The novel Branching-Ratio Method (BRM) for the determination of effective value <math altimg='si1.svg'><msubsup><mrow><mi>g</mi></mrow><mrow><mi mathvariant='normal'>A</mi></mrow><mrow><mi mathvariant='normal'>eff</mi></mrow></msubsup></math> of the weak axial coupling <math altimg='si2.svg'><msub><mrow><mi>g</mi></mrow><mrow><mi mathvariant='normal'>A</mi></mrow></msub></math> for forbidden non-unique (FNU) <math altimg='si3.svg'><msup><mrow><mi>&#946;</mi></mrow><mrow><mo linebreak='badbreak' linebreakstyle='after'>+</mo></mrow></msup></math>/electron-capture(EC) decays is introduced. The method offers new possibilities for testing the fitness of nuclear Hamiltonians in modeling the physics of complex <math altimg='si3.svg'><msup><mrow><mi>&#946;</mi></mrow><mrow><mo linebreak='badbreak' linebreakstyle='after'>+</mo></mrow></msup></math>/EC decays. The constraint of simultaneously reproducing the branching to <math altimg='si3.svg'><msup><mrow><mi>&#946;</mi></mrow><mrow><mo linebreak='badbreak' linebreakstyle='after'>+</mo></mrow></msup></math> and EC transitions offers an additional constraint in tackling the problem of <math altimg='si1.svg'><msubsup><mrow><mi>g</mi></mrow><mrow><mi mathvariant='normal'>A</mi></mrow><mrow><mi mathvariant='normal'>eff</mi></mrow></msubsup></math> determination. In the BRM the ambiguity in the choice of the values of <math altimg='si1.svg'><msubsup><mrow><mi>g</mi></mrow><mrow><mi mathvariant='normal'>A</mi></mrow><mrow><mi mathvariant='normal'>eff</mi></mrow></msubsup></math> and s-NME (small relativistic vector nuclear matrix element) is lifted when constraints of the branching ratios of <math altimg='si3.svg'><msup><mrow><mi>&#946;</mi></mrow><mrow><mo linebreak='badbreak' linebreakstyle='after'>+</mo></mrow></msup></math> and EC decays are applied. As an example, in the present work we apply BRM to the case of second FNU <math altimg='si3.svg'><msup><mrow><mi>&#946;</mi></mrow><mrow><mo linebreak='badbreak' linebreakstyle='after'>+</mo></mrow></msup></math>/EC decay of 59Ni. 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.

Finland

Aagrah Agnihotri

Romania

Jouni Suhonen

Novel way of evaluating <italic>g</italic> <inf><italic>A</italic></inf> quenching in <italic>&#946;</italic> +/EC decays: Introducing the Branching-Ratio Method (BRM)

