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Unformatted text preview: 1 NEUTRINOLESS DOUBLE- DECAY Revised August 2009 by P. Vogel (Caltech) and A. Piepke (University of Alabama). Neutrinoless double-beta (0 ) decay would signal vio- lation of total lepton number conservation. The process can be mediated by an exchange of a light Majorana neutrino, or by an exchange of other particles. However, the existence of -decay requires Majorana neutrino mass, no matter what the actual mechanism is. As long as only a limit on the lifetime is available, limits on the effective Majorana neutrino mass, on the lepton-number violating right-handed current or other possible mechanisms mediating 0 -decay can be obtained, independently of the actual mechanism. These limits are listed in the next three tables, together with a claimed 0 -decay signal reported by part of the Heidelberg-Moscow collaboration. A 6 excess of counts at the decay energy is used for a deter- mination of the Majorana neutrino mass. This signal has not yet been independently confirmed. In the following we assume that the exchange of light Majorana neutrinos ( m i 10 MeV) contributes dominantly to the decay rate. Besides a dependence on the phase space ( G ) and the nu- clear matrix element ( M ), the observable 0 -decay rate is proportional to the square of the effective Majorana mass m , ( T 1 / 2 ) 1 = G | M | 2 m 2 , with m 2 = | i U 2 ei m i | 2 . The sum contains, in general, complex CP-phases in U 2 ei , i.e. , cancellations may occur. For three neutrino avors, there are three physical phases for Majorana neutrinos and one for Dirac neutrinos. The two additional Majorana phase differences af- fect only processes to which lepton-number-changing amplitudes contribute. Given the general 3 3 mixing matrix for Majorana neutrinos, one can construct other analogous lepton number vi- olating quantities, m = i U i U i m i . However, these are currently much less constrained than m . Nuclear structure calculations are needed to deduce m from the decay rate. While G can be calculated reliably, the computation of M is subject to uncertainty. Indiscriminate averaging over all published matrix element values would result, for any given nuclide, in a factor of...
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