arXiv subjects

Lotta Jokiniemi

Publications and source records attributed to Lotta Jokiniemi.

9 recordsLinked to original sources

arXiv2026

Future directions in nuclear $β$ decay at FRIB and beyond

Motivated by the opportunities presented for studies relevant to nuclear structure, astrophysics, and fundamental symmetries with nuclear $β$ decay, the Facility for Rare Isotope Beams (FRIB) Theory Alliance topical program ``Future Directions in Nuclear $β$ Decays at FRIB'' was held in September of 2025. This white paper summarizes the main points of discussion over the two-week program, and it aims to provide a snapshot of the current status of the field while also highlighting important questions and opportunities for future work. We provide an overview of the experimental tools and techniques that enable modern $β$ decay studies, discuss the current state of nuclear many-body approaches used to study $β$ decays, and highlight the important science questions that can be addressed by weak decays.

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arXiv2026

Ab initio calculations of two-neutrino and neutrinoless double-$\boldsymbolβ$ decay of $^{48}$Ca and related Gamow-Teller strength distributions

We present ab initio calculations of two-neutrino double-beta ($2νββ$) decay of $^{48}$Ca and the related Gamow-Teller (GT) strength functions in $^{48}$Sc using the valence-space in-medium similarity renormalization group (VS-IMSRG) with nuclear interactions and electroweak currents based on chiral effective field theory. We find that the usual $pf$-shell valence space significantly underestimates the nuclear matrix element (NME) of $2νββ$ decay compared to experiment, while an enlarged $d_{3/2}pf$ valence space yields very good agreement with the experimental value without any adjustments. We trace this to an improved description of the involved GT strength distributions, so that the enlarged valence space captures important correlations. The enlarged $d_{3/2}pf$ valence space leads to neutrinoless $ββ$ NMEs of $^{48}$Ca that are twice as large compared to the $pf$-shell calculation. Our findings suggest that studies with different valence spaces and related GT strengths are important for assessing ab initio NME calculations of heavier $ββ$ decays.

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arXiv2025

Neutrinoless $ββ$ decay nuclear matrix elements complete up to N$^2$LO in heavy nuclei

We evaluate all nuclear matrix elements (NMEs) up to next-to-next-to leading order (N$^2$LO) in chiral effective field theory ($χ$EFT) for the neutrinoless double-beta ($0νββ$) decay of the nuclei most relevant for experiments, including $^{76}$Ge, $^{100}$Mo, and $^{136}$Xe. We use the proton-neutron quasiparticle random-phase approximation (pnQRPA) and the nuclear shell model to calculate the N$^2$LO NMEs from very low-momentum (ultrasoft) neutrinos and from loop diagrams usually neglected in $0νββ$ studies. Our results indicate that the overall N$^2$LO contribution is centered around $-(5$-$10)\%$ for the shell model and $+(10$-$15)\%$ for the pnQRPA, with sizable uncertainties due to the scale dependence of the ultrasoft NMEs and the short-range nature of the loop NMEs. The sign discrepancy between many-body methods is common to all studied nuclei and points to the different behaviour of the intermediate states of the $0νββ$ decay. Within uncertainties, our results for the ultrasoft NME are of similar size as contributions usually referred to as ``beyond the closure approximation''.

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arXiv2024

Muon Capture on $^{6}$Li, $^{12}$C, and $^{16}$O from $Ab~Initio$ Nuclear Theory

Muon capture on nuclei is one of the most promising probes of the nuclear electroweak current driving the yet-hypothetical neutrinoless double-beta ($0νββ$) decay. Both processes involve vector and axial-vector currents at finite momentum transfer, $q\sim 100$ MeV, as well as the induced pseudoscalar and weak-magnetism currents. Comparing measured muon-capture rates with reliable $ab~initio$ nuclear-theory predictions could help us validate these currents. To this end, we compute partial muon-capture rates for $^{6}$Li, $^{12}$C and $^{16}$O, feeding the ground and excited states in $^{6}$He, $^{12}$B and $^{16}$N, using $ab~initio$ no-core shell model with two- and three-nucleon chiral interactions. We remove the spurious center-of-mass motion by introducing translationally invariant operators and approximate the effect of hadronic two-body currents by Fermi-gas model. We solve the bound-muon wave function from the Dirac wave equations in the Coulomb field created by a finite nucleus. We find that the computed rates to the low-lying states in the final nuclei are in good agreement with the measured counterparts. We highlight sensitivity of some of the transitions to the sub-leading three-nucleon interaction terms. We also compare summed rates to several tens of final states with the measured total capture rates and note that we slightly underestimate the total rate with this simple approach due to limited range of excitation energies.