The proton-neutron quasiparticle random-phase approximation (QRPA) has been widely used to describe various nuclear processes, in particular the ordinary muon capture (OMC). In the present work, we apply for the first time the particle-number projected QRPA (PQRPA) to calculate the OMC on the <math altimg='si1.svg'><msup><mrow><mn>0</mn></mrow><mrow><mo linebreak='badbreak' linebreakstyle='after'>+</mo></mrow></msup></math> ground state of 136Ba and include a comparison with the results obtained using the standard QRPA, both in the proton-neutron channel. The nucleus 136Ba is the daughter nucleus in the neutrinoless double beta ( <math altimg='si2.svg'><mn>0</mn><mi>&#957;</mi><mi>&#946;</mi><mi>&#946;</mi></math>) decay <math altimg='si3.svg'><mmultiscripts><mrow><mtext mathvariant='normal'>Xe</mtext></mrow><mprescripts /><none /><mrow><mn>136</mn></mrow></mmultiscripts><mo stretchy='false'>(</mo><msup><mrow><mn>0</mn></mrow><mrow><mo linebreak='badbreak' linebreakstyle='after'>+</mo></mrow></msup><mo stretchy='false'>)</mo><mo stretchy='false'>&#8594;</mo><mspace width='0.2em' /><mmultiscripts><mrow><mtext mathvariant='normal'>Ba</mtext></mrow><mprescripts /><none /><mrow><mn>136</mn></mrow></mmultiscripts><mo stretchy='false'>(</mo><msup><mrow><mn>0</mn></mrow><mrow><mo linebreak='badbreak' linebreakstyle='after'>+</mo></mrow></msup><mo stretchy='false'>)</mo></math>, an important transition for searches of Majorana neutrinos in the present and future large-scale <math altimg='si2.svg'><mn>0</mn><mi>&#957;</mi><mi>&#946;</mi><mi>&#946;</mi></math>-decay experiments. Both the <math altimg='si2.svg'><mn>0</mn><mi>&#957;</mi><mi>&#946;</mi><mi>&#946;</mi></math> decay and OMC populate the ground and excited states of the <math altimg='si2.svg'><mn>0</mn><mi>&#957;</mi><mi>&#946;</mi><mi>&#946;</mi></math>-decay intermediate nucleus 136Cs in the same momentum-exchange region of <math altimg='si4.svg'><mi>q</mi><mo>&#8764;</mo><mn>100</mn></math> MeV, thus making the OMC a perfect probe of the <math altimg='si2.svg'><mn>0</mn><mi>&#957;</mi><mi>&#946;</mi><mi>&#946;</mi></math> decay. We compute OMC rates to nuclear states below 1 MeV of excitation energy in 136Cs, a region accessible to present-day OMC experiments, like MONUMENT. We use a Bonn G-matrix based effective interaction and a large no-core single-particle basis, also adding the chiral two-body meson-exchange currents (2BC) and using exact Dirac wave function for the captured <italic>s</italic>-orbital muon. It turns out that the OMC rates computed in the PQRPA framework are notably larger than those computed using QRPA for few very low-lying <math altimg='si5.svg'><msup><mrow><mn>3</mn></mrow><mrow><mo linebreak='badbreak' linebreakstyle='after'>+</mo></mrow></msup></math> states offering a strong incentive for future experimental studies of the OMC on 136Ba.

Argentina

Osvaldo Civitarese

Jenni Kotila

Application of the particle-number projected proton-neutron QRPA to the muon capture on 136Ba

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.

Zhuang Ge

Tommi Eronen

Vasile Alin Sevestrean

Ovidiu Niţescu

Sabin Stoica

Marlom Ramalho

Belgium

France

Antoine de Roubin

Dmitrii Nesterenko

Anu Kankainen

Pauline Ascher

Spain

Samuel Ayet San Andres

Olga Beliuskina

Pierre Delahaye

Mathieu Flayol

Mathias Gerbaux

Stéphane Grévy

Marjut Hukkanen

Arthur Jaries

Ari Jokinen

Audric Husson

Daid Kahl

Joel Kostensalo

United States

Iain Moore

Stylianos Nikas

Marek Stryjczyk

Ville Virtanen

High-precision measurements of the atomic mass and electron-capture decay Value of Tc

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 <math><msub><mrow><mi>g</mi></mrow><mrow><mi>A</mi></mrow></msub></math> 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 <math><msub><mrow><mi>g</mi></mrow><mrow><mi>A</mi></mrow></msub></math> 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: <math><msub><mrow><mover><mrow><mi>g</mi></mrow><mo>‾</mo></mover></mrow><mrow><mi>A</mi></mrow></msub><mo>(</mo><mtext>ISM</mtext><mo>)</mo><mo>=</mo><mn>0.907</mn><mo>±</mo><mn>0.064</mn></math>, <math><msub><mrow><mover><mrow><mi>g</mi></mrow><mo>‾</mo></mover></mrow><mrow><mi>A</mi></mrow></msub><mo>(</mo><mtext>MQPM</mtext><mo>)</mo><mo>=</mo><mn>0.993</mn><mo>±</mo><mn>0.063</mn></math> and <math><msub><mrow><mover><mrow><mi>g</mi></mrow><mo>‾</mo></mover></mrow><mrow><mi>A</mi></mrow></msub><mo>(</mo><mtext>IBFM-2</mtext><mo>)</mo><mo>=</mo><mn>0.828</mn><mo>±</mo><mn>0.140</mn></math>. 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 <math><msub><mrow><mi>g</mi></mrow><mrow><mi>A</mi></mrow></msub></math> in low-momentum exchange nuclear processes.

