Theory of neutrinoless double-beta decay

Abstract

Abstract. Neutrinoless double beta decay, which is a very old and yet elusive process, is reviewed. Its observation will signal that lepton number is not conserved and the neutrinos are Majorana particles. More importantly it is our best hope for determining the absolute neutrino mass scale at the level of a few tens of meV. To achieve the last goal certain hurdles have to be overcome involving particle, nuclear and experimental physics. Nuclear physics is important for extracting the useful information from the data. One must accurately evaluate the relevant nuclear matrix elements, a formidable task. To this end, we review the sophisticated nuclear structure approaches recently been developed, which give confidence that the needed nuclear matrix elements can be reliably calculated employing different methods: a) the various versions of the Quasiparticle Random Phase Approximations, b) the interacting boson model, c) the energy density functional method and d) the large basis Interacting Shell Model. It is encouraging that, for the light neutrino mass term at least, these vastly different approaches now give comparable results. From an experimental point of view it is challenging, since the life times are long and one has to fight against formidable backgrounds. One needs large isotopically enriched sources and detectors with high energy resolution, low thresholds and very low background. If a signal is found, it will be a tremendous accomplishment. Then, of course, the real task is going to be the extraction of the neutrino mass from the observations. This is not trivial, since current particle models predict the presence of many mechanisms other than the neutrino mass, which may contribute or even dominate this process. We will, in particular, consider the following processes: We will show that it is possible to disentangle the various mechanisms and unambiguously extract the important neutrino mass scale, if all the signatures of the reaction are searched in a sufficient number of nuclear isotopes.Neutrinoless double beta decay 2

Analysis of mechanisms that could contribute to neutrinoless double-beta decay

“…Usually, only the light left-handed neutrino-exchange mechanism (a.k.a. the mass mechanism) is taken into consideration, but other mechanisms could play a significant role [5]. One popular model that considers the right-handed currents contributions is the left-right symmetric model [19,20], which assumes the existence of heavy particles that are not part of the Standard Model (see also Ref.…”

“…[28]. Above, M 0ν and M 0N are the light and heavy neutrino-exchange nuclear matrix elements [5,6,10] (see their explicit decomposition in Appendix B), and X λ and X η represent combinations of NME and phase space factors that will be analyzed below. G 0ν 01 is a phase space factor [45] that can be calculated with relatively good precision in most cases [46,47], and g A = 1.27 (see also Appendix C).…”

“…Ref. [5] and references therein). A more general approach that includes a complete set of dimension 6 and dimension 9 operators to the SM Lagrangian, as well as R-parity violating SUSY contributions, Kaluza-Klein modes in higher dimensions [31,32], violation of Lorentz invariance and equivalence principle [33][34][35], is given in Refs.…”

“…Finally, some more exotic possibilities [5,29] leading to one-and two-pion exchange NME [30], were also calculated in the past within the interacting shell model approach [6,12], and quasiparticle random phase approximation (QRPA) (see e.g. Ref.…”

Neutrinoless ββ nuclear matrix elements using isovector spin-dipole Jπ=2− data

“…The neutrinoless double-beta (0νββ) decay of atomic nuclei is a promising way to access the physics beyond the standard model [1][2][3][4][5], as witnessed by the ever growing experimental interest in this decay mode. At the same time half-lives of the two-neutrino ββ (2νββ) decays of several nuclei have been measured with increased precision [6,7].…”