arXiv subjects

Jouni Suhonen

Publications and source records attributed to Jouni Suhonen.

At least 19 recordsLinked to original sources

arXiv2026

Precise determination of electron-capture $Q$ value of $^{113}$Sn decay related to electron neutrino mass measurements

A high-precision measurement of the electron-capture (EC) decay $Q$ value for the ground-state-to-ground-state (gs-to-gs) transition of $^{113}$Sn to $^{113}$In has been performed using the JYFLTRAP double Penning trap mass spectrometer. Employing the phase-imaging ion-cyclotron-resonance technique, the isomeric state of $^{113}$Sn at 77.389(19) keV was resolved, and the cyclotron frequency ratio measured between the isomer $^{113m}$Sn and the daughter nucleus $^{113}$In. This yielded an isomer-to-ground-state $Q$ value of 1116.64(19) keV and gs-to-gs $Q$ value of 1039.25(19) keV. The atomic mass excess of $^{113}$Sn was determined as $-$88327.87(27) keV/c$^2$, in excellent agreement with the Atomic Mass Evaluation 2020 (AME2020) but with a sixfold precision improvement. Using nuclear energy-level data for $^{113}$In, we identified two low $Q$-value transitions of the ground state of $^{113}$Sn to excited states of $^{113}$In at 1024.280(50) keV ($Q_{EC}^* = 14.97(20)$ keV, second forbidden non-unique) and 1029.650(50) keV ($Q_{EC}^* = 9.60(20)$ keV, allowed). The allowed transition exhibits small energy differences ($Δ_{L1} = 5.58(20)$ keV, $Δ_{L2} = 5.87(20)$ keV) from L1 and L2 shell binding energies, enhancing endpoint events. Partial half-lives and energy-release spectra were calculated using the self-consistent Dirac-Hartree-Fock-Slater (DHFS) method (including exchange, overlap, shake-up, and shake-off corrections) together with the nuclear shell model, show enhanced endpoint sensitivity for the allowed transition to the state at 1029.650 keV. Including subthreshold atomic states in the spectral function enhances the EC rate near the zero-neutrino-momentum region by a factor of five, enabling new approaches for low $Q$-value EC reactions in neutrino-mass studies.

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arXiv2025

High-precision direct decay energy measurements of the electron-capture decay of $^{97}$Tc

A direct measurement of the ground-state-to-ground-state electron-capture decay $Q$ ($Q_{\rm EC}$) value of $^{97}$Tc has been conducted employing the high resolving power phase-imaging ion-cyclotron-resonance technique with the double Penning trap mass spectrometer JYFLTRAP. The resulting $Q_{\rm EC}$ value for $^{97}$Tc is 324.82(21) keV, exhibiting a precision approximately 19 times higher than the value adopted in the newest Atomic Mass Evaluation (AME2020) and differing by 1.2$σ$. Furthermore, by combining this refined $Q$ value with nuclear energy-level data for the decay-daughter $^{97}$Mo, a potential ultra-low Q-value transition, possibly of allowed type, $^{97}$Tc (9/2$^{+}$, ground state) $\rightarrow$ $^{97}$Mo$^{*}$ (320(1) keV), was evaluated for future long-term neutrino-mass determination experiments. The ground-state-to-excited-state electron-capture decay $Q$ value ($Q^{*}_{\rm EC}$) of this transition was determined to be 4.8(10) keV, confirming it to be energetically allowed with a confidence level of exceeding 4$σ$. The captures of electrons occupying the L and higher shells for this transition are energetically allowed, giving a value of 2.0(10) keV for the closest distance of $Q^{*}_{\rm EC}$ to the allowed binding energy of the L1 shell. To predict partial half-lives and energy-release distributions for this transition, the atomic self-consistent many-electron Dirac--Hartree--Fock--Slater method and the nuclear shell model have been employed. Dominant correction terms such as exchange and overlap corrections, as well as shake-up and shake-off effects, were included in the final results. Moreover, the normalized distribution of released energy in the electron-capture decay of $^{97}$Tc to excited states of $^{97}$Mo, is compared with that of $^{163}$Ho, which is being used for electron-neutrino-mass determination.

