Large-Scale Shell-Model Analysis of the NeutrinolessββDecay ofCa48

Abstract

We present the nuclear matrix element for the neutrinoless double-beta decay of ^{48}Ca based on large-scale shell-model calculations including two harmonic oscillator shells (sd and pf shells). The excitation spectra of ^{48}Ca and ^{48}Ti, and the two-neutrino double-beta decay of ^{48}Ca are reproduced in good agreement to the experimental data. We find that the neutrinoless double-beta decay nuclear matrix element is enhanced by about 30% compared to pf-shell calculations. This reduces the decay lifetime by almost a factor of 2. The matrix-element increase is mostly due to pairing correlations associated with cross-shell sd-pf excitations. We also investigate possible implications for heavier neutrinoless double-beta decay candidates.

Measurement of the double-beta decay half-life and search for the neutrinoless double-beta decay ofCa48with the NEMO-3 detector

“…This result improves on the previously published best limit of 7.2 × 10 20 yr [44] obtained with data from [45]. An upper limit on the coupling between the ν e and the Majoron of hg ee i < ð1.0-4.3Þ × 10 −4 is extracted from the half-life limit using the phase space factor from [43] and matrix elements from [11][12][13][14][15][16][17].…”

“…However 48 Ca has a low natural abundance of 0.187% and is difficult to enrich. Recent calculations also indicate a relative suppression of 0νββ NMEs for 48 Ca compared to other isotopes [10][11][12][13][14][15][16][17]. The 2νββ decay mode of 48 Ca was first discovered in the Hoover Dam TPC experiment employing approximately 14 g of isotope [18].…”

“…Using the axial-vector coupling constant g A ¼ 1.27, the phase space factor from [41] and NMEs calculated in the shell-model framework [11,12,17] gives hm ββ i < 12-24 eV, whereas extending the NME selection to include calculations in the QRPA [10,14,15], interacting boson model [16] and energy density functional [13] frameworks yields a wider range: hm ββ i < 6.0-26 eV.…”

“…Shell-model calculations are expected to be the most reliable for 48 Ca. Nevertheless recent predictions for the two-neutrino half-life span a range of ð2.0-4.5Þ × 10 19 yr [12,[21][22][23]. Other NME schemes are less well suited to this isotope, although there is a quasiparticle randomphase approximation (QRPA) prediction of 4.7 × 10 19 yr [24].…”

Neutrinoless double- β decay matrix elements in large shell-model spaces with the generator-coordinate method

“…For the decay of 48 Ca, as noted, we reproduce the exact shell model results nearly perfectly in both one and two shells. For the KB3G [16], or SDPFMU-DB [4] interactions. The term pf sdg denotes the two-shell interaction used here for A ≈ 80 nuclei.…”