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arXiv2023

Correlations Between Neutrinoless Double-Beta, Double Gamow-Teller and Double-Magnetic Decays in the pnQRPA Framework

We explore the relation between the nuclear matrix elements of neutrinoless double-beta ($0νββ$) decay and two other processes: double Gamow-Teller (DGT) and double-magnetic dipole (M1M1) transitions, with focus on medium-mass to heavy nuclei studied with the proton-neutron quasiparticle random-phase approximation (pnQRPA) framework. We explore a wide span of isoscalar proton-neutron pairing strengths covering the typical range of values that describe well $β$- and two-neutrino $ββ$-decay data. Our results indicate good linear correlations between $0νββ$ and both DGT and M1M1 matrix elements. Together with future measurements of DGT and M1M1 transitions, these correlations could help constrain the values of the $0νββ$-decay nuclear matrix elements.

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arXiv2023

Neutrinoless $ββ$-decay nuclear matrix elements from two-neutrino $ββ$-decay data

We study two-neutrino ($2νββ$) and neutrinoless double-$β$ ($0νββ$) decays in the nuclear shell model and proton-neutron quasiparticle random-phase approximation (pnQRPA) frameworks. Calculating the decay half-life of several dozens of nuclei ranging from calcium to xenon with the shell model, and of $ββ$ emitters with a wide range of proton-neutron pairing strengths in the pnQRPA, we observe good linear correlations between $2νββ$- and $0νββ$-decay nuclear matrix elements for both methods. We then combine the correlations with measured $2νββ$-decay half-lives to predict $0νββ$-decay matrix elements with theoretical uncertainties based on our systematic calculations. Our results include two-body currents and the short-range $0νββ$-decay operator.

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arXiv2023

Two-neutrino $ββ$ decay of $^{136}$Xe to the first excited $0^+$ state in $^{136}$Ba

We calculate the nuclear matrix element for the two-neutrino $ββ$ decay of $^{136}$Xe into the first excited $0^+$ state of $^{136}$Ba. We use different many-body methods: the quasiparticle random-phase approximation (QRPA) framework, the nuclear shell model, the interacting boson model (IBM-2), and an effective field theory (EFT) for $β$ and $ββ$ decays. While the QRPA suggests a decay rate at the edge of current experimental limits, the shell model points to a half-life about two orders of magnitude longer. The predictions of the IBM-2 and the EFT lie in between, and the latter provides systematic uncertainties at leading order. An analysis of the running sum of the nuclear matrix element indicates that subtle cancellations between the contributions of intermediate states can explain the different theoretical predictions. For the EFT, we also present results for two-neutrino $ββ$ decays to the first excited $0^+$ state in other nuclei.

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arXiv2021

Impact of the leading-order short-range nuclear matrix element on the neutrinoless double-beta decay of medium-mass and heavy nuclei

We evaluate the leading-order short-range nuclear matrix element for the neutrinoless double-beta ($0νββ$) decay of the nuclei most relevant for experiments, including $^{76}$Ge, $^{100}$Mo, $^{130}$Te and $^{136}$Xe. In our calculations, performed with the nuclear shell model and proton-neutron quasiparticle random-phase approximation (pnQRPA) methods, we estimate the coupling of this term by the contact charge-independence-breaking coupling of various nuclear Hamiltonians. Our results suggest a significant impact of the short-range matrix element, which is about $15\%-50\%$ and $30\%-80\%$ of the standard $0νββ$-decay long-range matrix element for the shell model and pnQRPA, respectively. Combining the full matrix elements with the results from current $0νββ$-decay experiments we find that, if both matrix elements carry the same sign, these searches move notably toward probing the inverted mass ordering of neutrino masses.

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arXiv2016

Magnetic hexadecapole gamma transitions and neutrino-nuclear responses in medium heavy nuclei

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, $M_{\rm EXP}$(M4), deduced from observed M4 $γ$ transition half-lives are compared with the single-quasiparticle (QP) NMEs, $M_{\rm QP}$(M4), and the microscopic quasiparticle-phonon model (MQPM) NMEs $M_{\rm MQPM}$(M4). The experimentally driven M4 NMEs are found to be reduced by a coefficient $k \approx 0.29 $ with respect to $M_{\rm QP}$(M4) and by $k \approx 0.33$ with respect to $M_{\rm MQPM}$(M4). The M4 NMEs are reduced a little by the quasiparticle-phonon correlations of the MQPM wave functions but mainly by other nucleonic and non-nucleonic 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 $γ$ transitions and Gamow-Teller (GT) and spin-dipole (SD) $β$ transitions. The impact of the found reduction coefficients on the magnitudes of the NMEs involved in astroneutrino interactions and neutrinoless double beta decays are discussed.

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