dalitz plot representation of the phase space of neutron $\beta$-decay for all electrons >100 keV. In addition, new constraints are placed on a possible excited neutron state, hypothesized to explain the disagreement between the appearance and disappearance neutron lifetime techniques.

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arXiv2025

CONFLUX: A Standardized Framework to Calculate Reactor Antineutrino Flux

Nuclear fission reactors are abundant sources of antineutrinos. The flux and spectrum of antineutrinos emitted by a reactor can indicate its activity and composition, suggesting potential applications of neutrino measurements beyond fundamental scientific studies that may be valuable to society. The utility of reactor antineutrinos for applications and fundamental science is dependent on the availability of precise predictions of these emissions. For example, in the last decade, disagreements between reactor antineutrino measurements and models have inspired revision of reactor antineutrino calculations and standard nuclear databases as well as searches for new fundamental particles not predicted by the Standard Model of particle physics. Past predictions and descriptions of the methods used to generate them are documented to varying degrees in the literature, with different modeling teams incorporating a range of methods, input data, and assumptions. The resulting difficulty in accessing or reproducing past models and reconciling results from differing approaches complicates the future study and application of reactor antineutrinos. The CONFLUX (Calculation Of Neutrino FLUX) software framework is a neutrino prediction tool built with the goal of simplifying, standardizing, and democratizing the process of reactor antineutrino flux calculations. CONFLUX include three primary methods for calculating the antineutrino emissions of nuclear reactors or individual beta decays that incorporate common nuclear data and beta decay theory. The software is prepackaged with the current nuclear database. It includes the capability to predict time-dependent neutrino model, adjust decay information entries, and propagate uncertainties. This paper describes the software structure, details the methods used for flux and spectrum calculations, and talks about potential use cases.

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arXiv2024

Impact of reactor neutrino uncertainties on coherent scattering's discovery potential

Nuclear power reactors are the most intense man-made source of antineutrino's and have long been recognized as promising sources for coherent elastic neutrino-nucleus scattering (CE$ν$NS) studies. Its observation and the spectral shape of the associated recoil spectrum is sensitive to a variety of exotic new physics scenarios and many experimental efforts are underway. Within the context of the reactor antineutrino anomaly, which initially indicated eV-scale sterile neutrino's, the modeling of the reactor antineutrino spectrum has seen a significant evolution in the last decade. Even so, uncertainties remain due to a variety of nuclear structure effects, incomplete information in nuclear databases and fission dynamics complexities. Here, we investigate the effects of these uncertainties on one's ability to accurately distinguish new physics signals. For the scenarios discussed here, we find that reactor spectral uncertainties are similar in magnitude to the projected sensitivities pointing towards a need for $β$ spectroscopy measurements below the inverse $β$ decay threshold.

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arXiv2024

Opportunities and open questions in modern $\beta$ decay

For well over half a century, precision studies of neutron and nuclear $\beta$ decays have been at the forefront of searches for exotic electroweak physics. Recent advances in nuclear ab initio theory and the widespread use of effective field theories means that its modern understanding is going through a transitional phase. This has been propelled by current tensions in the global data set leading to renewed scrutiny of its theoretical ingredients. In parallel, a host of novel techniques and methods are being investigated that are able to sidestep many traditional systematic uncertainties and require a diverse palette of skills and collaboration with material science and condensed matter physics. We highlight the current opportunities and open questions with the aim of facilitating the transition to a more modern understanding of $\beta$ decay.

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arXiv2022

Precision pulse shape simulation for proton detection at the Nab experiment

The Nab experiment at Oak Ridge National Laboratory, USA, aims to measure the beta-antineutrino angular correlation following neutron $β$ decay to an anticipated precision of approximately 0.1\%. The proton momentum is reconstructed through proton time-of-flight measurements, and potential systematic biases in the timing reconstruction due to detector effects must be controlled at the nanosecond level. We present a thorough and detailed semiconductor and quasiparticle transport simulation effort to provide precise pulse shapes, and report on relevant systematic effects and potential measurement schemes.