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arXiv2025

Inelastic neutrino-nucleus scattering off $^{203/205}$Tl in terms of the nuclear recoil energy using a hybrid nuclear model

Nuclear structure calculations in the context of a novel hybrid nuclear model, combining the nuclear shell model and the microscopic quasiparticle-phonon model are presented. The predictivity of the hybrid model is tested by computing inelastic neutral-current neutrino-nucleus scattering cross sections off the stable thallium isotopes. The cross sections are presented in terms of the incoming neutrino energy, taking also into account the effect of nuclear recoil energy. Also reported are the expected event rates assuming neutrinos emerging from pion-decay at rest and the diffuse supernova neutrino background. Regarding solar neutrino rates, new results are presented in the context of the hybrid model and compared with previously reported results based solely on nuclear shell model calculations, demonstrating the improved accuracy of the adopted hybrid model at higher neutrino energies.

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arXiv2025

Novel way of evaluating $g_A$ quenching in $β^+$/EC decays : Introducing the Branching-Ratio Method (BRM)

The novel Branching-Ratio Method (BRM) for the determination of effective value $g_{\rm A}^{\rm eff}$ of the weak axial coupling $g_{\rm A}$ for forbidden non-unique (FNU) $β^+$/electron-capture(EC) decays is introduced. The method offers new possibilities for testing the fitness of nuclear Hamiltonians in modeling the physics of complex $β^+$/EC decays. The constraint of simultaneously reproducing the branching to $β^+$ and EC transitions offers an additional constraint in tackling the problem of $g_{\rm A}^{\rm eff}$ determination. In the BRM the ambiguity in the choice of the values of $g_{\rm A}^{\rm eff}$ and s-NME (small relativistic vector nuclear matrix element) is lifted when constraints of the branching ratios of $β^+$ and EC decays are applied. As an example, in the present work we apply BRM to the case of second FNU $β^+$/EC decay of $^{59}$Ni. This decay is treated with three different nuclear shell-model (NSM) Hamiltonians demonstrating the effects of different nuclear-structure aspects in application of the BRM.

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arXiv2024

Constraints for rare electron-capture decays mimicking detection of dark-matter particles in nuclear transitions

We give for the first time, theoretical estimates of unknown rare electron-capture (EC) decay branchings of $^{44}$Ti, $^{57}$Co, and $^{139}$Ce, relevant for searches of (exotic) dark-matter particles. The nuclear-structure calculations have been done exploiting the nuclear shell model (NSM) with well-established Hamiltonians and an advanced theory of $β$ decay. In the absence of experimental measurements of these rare branches, these estimates are of utmost importance for terrestrial searches of dark-matter particles, such as axionic dark matter in the form of axion-like particles (ALPs), anapole dark matter, and dark photons in nuclear transitions. Predictions are made for EC-decay rates of 2$^{nd}$-forbidden unique (FU) and 2$^{nd}$-forbidden non-unique (FNU) EC transitions that can potentially mimic dark-matter-particle detection in dedicated underground experiments designed to observe the absence of the corresponding nuclear electromagnetic transitions.

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arXiv2024

Systematic shell-model analysis of $2νββ$ decay of $^{76}$Ge and $^{96}$Zr to the ground and excited states of $^{76}$Se and $^{96}$Mo

In this work, we have studied the $2νββ$ decay of $^{76}$Ge and $^{96}$Zr isotopes utilizing large-scale shell-model calculations. The GWBXG effective interaction has been employed in the calculation of $2νββ$-decay nuclear matrix elements (NMEs). We have tested the effective interaction by comparing the predicted spectroscopic properties, such as energy spectra and transition probabilities, with the available experimental data. The variation of cumulative NMEs with respect to the $1^+$ state energies of the intermediate nucleus is also studied, corresponding to $0^+_{\rm g.s.}\rightarrow0^+_{\rm g.s.}$, $0^+_{\rm g.s.}\rightarrow0^+_{2}$, and $0^+_{\rm g.s.}\rightarrow2^+_{1}$ transitions between the parent and grand-daughter nuclei. The effective values of axial-vector coupling strength ($g_A^{\rm eff}$) are calculated using the predicted NMEs and experimental half-lives for $0^+_{\rm g.s.}\rightarrow0^+_{\rm g.s.}$ transitions. The extracted half-lives for $0^+_{\rm g.s.}\rightarrow0^+_{2}$, and $0^+_{\rm g.s.}\rightarrow2^+_{1}$ transitions using the shell-model predicted NMEs are consistent with the recent experimental data. The comparison of the shell-model predicted NMEs with previous NMEs available in the literature is discussed. Also, the computed branching ratios for the $2νββ$ decay of $^{76}$Ge and both the $2νββ$ and single-$β$ decay of $^{96}$Zr are reported corresponding to the calculated $g_A^{\rm eff}$ values.

