Competition between standard and exotic double beta decays

We evaluate the contributions of higher order terms in weak Hamiltonian to the standard twoneutrino double beta decay. It is shown that only the first-forbidden unique transitions can alter the two-electron energy spectrum. Yet, their effect is too small to screen the detection of exotic neutrinoless double beta decays, which are candidates for testing the physics beyond the standard model. PACS: 13.15+q;14.80;21.60Jz;23.40 Fellow of the CONICET from Argentina. The double beta (ββ) experiments furnish a unique window onto whatever new physics may replace the Standard Electroweak Model (SM). To be observable the new physics should: i) violate the electron-lepton-number (Le) conservation, that is fulfilled in the SM, and/or ii) fit scalar particles (currently called Majorons), light enough to be produced in the ββ decay. The quantity that is used to discern experimentally between the ordinary SM two-neutrino decays (ββ2ν) and the exotic neutrinoless ββ events, both without (ββ0ν) and with Majoron emissions (ββM), is the electron energy spectrum dΓ/dǫ of the decay rate Γ, as a function of the sum ǫ = ǫ1 + ǫ2 of the energies of the two emitted electrons. For a transition from the initial state |0I〉 in the (N,Z) nucleus to the final state |0F 〉 in the (N − 2, Z + 2) nucleus (with energies EI and EF and spins and parities J = 0) the differential decay rate reads dΓ(0 I →0 F ) = 4Gḡ 15π ∣