The observation of neutrinoless double-beta decay (0νββ) would show that lepton number is violated, reveal that neu-trinos are Majorana particles, and provide information on neutrino mass. A discovery-capable experiment covering the inverted ordering region, with effective Majorana neutrino masses of 15 − 50 meV, will require a tonne-scale experiment with excellent energy resolution and extremely low backgrounds, at the level of ∼0.1 count /(FWHM·t·yr) in the region of the signal. The current generation ⁷⁶Ge experiments GERDA and the Majorana Demonstrator, utilizing high purity Germanium detectors with an intrinsic energy resolution of 0.12%, have achieved the lowest backgrounds by over an order of magnitude in the 0νββ signal region of all 0νββ experiments. Building on this success, the LEGEND collaboration has been formed to pursue a tonne-scale ⁷⁶Ge experiment. The collaboration aims to develop a phased 0νββ experimental program with discovery potential at a half-life approaching or at 10²⁸ years, using existing resources as appropriate to expedite physics results.

The Majorana Collaboration is operating an array of high purity Ge detectors to search for neutrinoless double-$\beta$ decay in ${}^{76}\mathrm{Ge}$. The Majorana Demonstrator comprises 44.1 kg of Ge detectors (29.7 kg enriched in ${}^{76}\mathrm{Ge}$) split between two modules contained in a low background shield at the Sanford Underground Research Facility in Lead, South Dakota. Here we present results from data taken during construction, commissioning, and the start of full operations. We achieve unprecedented energy resolution of 2.5 keV FWHM at $Q_{\beta \beta }$ and a very low background with no observed candidate events in 9.95 kg yr of enriched Ge exposure, resulting in a lower limit on the half-life of $1.9\times {10}^{25}\mathrm{yr}$ (90% C.L.). This result constrains the effective Majorana neutrino mass to below 240–520 meV, depending on the matrix elements used. In our experimental configuration with the lowest background, the background is ${4.0}_{-2.5}^{+3.1}\mathrm{counts}/(\mathrm{FWHM}\mathrm{t}\mathrm{yr})$.

The MajoranaDemonstratorwill search for the neutrinoless double-beta(ββ0ν)decay of the isotopeGe with a mixed array of enriched and natural germanium detectors. The observation of this rare decay would indicate that the neutrino is its own antiparticle, demonstrate that lepton number is not conserved, and provide information on the absolute mass scale of the neutrino. The Demonstratoris being assembled at the 4850-foot level of the Sanford Underground Research Facility in Lead, South Dakota. The array will be situated in a low-background environment and surrounded by passive and active shielding. Here we describe the science goals of the Demonstratorand the details of its design.

The ground-state band of the $Z=102\mathrm{}$ isotope ${}^{254}\mathrm{No}$ has been identified up to spin 14, indicating that the nucleus is deformed. The deduced quadrupole deformation, $\beta =0.27\mathrm{}$, is in agreement with theoretical predictions. These observations confirm that the shell-correction energy responsible for the stability of transfermium nuclei is partly derived from deformation. The survival of ${}^{254}\mathrm{No}$ up to spin 14 means that its fission barrier persists at least up to that spin.

Neutron-rich Zr and Mo isotopes were populated as fission fragments produced by the ${}^{238}\mathrm{U}\left(\alpha ,\mathit{f}\right)$ fusion-fission reaction. The level schemes of these nuclei have been extended beyond the first band crossing region, which can be ascribed to the $h_{11∕2}$ neutron pair alignment. The spin alignment and signature splitting for the $\nu h_{11∕2}$ orbitals in term of triaxiality is addressed. The crossing frequency of the aligned bands in even Zr and Mo isotopes can be reproduced well by calculations using the cranked shell model. Compared to the Zr isotopes, band crossings in Mo isotopes shift to a lower rotational frequency due to the emerging importance of the $\gamma$ degree of freedom. Within the framework of particle-rotor model, the difference in the signature splitting observed for the $\nu h_{11∕2}$ bands between the odd Zr and Mo isotopes can be attributed to the triaxial degree of freedom in the Mo isotopes. The surprising shift of the band crossing to a higher rotational frequency in ${}^{106}\mathrm{Mo}$ is interpreted as a manifestation of the deformed subshell closure at $N=64$.