Abstract
One of the key ingredients needed to extract quantitative information on neutrino absolute mass scale from the possible measurement of the neutrinoless double-beta (0νββ) decay half-lives is the nuclear matrix element (NME) characterizing such transitions. NMEs are not physical observables and can only be deduced by theoretical calculations. However, since the atomic nuclei involved in the decay are many-body systems, only approximated values are available to date. In addition, the value of the coupling constants to be used for the weak interaction vertices is still an open question, which introduces a further indetermination in the calculations of NMEs. Several experimental approaches were developed in the years with the aim of providing useful information to further constrain the theory. Here we give an overview of the role of charge exchange reactions in this scenario, focusing on second-order processes, namely the double charge exchange (DCE) reactions.
Highlights
Neutrinoless double beta decay (0νββ) is a hypothetic class of nuclear processes where a parent nucleus is transformed into an isobar daughter differing by two unit charges, and two electrons are emitted
The 0νββ decay rate [T1/2 ]−1 is typically expressed as the product of three main factors: (i) a phase-space parameter G0ν, (ii) a nuclear matrix element (NME) M0ν and (iii) a function f of the neutrino masses mi, the mixing coefficients Uei and the Majorana phases ζi
If the NMEs are established with sufficient precision, the f factor, containing physics beyond the standard model, can be accessed from 0νββ decay rate measurements [2]
Summary
Neutrinoless double beta decay (0νββ) is a hypothetic class of nuclear processes where a parent nucleus is transformed into an isobar daughter differing by two unit charges, and two electrons (or positrons) are emitted. “multi-messenger” approaches, where several observables are determined all together in the same experiment and analyzed in a consistent theoretical framework, have attracted interest [41] In this framework, key information could be provided by the study of DCE reactions, which change the nuclear charge by two units keeping the mass number unvaried, in analogy to the ββ-decay. The spectral shape of DCE cross-section has attracted recent interest due to its connection to the nuclear response to double Fermi and double Gamow–Teller operators, giving access to the experimental scrutiny of model-independent sum-rules and providing supplementary hints to NME of 0νββ decay [42,43].
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