Germany

Juliane Volkmer

Stefan Zatschler

Kai Zuber

Confirmation of Quenching Using the Revised Spectrum-shape Method for the Analysis of the Cd -decay as measured with the COBRA demonstrator

We investigate the prospects for real-time detection of solar neutrinos via the charged-current neutrino-nucleus scattering process in liquid xenon time projection chambers. We use a nuclear shell model, benchmarked with experimental data, to calculate the cross sections for populating specific excited states of the cesium nuclei produced by neutrino capture on <math><mrow><mmultiscripts><mrow><mi>Xe</mi></mrow><mprescripts></mprescripts><none></none><mrow><mn>131</mn></mrow></mmultiscripts></mrow></math> and <math><mrow><mmultiscripts><mrow><mi>Xe</mi></mrow><mprescripts></mprescripts><none></none><mrow><mn>136</mn></mrow></mmultiscripts></mrow></math>. The shell model is further used to compute the decay schemes of the low-lying <math><msup><mn>1</mn><mo>+</mo></msup></math> excited states of <math><mrow><mmultiscripts><mrow><mi>Cs</mi></mrow><mprescripts></mprescripts><none></none><mrow><mn>136</mn></mrow></mmultiscripts></mrow></math>, for which there is sparse experimental data. We explore the possibility of tagging the characteristic deexcitation <math><mi>γ</mi></math> rays/conversion electrons using two techniques: spatial separation of their energy deposits using event topology and their time separation using delayed coincidence. The efficiencies in each case are evaluated within a range of realistic detector parameters. We find that the topological signatures are likely to be dominated by radon backgrounds, but that a delayed-coincidence signature from long-lived states predicted in <math><mrow><mmultiscripts><mrow><mi>Cs</mi></mrow><mprescripts></mprescripts><none></none><mrow><mn>136</mn></mrow></mmultiscripts></mrow></math> may enable background-free detection of CNO neutrino interactions in next-generation experiments with smaller uncertainty than current measurements. We also estimate the sensitivity as a function of exposure for detecting the solar-temperature-induced line shift in <math><mrow><mmultiscripts><mrow><mi>Be</mi></mrow><mprescripts></mprescripts><none></none><mrow><mn>7</mn></mrow></mmultiscripts></mrow></math> neutrino emission, which may provide a new test of solar models.

Scott Haselschwardt

Brian Lenardo

Pekka Pirinen

Solar neutrino detection in liquid xenon detectors via charged-current scattering to excited states

Two-neutrino double-beta-decay matrix elements <math><msub><mrow><mi>M</mi></mrow><mrow><mn>2</mn><mi>ν</mi></mrow></msub></math> and single beta-decay branching ratios were calculated for $^{48}$Ca and $^{96}$Zr in the interacting nuclear shell model using large single-particle valence spaces with well-tested two-body Hamiltonians. For $^{48}$Ca the matrix element <math><msub><mrow><mi>M</mi></mrow><mrow><mn>2</mn><mi>ν</mi></mrow></msub><mo>=</mo><mn>0.0511</mn></math> is obtained, which is 5.5% smaller than the previously reported value of 0.0539. For $^{96}$Zr this work reports the first large-scale shell-model calculation of the nuclear matrix element, yielding a value <math><msub><mrow><mi>M</mi></mrow><mrow><mn>2</mn><mi>ν</mi></mrow></msub><mo>=</mo><mn>0.0747</mn></math> with extreme single-state dominance. These matrix elements, combined with the available ββ-decay half-life data, yield effective values of the weak axial coupling which in turn are used to produce in a consistent way the β-decay branching ratios of <math><mo>(</mo><mn>7.5</mn><mo>±</mo><mn>2.8</mn><mo>)</mo></math>% for $^{48}$Ca and <math><mo>(</mo><mn>18.4</mn><mo>±</mo><mn>0.09</mn><mo>)</mo></math>% for $^{96}$Zr. These are larger than obtained in previous studies, implying that the detection of the β-decay branches could be possible in dedicated experiments sometime in the (near) future.

