Session Q04: Nuclear Data for Neutrinoless Double Beta Decay

Live

The rate of neutrinoless double-beta decay ($0\nu\beta\beta$) depends on three components: the phase-space factor for the emission of two electrons, the effective Majorana mass of the electron neutrino, and the nuclear matrix element (NME). The NMEs cannot be measured experimentally and must be calculated. Various nuclear structure models have been used for this purpose including the shell model, interacting boson model, quasiparticle random phase approximation, and energy density functional theory, the results of which differ by factors of 2-3 for individual nuclei. Increasing the accuracy of and reducing the uncertainty in the NMEs is considered crucial for extracting the neutrino mass if the half-life of $0\nu\beta\beta$ is measured. To test these calculations and constrain the models, theoretical results are compared with nuclear structure data for reproducibility. Data of importance include level energies, spins and parities, branching ratios, mixing ratios, and transition probabilities. While the structure of the double-beta decay parent and daughter are paramount, many of these nuclei lie in complex regions of nuclear structure and it is necessary to understand the properties of other nuclei in the isotopic chain as well in order to fully characterize the structures involved. Unfortunately, systematic data are not always available to test the models. While improvements in our knowledge of the structure of many $0\nu\beta\beta$ candidates are ongoing, the Ge nuclei have been the focus of numerous recent studies. Various experimental techniques including Coulomb excitation, photon scattering, inelastic neutron scattering, beta decay, and others have been employed to study $^{76}$Ge (the $0\nu\beta\beta$ candidate) and other nuclei in the isotopic chain. In this presentation, I will describe recent experimental advances in this region as well as the results of model calculations for comparison.   

Live

Precise determinations of the energy distribution of recoil ions emerging from nuclear $\beta $-decay enables the determination of branching fractions to various excited states and angular correlations between the decay products. In this talk I will review ongoing and planned experiments in this field, and expand on two opportunities to make a significant impact with recoil measurements. The first is to search for, or exclude, new tensor interactions coupled to right-handed neutrinos. The second is to extract the Vud CKM matrix element from the Ft values of mirror and Fermi transitions. For measurements in isotopes decaying to a stable or long-lived daughter, it is necessary to deduce the energy distributions from the kinematics of the recoil daughter nuclei, requiring the use of ion or atom traps. A major systematic uncertainty in such experiments is caused by discrepancies in the determination of the trap position and size. I will show how a new imaging technique, the nuclear microscope [1], maximizes the sensitivity of a kinematic measurement to the underlying energy distributions, and reduces the systematic uncertainty contributions. This technique was demonstrated by measuring branching ratios and recoil-ion energy distributions for ionization processes in optical collisions of cold metastable neon [2]. [1] Physical Review C 101.3 (2020): 035501 [2] Physical Review Letters 123.6 (2019): 063401   

Live

In the search for physics beyond the standard model, the unitarity test of the Cabibbo-Kobayashi-Maskawa (CKM) matrix is one of the most demanding. Superallowed $\beta $ transitions between $J^{\pi }=0^{+}$, $T=1$ analog states currently provide the most precise value for $V_{\mathrm{ud}}$, the up-down quark mixing element of the CKM-matrix. Since no axial current can contribute in first order to these transitions, they give a direct access to the vector coupling constant $G_{\mathrm{V}}$ of the weak interaction. The current value of $V_{\mathrm{ud}}$ is $\pm 0.03\% $ accurate [1] and is obtained from fifteen $ft$-values for superallowed $\beta $ decays, all measured with a precision of $0.3\% $ or better. There are four small theoretical corrections (all of the order of $1\% )$ required in the $V_{\mathrm{ud}}$ extraction. The current result's error is dominated by these theoretical corrections. From the experimentalist's perspective, precision can be further improved by testing the reliability of the predicted corrections. A powerful experimental test comes from measurements of mirror pair superallowed transitions [1,2] In these transitions the predicted corrections are relatively large and the ratio of their $ft$-values is very sensitive to the model calculation of the isospin-symmetry-breaking corrections $\delta_{NS} $ and $\delta_{C} $. The talk will focus on the experimental effort required, exemplifying with the mirror pair of superallowed $0^{+}\to 0^{+}\beta $ transitions ${ }^{34}\mbox{Ar}\to { }^{34}\mbox{Cl}$ and ${ }^{34}\mbox{Cl}\to { }^{34}\mbox{S.}$ [1] J. J. C. Hardy and I. S. Towner, Phys. Rev. C \textbf{102}, 045501 (2020) [2] V.E. Iacob \textit{et al.} Phys. Rev. C \textbf{101}, 045501 (2020)