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arXiv2022

Ab initio calculation of the $β$ decay spectrum of $^6$He

We calculate the $β$ spectrum in the decay of $^6$He using Quantum Monte Carlo methods with nuclear interactions derived from chiral Effective Field Theory and consistent weak vector and axial currents. We work at second order in the multipole expansion, retaining terms suppressed by $\mathcal O(q^2/m_π^2)$, where $q$ denotes low-energy scales such as the reaction's $\mathcal Q$-value or the electron energy, and $m_π$ the pion mass. We go beyond the impulse approximation by including the effects of two-body vector and axial currents. We estimate the theoretical error on the spectrum by using four potential models in the Norfolk family of local two- and three-nucleon interactions, which have different cut-off, fit two-nucleon data up to different energies and use different observables to determine the couplings in the three-body force. We find the theoretical uncertainty on the $β$ spectrum, normalized by the total rate, to be well below the permille level, and to receive contributions of comparable size from first and second order corrections in the multipole expansion. We consider corrections to the $β$ decay spectrum induced by beyond-the-Standard Model charged-current interactions in the Standard Model Effective Field Theory, with and without sterile neutrinos, and discuss the sensitivity of the next generation of experiments to these interactions.

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arXiv2022

Pion-induced radiative corrections to neutron beta-decay

We compute the electromagnetic corrections to neutron beta decay using a low-energy hadronic effective field theory. We identify and compute new radiative corrections arising from virtual pions that were missed in previous studies. The largest correction is a percent-level shift in the axial charge of the nucleon proportional to the electromagnetic part of the pion-mass splitting. Smaller corrections, comparable to anticipated experimental precision, impact the $β$-$ν$ angular correlations and the $β$-asymmetry. We comment on implications of our results for the comparison of the experimentally measured axial charge with first-principle computations using lattice QCD and on the potential of $β$-decay experiments to constrain beyond-the-Standard-Model interactions.

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arXiv2021

Standard Model $\mathcal{O}(α)$ renormalization of $g_A$ and its impact on new physics searches

We present an $\mathcal{O}(α)$ Standard Model calculation of the inner radiative corrections to Gamow-Teller $β$ decays. We find that \textit{a priori} contributions arise from the photonic vertex correction and $γW$ box diagram. Upon evaluation most elastic contributions vanish due to crossing symmetry or cancellation between isoscalar and isovector photonic contributions, leaving only the polarized parity-odd contribution, i.e., the Gamow-Teller equivalent of the well-known axial $γW$ box contribution for Fermi decays. We show that weak magnetism contributes significantly to the Born amplitude, and consider additional hadronic contributions at low energy using a holomorphic continuation of the polarized Bjorken sum rule constrained by experimental data. We perform the same procedure for the Fermi inner radiative correction through a combination of the running of Bjorken and Gross-Llewellyn Smith sum rules. We discuss heavy flavor, higher-twist, and target mass corrections and find a significant increase at low momentum from the latter. We find $Δ_R^A = 0.02532(22)$ and $Δ_R^V = 0.02473(27)$ for axial and vector inner radiative corrections, respectively, resulting in $Δ_R^A-Δ_R^V=0.60(5) \times 10^{-3}$, which allows us to extract $g_A^0$ for the first time to our knowledge. We discuss consequences for comparing experimental data to lattice calculations in beyond Standard Model fits. Further, we show how some traditional $β$ decay calculations contain part of this effect but fail to account for cancellations in the full $\mathcal{O}(α)$result. Finally, we correct for a double-counting instance in the isospin $T=1/2$ mirror decay extraction of $|V_{ud}|$, the up-down matrix element of the Cabibbo-Kobayashi-Maskawa matrix, resolving a long-standing tension and leading to increased precision.

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arXiv2021

Radiative corrections to nucleon weak charges and Beyond Standard Model impact

The nucleon axial charge is a central ingredient in nuclear and particle physics, and a key observable in precision tests of the electroweak Standard Model sector and beyond. We report on the first complete calculation of its electroweak quantum corrections up to $\mathcal{O}(α)$, using a combination of current algebra techniques and QCD sum rules. We find a substantial enhancement due to the weak magnetism contribution in the elastic channel, and include higher-twist and target mass corrections at low $Q^2$ to find $Δ_R^A = 0.02881(22)$. Using analogous methods, we determine a new value for the vector charge renormalization, $Δ_R^V = 0.02474(27)$, and show how the two most recent calculations can be brought into agreement. This allows us to determine a corrected experimental $g_A^0 = 1.2730(13)$, which is a $>2σ$ shift away from the commonly quoted value. We use this new result to set constraints on exotic right-handed currents by comparing to lattice QCD results, and resolve a double-counting issue in the $|V_{ud}|$ extraction from mirror decays.

