When three Majorana neutrinos describe the solar and atmospheric neutrino data via oscillations , a nonzero measurement of neutrinoless double beta ( 0 \nu \beta \beta ) decay can determine the sum of neutrino masses \sum m _ { \nu } if the solar solution has small-angle mixing , and place a lower bound on \sum m _ { \nu } for large-angle solar mixing .
If in addition a nonzero \sum m _ { \nu } is deduced from cosmology , the neutrino mass spectrum may be uniquely specified for some ranges of neutrino parameters .
For \sum m _ { \nu } > 0.75 eV , the small-angle solar solution is excluded by the current upper limit on neutrinoless double beta decay .
In models with maximal solar mixing the CP phases of the neutrinos may be strongly constrained by stringent upper bounds on 0 \nu \beta \beta decay .