= Light-Majorana exchange in neutrinoless double beta decay
Two charged-current beta vertices can be connected by a virtual <Majorana neutrino>. With light exchanged <masses> relative to nuclear <momentum> scales, the <chirality> connection makes the amplitude proportional to $\sum_iU_{ei}^2m_i$, not an incoherent <mass> average. Squaring introduces the <effective Majorana mass>, <nuclear matrix elements> and <phase space>. Other lepton-number-violating operators can also mediate the same final state, so translating a rate into a <mass> requires the stated mechanism.
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