Small finite Majorana masses assume very heavy mass scale symmetry, considered in mainstream theories, but the predicted values of light neutrino masses from the necessary see-saw mechanism are uncertain.
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In the framework of the flavor-geometric semi-empirical phenomenology of Standard Model particle mass and mixing hierarchies (arXiv:1212.1417) small finite Dirac masses assume zero scale of smallest neutrino mass, probably from new symmetry, with definite values of light neutrino masses that follow from solar and atmospheric mass squared differences: m1 = ~ 0, m2 = ~9 x10^-3 eV, m3 = ~ 5 x10^-2 eV. The mass hierarchy of that neutrino mass pattern, m2 /m1 >>1, m3 /m2 = ~ 6, is similar to the known Dirac mass hierarchies of charged leptons, m(muon)/m(electron) = ~ 200, m(tau) /m(muon) = ~ 17, and quarks. That special statement is suggested by the obtained in the mentioned arXiv-research new interesting unexpected result that all SM hierarchies may be characterized by defined ‘mass hierarchy angles’ of nearly universal form including all Dirac particle masses and neutrino mixing angles. So, Dirac neutrino mass pattern is a probable explanation.
Thus the question in title above may be answered by coming accurate experimental data on neutrino mass hierarchy of the masses m1 and m2. Data indication m1 << m2 means likely Dirac neutrinos, while near degenerate masses m1= ~ m2 means likely Majorana neutrinos. /* Style Definitions */ table.MsoNormalTable {mso-style-name:"Table Normal"; mso-tstyle-rowband-size:0; mso-tstyle-colband-size:0; mso-style-noshow:yes; mso-style-parent:""; mso-padding-alt:0in 5.4pt 0in 5.4pt; mso-para-margin:0in; mso-para-margin-bottom:.0001pt; mso-pagination:widow-orphan; font-size:10.0pt; font-family:"Times New Roman";}
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