Consistent large-scale shell-model analysis of the two-neutrino And Single Branchings in Ca and Zr

A dedicated study of the quenching of the weak axial-vector coupling strength <math><msub><mrow><mi>g</mi></mrow><mrow><mi>A</mi></mrow></msub></math> in nuclear processes has been performed by the COBRA collaboration. This investigation is driven by nuclear model calculations which show that the β-spectrum shape of the fourfold forbidden non-unique decay of $^{113}$Cd strongly depends on the effective value of <math><msub><mrow><mi>g</mi></mrow><mrow><mi>A</mi></mrow></msub></math>. Using an array of CdZnTe semiconductor detectors, 45 independent $^{113}$Cd spectra were obtained and interpreted in the context of three nuclear models. The resulting effective mean values are <math><msub><mrow><mover><mrow><mi>g</mi></mrow><mo>‾</mo></mover></mrow><mrow><mi>A</mi></mrow></msub><mo>(</mo><mtext>ISM</mtext><mo>)</mo><mo>=</mo><mn>0.915</mn><mo>±</mo><mn>0.007</mn></math>, <math><msub><mrow><mover><mrow><mi>g</mi></mrow><mo>‾</mo></mover></mrow><mrow><mi>A</mi></mrow></msub><mo>(</mo><mtext>MQPM</mtext><mo>)</mo><mo>=</mo><mn>0.911</mn><mo>±</mo><mn>0.013</mn></math> and <math><msub><mrow><mover><mrow><mi>g</mi></mrow><mo>‾</mo></mover></mrow><mrow><mi>A</mi></mrow></msub><mo>(</mo><mtext>IBFM-2</mtext><mo>)</mo><mo>=</mo><mn>0.955</mn><mo>±</mo><mn>0.022</mn></math>. These values agree well within the determined uncertainties and deviate significantly from the free value of <math><msub><mrow><mi>g</mi></mrow><mrow><mi>A</mi></mrow></msub></math>. This can be seen as a first step towards answering the long-standing question regarding quenching effects related to <math><msub><mrow><mi>g</mi></mrow><mrow><mi>A</mi></mrow></msub></math> in low-energy nuclear processes.

Lucas Bodenstein-Dresler

Yingjie Chu

Daniel Gehre

Claus Gößling

Arne Heimbold

Christian Herrmann

Czechia

Rastislav Hodak

Kevin Kröninger

Julia Küttler

Christian Nitsch

Thomas Quante

Ekaterina Rukhadze

Ivan Stekl

Jan Tebrügge

Robert Temminghoff

Quenching of Deduced From the -spectrum Shape of Cd measured with the COBRA experiment

The effects of the enhancement of the axial-charge matrix element <math><msub><mrow><mi>γ</mi></mrow><mrow><mn>5</mn></mrow></msub></math> were studied in medium heavy and heavy nuclei for first-forbidden <math><msup><mrow><mi>J</mi></mrow><mrow><mo>+</mo></mrow></msup><mo>↔</mo><msup><mrow><mi>J</mi></mrow><mrow><mo>−</mo></mrow></msup></math> decay transitions using the nuclear shell model. Noticeable dependence on the enhancement <math><msub><mrow><mi>ϵ</mi></mrow><mrow><mi>MEC</mi></mrow></msub></math> of the axial-charge matrix element, as well as on the value of the axial-vector coupling constant <math><msub><mrow><mi>g</mi></mrow><mrow><mi>A</mi></mrow></msub></math> was found in the spectral shapes of $^{93}$Y, $^{95}$Sr, and $^{97}$Y. The importance of the spectrum of $^{138}$Cs in the determination of <math><msub><mrow><mi>g</mi></mrow><mrow><mi>A</mi></mrow></msub></math> is discussed. Half-life analyses in the <math><mi>A</mi><mo>≈</mo><mn>95</mn></math> and <math><mi>A</mi><mo>≈</mo><mn>135</mn></math> regions were done, and consistent results <math><msub><mrow><mi>g</mi></mrow><mrow><mi>A</mi></mrow></msub><mo>≈</mo><mn>0.90</mn></math>, 0.75, and 0.65, corresponding to the three enhancement scenarios <math><msub><mrow><mi>ϵ</mi></mrow><mrow><mi>MEC</mi></mrow></msub><mo>=</mo><mn>1.4</mn></math>, 1.7, and 2.0, were obtained. Connection to the reactor-antineutrino anomaly is pointed out.

