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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 doublebeta (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 leptonnumber violating righthanded 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 HeidelbergMoscow 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 CPphases 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 leptonnumberchanging 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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 Spring '11
 Kutter

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