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arXiv2020

A look into mirrors: A measurement of the $β$-asymmetry in $^{19}$Ne decay and searches for new physics

High precision measurements of isospin $T=1/2$ decays in the neutron and nuclei provide strong model-independent constraints on extensions to the standard model of particle physics. A measurement of the $β$-asymmetry in $^{19}$Ne decay between the initial nuclear spin and the direction of the emitted positron is presented which establishes this decay as the most precisely characterized nuclear mirror and fixes the Fermi-to-Gamow-Teller mixing ratio to $ρ= 1.6014(+21/-28)_{sys}(8)_{stat}$. This is consistent with the previous, most precise measurement, produces a value of the CKM unitarity parameter $V_{ud}$ in agreement with the nuclear mirror, neutron and superallowed $β$-decay data sets, shows no evidence for second class currents, and can be effectively used with neutron decay data to place a limits on exotic tensor couplings.

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arXiv2020

Consistent description of angular correlations in $β$ decay for Beyond Standard Model physics searches

Measurements of angular correlations between initial and final particles in $β$ decay remain one of the most promising ways of probing the Standard Model and looking for new physics. As experiments reach unprecedented precision well into the per-mille regime, proper extraction of results requires one to take into account a great number of nuclear structure and radiative corrections in a procedure which becomes dependent upon the experimental geometry. We provide here a compilation and update of theoretical results which describe all corrections in the same conceptual framework, point out pitfalls and review the influence of the experimental geometry. Finally, we summarize the potential for new physics reach.

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arXiv2020

Detailed $β$ spectrum calculations of $^{214}$Pb for new physics searches in liquid Xenon

We present a critical assessment of the calculation and uncertainty of the $^{214}$Pb $\to$ $^{214}$Bi ground state to ground state $β$ decay, the dominant source of background in liquid Xenon dark matter detectors, down to below 1 keV. We consider contributions from atomic exchange effects, nuclear structure and radiative corrections. For each of these, we find changes much larger than previously estimated uncertainties and discuss shortcomings of the original calculation. Specifically, through the use of a self-consistent Dirac-Hartree-Fock-Slater calculation, we find that the atomic exchange effect increases the predicted flux by $10(3)\%$ at 1 keV relative to previous exchange calculations. Further, using a shell model calculation of the nuclear structure contribution to the shape factor, we find a strong disagreement with the allowed shape factor and discuss several sources of uncertainty. In the 1-200 keV window, the predicted flux is up to 20$\%$ lower. Finally, we discuss omissions and detector effects in previously used QED radiative corrections, and find small changes in the slope at the $\gtrsim 1\%$ MeV$^{-1}$ level, up to $3\%$ in magnitude due to omissions in $\mathcal{O}(Zα^2, Z^2α^3)$ corrections and $3.5\%$ uncertainty from the neglect of as of yet unavailable higher-order contributions. Combined, these give rise to an increase of at least a factor 2 of the uncertainty in the 1-200 keV window. We comment on possible experimental schemes of measuring this and related transitions.

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arXiv2020

$Q_{\textrm{EC}}$-value determination for $^{21}$Na$\rightarrow^{21}$Ne and $^{23}$Mg$\rightarrow^{23}$Na mirror-nuclei decays using high-precision mass spectrometry with ISOLTRAP at ISOLDE/CERN

We report on high-precision $Q_{\textrm{EC}}$ values of the $^{21}$Na$\rightarrow^{21}$Ne and $^{23}$Mg$\rightarrow^{23}$Na mirror $β$-transitions from mass measurements with ISOLTRAP at ISOLDE/CERN. A precision of $δm/m = 9 \cdot 10^{-10}$ and $δm/m = 1.5 \cdot 10^{-9}$ was reached for the masses of $^{21}$Na and $^{23}$Mg, respectively. We reduce the uncertainty of the $Q_{\textrm{EC}}$ values by a factor five, making them the most precise experimental input data for the calculation of the corrected $\mathcal{F} t$-value of these mixed Fermi/Gamow-Teller transitions. For the $^{21}$Na$\rightarrow^{21}$Ne $Q_{\textrm{EC}}$ value, a $2.3 σ$ deviation from the literature $Q_{\textrm{EC}}$-value was found.