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arXiv2024

Incoherent solar-neutrino scattering off the stable Tl-isotopes

Nuclear-structure calculations for the description of low-energy neutral-current neutrino scattering off the stable $^{203,205}$Tl isotopes are performed in the context of the nuclear shell model using the model space jj56pn. Cross-section and event-rate calculations focusing on inelastic solar-neutrino scattering off $^{203,205}$Tl are performed. The individual contributions of the various nuclear responses are presented and discussed, and the results are also illustrated in terms of the nuclear recoil energy. Analytical expressions entering the cross sections are given in order to achieve a direct connection with experimental observables.

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arXiv2024

High-precision measurements of the atomic mass and electron-capture decay $Q$ value of $^{95}$Tc

A direct measurement of the ground-state-to-ground-state electron-capture decay $Q$ value of $^{95}$Tc has been performed utilizing the double Penning trap mass spectrometer JYFLTRAP. The $Q$ value was determined to be 1695.92(13) keV by taking advantage of the high resolving power of the phase-imaging ion-cyclotron-resonance technique to resolve the low-lying isomeric state of $^{95}$Tc (excitation energy of 38.910(40) keV) from the ground state. The mass excess of $^{95}$Tc was measured to be $-$86015.95(18) keV/c$^2$, exhibiting a precision of about 28 times higher and in agreement with the value from the newest Atomic Mass Evaluation (AME2020). Combined with the nuclear energy-level data for the decay-daughter $^{95}$Mo, two potential ultra-low $Q$-value transitions are identified for future long-term neutrino-mass determination experiments. The atomic self-consistent many-electron Dirac--Hartree--Fock--Slater method and the nuclear shell model have been used to predict the partial half-lives and energy-release distributions for the two transitions. The dominant correction terms related to those processes are considered, including the exchange and overlap corrections, and the shake-up and shake-off effects. The normalized distribution of the released energy in the electron-capture decay of $^{95}$Tc to excited states of $^{95}$Mo is compared to that of $^{163}$Ho currently being used for electron-neutrino-mass determination.

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arXiv2024

Analysis of $^{115}$In $β$ decay through the spectral moment method

We analyze the $^{115}$In $β$-decay energy spectrum through the spectral moment method (SMM), previously introduced in the context of $^{113}$Cd $β$ decay. The spectral moments $μ_n$ are defined as averaged $n^{\rm th}$ powers of the $β$ particle energy, characterizing the spectrum normalization ($n=0$) and shape ($n\geq 1$) above a given threshold. For $^{115}$In, we consider three independent datasets characterized by different thresholds. We also consider three nuclear model calculations with two free parameters: the ratio of axial-vector to vector couplings, $r=g_{\rm A}/g_{\rm V}$, and the small vector-like relativistic nuclear matrix element (NME), $s=s$-NME. By using the most recent of the three datasets, we show that the first few spectral moments can determine $(r,\, s)$ values in good agreement with those obtained by full-fledged experimental fits. We then work out the SMM results for the other datasets. We find that, although $g_{\rm A}$ quenching is generally favored, the preferred quenching factors may differ considerably depending on the chosen experimental data and nuclear models. We discuss various issues affecting both the overall normalization and the low-energy behaviour of the measured and computed spectra, and their joint effects on the experimentally quoted half-life values. Further $^{115}$In $β$-decay data at the lowest possible energy threshold appear to be crucial to clarify these issues.

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arXiv2024

The MONUMENT Experiment: Ordinary Muon Capture studies for 0$νββ$ decay

The MONUMENT experiment measures ordinary muon capture (OMC) on isotopes relevant for neutrinoless double-beta (0$νββ$) decay and nuclear astrophysics. OMC is a particularly attractive tool for improving the theoretical description of 0$νββ$ decay. It involves similar momentum transfers and allows testing the virtual transitions involved in 0$νββ$ decay against experimental data. During the 2021 campaign, MONUMENT measured OMC on $^{76}$Se and $^{136}$Ba, the isotopes relevant for next-generation 0$νββ$ decay searches, like LEGEND and nEXO. The experimental setup has been designed to accurately extract the total and partial muon capture rates, which requires precise reconstruction of energies and time-dependent intensities of the OMC-related $γ$ rays. The setup also includes a veto counter system to allow selecting a clean sample of OMC events. This work provides a detailed description of the MONUMENT setup operated during the 2021 campaign, its two DAQ systems, calibration and analysis approaches, and summarises the achieved detector performance. Future improvements are also discussed.

