New determination of double-β-decay properties in48Ca: High-precisionQββ-value measurement and improved nuclear matrix element cal

Abstract

We report a direct measurement of the Q ββ -value of the neutrinoless double-β-decay candidate 48 Ca at the TITAN Penning-trap mass spectrometer, with the result that Q ββ = 4267.98(32) keV. We measured the masses of both the mother and daughter nuclides, and in the latter case found a 1 keV deviation from the literature value. In addition to the Q ββ -value, we also present results of a new calculation of the neutrinoless double-β-decay nuclear matrix element of 48 Ca. Using diagrammatic many-body perturbation theory to second order to account for physics outside the valence space, we constructed an effective shell-model double-β-decay operator, which increased the nuclear matrix element by about 75% compared with that produced by the bare operator. The new Q ββ -value and matrix element strengthen the case for a 48 Ca double-β-decay experiment.The discovery of neutrino oscillations represents the first evidence for new physics beyond the Standard Model [1,2]. The oscillations conclusively demonstrate that neutrinos have mass, that flavor eigenstates are mixtures of mass eigenstates, and that neutrino physics is more complicated than we had thought. The observation of neutrinoless double-β (0νββ) decay, extremely rare if it exists, would at once fill multiple gaps in our understanding of the neutrino's nature and would represent a major breakthrough for particle physics. Since this lepton-number-violating process can occur only if the neutrino is its own antiparticle, its discovery would unambiguously confirm the neutrino as a Majorana particle, while a measured lifetime would provide a value for the neutrino mass scale [3]. In order to extract that value from the 0νββ-decay half-life, however, two quantities must be accurately determined: a phase-space factor, which depends on the Q ββ -value of the decay, and a nuclear matrix element, which is not observable and therefore must be obtained from nuclear structure theory.The twelve nuclides that have been observed to undergo two-neutrino double-β (2νββ) decay [4,5] are the basis for a number of large-scale experimental 0νββdecay searches currently underway. Of these nuclides, 48 Ca possesses the largest Q ββ -value of 4.3 MeV [6], giv-ing it several distinct experimental advantages. Because the Q ββ -value lies well above the energy of naturally occurring background, a good signal-to-noise ratio is ensured, while the large phase-space factor enhances the 0νββ-decay rate. The low isotopic abundance of 48 Ca, however, requires enrichment. 48 Ca is currently being measured at NEMO-III [7] and studied at CANDLES [8] and CARVEL [9]. The Q ββ -value provides vital input for the simulation of signal and background, the analysis of current data, and the design of future detectors. In order for the uncertainty of the Q ββ -value to be negligible in these studies, the required precision has to be better than the intrinsic resolution of the detector.The deep implications of massive neutrinos have led to a concentrated effort to calculate the nuclear matrix element for 0νββ-...

Isotope dependence of the Zeeman effect in lithium-like calcium

“…The resulting atomic mass agrees within its uncertainty with the previous less accurate measurements3233.…”

Comparison between variational Monte Carlo and shell model calculations of neutrinoless double beta decay matrix elements in light nuclei

“…Consequently, different theoretical models can give systematically different results. For example, the nuclear matrix elements of 48 Ca have been calculated using the shell model [15][16][17][18], energy density functionals [19], the quasiparticle random-phase approximation (QRPA) [20], and the interacting boson model (IBM) [21], leading to results that differ by a factor of two or three. Similar variations are observed for other 0νββ candidates [22].…”

Shell model predictions forSn124double-βdecay

“…Table V presents our new 124 Sn light neutrinoexchange 0νββ decay NME results with two SRC parametrizations (Argonne-V18 and CD-Bonn), together with the NME of five other nuclei from our group (ISM-CMU), 48 [40,48], compared with the most recent results that were obtained with nuclear structure methods that preserve the isospin symmetry and provide NME for both mechanisms. Included in Figure 6 (but not in Table V) are also shell model light neutrino-exchange NME for 48 Ca [87], 76 Ge, and 82 Se [88] that are using the same shell model calculations but an effective transition operator obtained in many-body perturbation theory. The updated IBM-2 NME [57] are only available for the Argonne-V18 SRC.…”