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arXiv2019

Radiative corrections to Gamow-Teller decays

Radiative corrections in the electroweak sector constitute an essential component in the ability to disentangle Beyond Standard Model physics from experimental data. This is particularly relevant for strongly bound systems undergoing weak decays such as nuclear $β$ decay, where its contribution to top-row CKM unitarity tests is essential. In this Letter we note the need for an additional radiative correction to the Gamow-Teller form factor in allowed decays. It concerns a combination of electrostatic final state interactions and QCD-induced currents. We review the basic derivation and report analytical results. Due to differences in their theoretical treatment in the literature, effects on the neutron and mirror systems are distinct. Significant consequences appear for a comparison of the former with lattice QCD, while changes occur in the $|V_{ud}|$ extraction in the latter. We discuss new limits on right-handed currents and provide a new value for $|V_{ud}|$ from mirror decays.

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arXiv2019

Interfacing Geant4, Garfield++ and Degrad for the Simulation of Gaseous Detectors

For several years, attempts have been made to interface Geant4 and other software packages with the aim of simulating the complete response of a gaseous particle detector. In such a simulation, Geant4 is always responsible for the primary particle generation and the interactions that occur in the non-gaseous detector material. Garfield++ on the other hand always deals with the drift of ions and electrons, amplification via electron avalanches and finally signal generation. For the ionizing interaction of particles with the gas, different options and physics models exist. The present paper focuses on how to use Geant4, Garfield++ (including its Heed and SRIM interfaces) and Degrad to create the ionization electron-ion pairs in the gas. Software-wise, the proposed idea is to use the Geant4 physics parameterization feature, and to implement a Garfield++ or Degrad based detector simulation as an external model. With a Degrad model, detailed simulations of the X-ray interaction in gaseous detectors, including shell absorption by photoelectric effect, subsequent Auger cascade, shake-off and fluorescence emission, become possible. A simple Garfield++ model can be used for photons (Heed), heavy ions (SRIM) and relativistic charged particles or MIPs (Heed). For non-relativistic charged particles, more effort is required, and a combined Geant4/Garfield++ model must be used. This model, the Geant4/Heed PAI model interface, uses the Geant4 PAI model in conjunction with the Heed PAI model. Parameters, such as the lower production cut of the Geant4 PAI model and the lowest electron energy limit have to be set correctly. The paper demonstrates how to determine these parameters for certain values of the W parameter and Fano factor of the gas mixture. The simulation results of this Geant4/Heed PAI model interface are then verified against the results obtained with the stand-alone software packages.

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arXiv2019

Beta Spectrum Generator: High precision allowed $β$ spectrum shapes

Several searches for Beyond Standard Model physics rely on an accurate and highly precise theoretical description of the allowed $β$ spectrum. Following recent theoretical advances, a C++ implementation of an analytical description of the allowed beta spectrum shape was constructed. It implements all known corrections required to give a theoretical description accurate to a few parts in $10^4$. The remaining nuclear structure-sensitive input can optionally be calculated in an extreme single-particle approximation with a variety of nuclear potentials, or obtained through an interface with more state-of-the-art computations. Due to its relevance in modern neutrino physics, the corresponding (anti)neutrino spectra are readily available with appropriate radiative corrections. In the interest of user-friendliness, a graphical interface was developed in Python with a coupling to a variety of nuclear databases. We present several test cases and illustrate potential usage of the code. Our work can be used as the foundation for current and future high-precision experiments related to the beta decay process. Source code: https://github.com/leenderthayen/BSG Documentation: http://bsg.readthedocs.io

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