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arXiv2024

$g_{\rm A}$-sensitive $β$ spectral shapes in the mass $A=86-99$ region assessed by the nuclear shell model

Recent years have witnessed an expanding interest in experimental studies of $β$ electrons (electrons emitted in $β^-$ decay transitions) and their energy distributions, the so-called $β$-electron spectra. These experiments are interested mainly in $β$ transitions with electron spectra sensitive to the effective value of the weak axial coupling $g_{\rm A}$. In the present paper we make an extensive search for $g_{\rm A}$ sensitive $β$ spectral shapes in the $A=86-99$ region using the nuclear shell model with the well established Hamiltonians $\textit{glekpn}$ and $\textit{jj45pnb}$, designed to render a good description of the spectroscopic properties of nuclei in this mass region. We have found eight $β^-$ decay transitions with various degrees of $g_{\rm A}$ sensitivity. Moreover, these transitions are also important in pinning down the value of the so-called small relativistic vector nuclear matrix element, sNME. In addition, some of the corresponding mother nuclei are important contributors to the antineutrino flux from nuclear reactors. All this means that the found $β$ transitions are potentially of great interest for future rare-events experiments.

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arXiv2023

Shell-model treatment of the $β$ decay of $^{99}$Tc

In the present paper we treat the second-forbidden non-unique (2nd-nu) ground-state-to-ground-state $β^-$ decay $^{99}\textrm{Tc}(9/2^+)\to\,^{99}\textrm{Ru}(5/2^+)$, with a 100$\%$ branching ratio, within the framework of the nuclear shell model (NSM). The energy spectrum of the electrons emitted in this $β$-decay transition ($β$-electron spectrum) is sensitive to the wave functions of the involved initial ($9/2^+$) and final ($5/2^+$) nuclear ground states through the many involved nuclear matrix elements (NME). The $β$-electron spectrum of this transition is potentially indicative of the effective value, $g_{\rm A}^{\rm eff}$, of the weak axial coupling, $g_{\rm A}$, of crucial importance for extraction of information on beyond-the-standard-model physics from the results of the present and future rare-events experiments. We describe the $β$ spectral shape of this decay by using a state-of-the-art $β$-decay formalism and compute the many involved NME using the well established NSM Hamiltonians $jj45pnb$ and $glekpn$. We have found a strong dependence of the $β$ spectral shape on the value of $g_{\rm A}$ making it a good candidate for determination of the value of $g_{\rm A}^{\rm eff}$ through comparison with the corresponding experimental $β$ spectral shape. We have also found an interesting dependence of the $β$ spectral shape on the value of the so-called small relativistic vector NME, sNME, used to match the computed half-life with the measured one.

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arXiv2023

$^{113}$Cd $β$-decay spectrum and $g_{\rm A}$ quenching using spectral moments

We present an alternative analysis of the $^{113}$Cd $β$-decay electron energy spectrum in terms of spectral moments $μ_n$, corresponding to the averaged values of $n^{\rm th}$ powers of the $β$ particle energy. The zeroth moment $μ_0$ is related to the decay rate, while higher moments $μ_n$ are related to the spectrum shape. The here advocated spectral-moment method (SMM) allows for a complementary understanding of previous results, obtained using the so-called spectrum-shape method (SSM) and its revised version, in terms of two free parameters: $r=g_{\rm A}/g_{\rm V}$ (the ratio of axial-vector to vector couplings) and $s$ (the small vector-like relativistic nuclear matrix element, $s$-NME). We present numerical results for three different nuclear models with the conserved vector current hypothesis (CVC) assumption of $g_{\rm V}=1$. We show that most of the spectral information can be captured by the first few moments which are simple quadratic forms (conic sections) in the $(r,\,s)$ plane: an ellipse for $n=0$ and hyperbolae for $n\geq 1$, all being nearly degenerate as a result of cancellations among nuclear matrix elements. The intersections of these curves, as obtained by equating theoretical and experimental values of $μ_n$, identify the favored values of $(r,\,s)$ at a glance, without performing detailed fits. In particular, we find that values around $r\sim 1$ and $s\sim 1.6$ are consistently favored in each nuclear model, confirming the evidence for $g_{\rm A}$ quenching in $^{113}$Cd, and shedding light on the role of the $s$-NME. We briefly discuss future applications of the SMM to other forbidden $β$-decay spectra sensitive to $g_{\rm A}$.