Mesonic enhancement of the weak axial charge and its effect on the half-lives and spectral shapes of first-forbidden ↔ Decays

A comprehensive analysis of the structure of the nuclear matrix elements (NMEs) of neutrinoless double beta-minus (<math id="M2"><mn>0</mn><mi>ν</mi><msup><mrow><mi>β</mi></mrow><mrow><mo>-</mo></mrow></msup><msup><mrow><mi>β</mi></mrow><mrow><mo>-</mo></mrow></msup></math>) decays to the <math id="M3"><mrow><msup><mrow><mn>0</mn></mrow><mrow><mo>+</mo></mrow></msup></mrow></math> ground and first excited states is performed in terms of the contributing multipole states in the intermediate nuclei of <math id="M4"><mn>0</mn><mi>ν</mi><msup><mrow><mi>β</mi></mrow><mrow><mo>-</mo></mrow></msup><msup><mrow><mi>β</mi></mrow><mrow><mo>-</mo></mrow></msup></math> transitions. We concentrate on the transitions mediated by the light (l-NMEs) Majorana neutrinos. As nuclear model we use the proton-neutron quasiparticle random-phase approximation (pnQRPA) with a realistic two-nucleon interaction based on the Bonn one-boson-exchange <math id="M5"><mrow><mi>G</mi></mrow></math> matrix. In the computations we include the appropriate short-range correlations, nucleon form factors, and higher-order nucleonic weak currents and restore the isospin symmetry by the isoscalar-isovector decomposition of the particle-particle proton-neutron interaction parameter <math id="M6"><mrow><msub><mrow><mi>g</mi></mrow><mrow><mi>p</mi><mi>p</mi></mrow></msub></mrow></math>.

Juhani Hyvärinen

Analysis of the Intermediate-State Contributions to Neutrinoless Double β− Decays

Neutrino-nuclear responses in the form of squares of nuclear matrix elements, NMEs, are crucial for studies of neutrino-induced processes in nuclei. In this work we investigate magnetic hexadecapole (M4) NMEs in medium-heavy nuclei. The experimentally derived NMEs, <math id="M2"><mrow><msub><mrow><mi>M</mi></mrow><mrow><mi>E</mi><mi>X</mi><mi>P</mi></mrow></msub></mrow></math>(M4), deduced from observed M4 <math id="M3"><mrow><mi>γ</mi></mrow></math> transition half-lives are compared with the single-quasiparticle (QP) NMEs, <math id="M4"><mrow><msub><mrow><mi>M</mi></mrow><mrow><mi>Q</mi><mi>P</mi></mrow></msub></mrow></math>(M4), and the microscopic quasiparticle-phonon model (MQPM) NMEs <math id="M5"><mrow><msub><mrow><mi>M</mi></mrow><mrow><mi>M</mi><mi>Q</mi><mi>P</mi><mi>M</mi></mrow></msub></mrow></math>(M4). The experimentally derived M4 NMEs are found to be reduced by a coefficient <math id="M6"><mi>k</mi><mo>≈</mo><mn>0.29</mn></math> with respect to <math id="M7"><mrow><msub><mrow><mi>M</mi></mrow><mrow><mi>Q</mi><mi>P</mi></mrow></msub></mrow></math>(M4) and by <math id="M8"><mi>k</mi><mo>≈</mo><mn>0.33</mn></math> with respect to <math id="M9"><mrow><msub><mrow><mi>M</mi></mrow><mrow><mi>M</mi><mi>Q</mi><mi>P</mi><mi>M</mi></mrow></msub></mrow></math>(M4). The M4 NMEs are reduced a little by the quasiparticle-phonon correlations of the MQPM wave functions but mainly by other nucleonic and nonnucleonic correlations which are not explicitly included in the MQPM. The found reduction rates are of the same order of magnitude as those for magnetic quadrupole <math id="M10"><mrow><mi>γ</mi></mrow></math> transitions and Gamow-Teller (GT) and spin-dipole (SD) <math id="M11"><mrow><mi>β</mi></mrow></math> transitions. The impacts of the found reduction coefficients on the magnitudes of the NMEs involved in astroneutrino interactions and neutrinoless double beta decays are discussed.

Lotta Jokiniemi

Japan

Hiroyasu Ejiri

Magnetic Hexadecapole Transitions and Neutrino-Nuclear Responses in Medium-Heavy Nuclei