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arXiv2022

The first large-scale shell-model calculation of the two-neutrino double beta decay of $^{76}$Ge to the excited states in $^{76}$Se

Large-scale shell-model calculations were carried out for the half-lives and branching ratios of the $2νββ$ decay of $^{76}$Ge to the ground state and the lowest three excited states $2_1^+$, $0_2^+$ and $2_2^+$ in $^{76}$Se. In total, the wave functions of more than 10,000 intermediate $1^+$ states in $^{76}$As were calculated in a three-step procedure allowing an efficient use of the available computer resources. In the first step, 250 lowest states, below some 5 MeV of excitation energy, were calculated without truncations within a full major shell $0f_{5/2}-1p-0g_{9/2}$ for both protons and neutrons. The wave functions of the rest of the states, up to some 30 MeV, were computed in two more steps by introducing two consecutive stages of truncation. The computed magnitudes of the $2νββ$ nuclear matrix elements (including the value of the axial-vector coupling $g_{\rm A}$), $\vert M_{2ν}\vert g_{\rm A}^2$, converged to the values 0.168$g_{\rm A}^2$, $1.2\times10^{-3}$$g_{\rm A}^2$, 0.121$g_{\rm A}^2$, and $3.1\times10^{-3}$$g_{\rm A}^2$ for the $0^+_{\rm g.s.}$, $2^+_1$, $0^+_2$, and $2^+_2$ states, respectively. Using up-to-date phase-space integrals, the corresponding branching ratios were derived to be 99.926\%, 4.4$\times10^{-5}$\%, 0.074\% and 2.5$\times10^{-7}$\%. The experimental half-life $(1.926\pm0.094)\times10^{21}$ yr of the ground-state transition was used to derive the value $g_{\rm A}=0.80\pm0.01$ for the axial-vector coupling, which is consistent with other shell-model calculations suggesting a quenched value of $g_{\rm A}$. Using this value of $g_{\rm A}$, predictions for the transition half-lives were derived.

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arXiv2022

Microscopic calculation of the $β^-$ decays of $^{151}$Sm, $^{171}$Tm, and $^{210}$Pb with implications to detection of the cosmic neutrino background

The electron spectral shapes corresponding to the low-$Q$ $β^-$-decay transitions $^{151}$Sm$(5/2^-_{\rm g.s.})\to\,^{151}\textrm{Eu}(5/2^+_{\rm g.s.})$, $^{151}$Sm$(5/2^-_{\rm g.s.})\to\,^{151}\textrm{Eu}(7/2^+_{1})$, $^{171}$Tm$(1/2^+_{\rm g.s.})\to\,^{171}\textrm{Yb}(1/2^-_{\rm g.s.})$, $^{171}$Tm$(1/2^+_{\rm g.s.})\to\,^{171}\textrm{Yb}(3/2^-_{1})$, $^{210}\textrm{Pb}(0^+_{\rm g.s.})\to\,^{210}\textrm{Bi}(1^-_{\rm g.s.})$, and $^{210}\textrm{Pb}(0^+_{\rm g.s.})\to\,^{210}\textrm{Bi}(0^-_{1})$ have been computed using beta-decay theory with several refinements for these first-forbidden nonunique (ff-nu) $β^-$ transitions. These ff-nu $β^-$ transitions have non-trivial electron spectral shapes with transition nuclear matrix elements (NMEs) computed by using the microscopic Interacting Boson-Fermion Model (IBFM-2) for the decays of $^{151}$Sm and $^{171}$Tm, and the nuclear shell model (NSM) for the decay of $^{210}$Pb. Within the respective $Q$ windows, the computed ff-nu electron spectral shapes deviate maximally at sub-percent level from the universal allowed shape, except for the transition $^{210}\textrm{Pb}(0^+_{\rm g.s.})\to\,^{210}\textrm{Bi}(1^-_{\rm g.s.})$, where the maximal deviation is some 2.7$\%$. This confirms that the so-called $ξ$ approximation is fairly good for most of these low-$Q$ $β^-$ transitions and thus the allowed shape is a rather good first approximation. Our computed spectral shapes could be of interest for experiments aiming to measure the cosmic neutrino background (C$ν$B), like the PTOLEMY experiment. We have also derived C$ν$B cross sections for the ground-state transitions of the considered nuclei at the $β$ endpoint. Our findings indicate that more work on the atomic mismatch correction is needed in the future in order to extract reliable and precise C$ν$B cross sections for any nuclear target.

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arXiv2021

Confirmation of $g_{\rm A}$ quenching using the revised spectrum-shape method for the analysis of the $^{113}$Cd $β$-decay as measured with the COBRA demonstrator

In this article we present an updated spectrum-shape analysis of the $^{113}$Cd fourfold forbidden non-unique $β$-decay transition in order to address the quenching of the weak axial-vector coupling $g_{\rm A}$ in low-momentum exchange nuclear processes. The experimental data were collected in a dedicated low-threshold run with the COBRA demonstrator at the LNGS and resulted in 44 individual $^{113}$Cd spectra. These data are evaluated in the context of three nuclear model frameworks based on a revised version of the spectrum-shape method and the conserved vector current hypothesis. The novel idea devised in the present work is to fit the value of the small relativistic nuclear matrix element (s-NME) driving the nuclear model calculations, which remained essentially as a free parameter in previous studies. This is done by tuning the nuclear structure calculations and making use of the interplay of $g_{\rm A}$ and the s-NME such that the experimentally known $^{113}$Cd half-life gets reproducible by the different frameworks. In this way, a best fit s-NME value can be derived for each of the considered nuclear models, which finally enters the template calculations used to perform the spectrum-shape analysis for each of the obtained $^{113}$Cd spectra. The primary analysis strategy results in significantly quenched values of the axial-vector coupling for all three nuclear models: $\overline{g}_{\rm A}(\text{ISM}) = 0.907 \pm 0.064$, $\overline{g}_{\rm A}(\text{MQPM}) = 0.993 \pm 0.063$ and $\overline{g}_{\rm A}(\text{IBFM-2}) = 0.828 \pm 0.140$. Moreover, with our data-driven approach one of the main shortcomings of the spectrum-shape method has been resolved. This achievement is a milestone in the description of strongly forbidden $β$-decays and adds to the indications for the existence of a quenching of $g_{\rm A}$ in low-momentum exchange nuclear processes.

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arXiv2021

Second-forbidden nonunique $β^-$ decays of $^{59,60}$Fe: Possible candidates for $g_A$ sensitive electron spectral-shape measurements

In this work, we present a theoretical study of the electron spectral shapes for the second-forbidden nonunique $β^-$-decay transitions $^{59}\textrm{Fe}(3/2^-)\to\,^{59}\textrm{Co}(7/2^-)$ and $^{60}\textrm{Fe}(0^+)\to\,^{60}\textrm{Co}(2^+)$ in the framework of the nuclear shell model. We have computed the involved wave functions by carrying out a complete $0\hbarω$ calculation in the full $fp$ model space using the KB3G and GXPF1A effective interactions. When compared with the available data, these interactions predict the low-energy spectra and electromagnetic properties of the involved nuclei quite successfully. This success paves the way for the computations of the $β$-decay properties, and comparison with the available data. We have computed the electron spectral shapes of the mentioned decay transitions as functions of the value of the weak axial coupling $g_{\rm A}$. By comparing these computed shapes with the measured spectral shapes allows then to extract the effective value of $g_{\rm A}$ for these decay transitions. This procedure, coined the spectrum-shape method (SSM) in several earlier studies, complements the method of determining the value of $g_{\rm A}$ by reproducing the (partial) half-lives of decay transitions. Here we have enhanced the original SSM by constraining the value of the relativistic vector matrix element, $^V\mathcal{M}^{(0)}_{KK-11}$, using the conserved vector-current hypothesis (CVC) as a starting point. We hope that this finding would be a strong incentive to measure the spectral shapes in the future.

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arXiv2020

Improved calculations of beta decay backgrounds to new physics in liquid xenon detectors

We present high-precision theoretical predictions for the electron energy spectra for the ground-state to ground-state $β$ decays of $^{214}$Pb, $^{212}$Pb, and $^{85}$Kr most relevant to the background of liquid xenon dark matter detectors. The effects of nuclear structure on the spectral shapes are taken into account using large-scale shell model calculations. Final spectra also include atomic screening and exchange effects. The impact of nuclear structure effects on the $^{214}$Pb and $^{212}$Pb spectra below $\approx100$ keV, pertinent for several searches for new physics, are found to be comparatively larger than those from the atomic effects alone. We find that the full calculation for $^{214}$Pb ($^{212}$Pb) predicts 15.0-23.2% (12.1-19.0%) less event rate in a 1-15 keV energy region of interest compared to the spectrum calculated as an allowed transition when using values of the weak axial vector coupling in the range $g_{\rm A}=0.7-1.0$. The discrepancy highlights the importance of both a proper theoretical treatment and the need for direct measurements of these spectra for a thorough understanding of $β$ decay backgrounds in future experiments.

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