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The possibility to probe new physics scenarios of light Majorana neutrino exchange and right-handed currents at the planned next generation neutrinoless double β decay experiment SuperNEMO is discussed. Its ability to study different isotopes and track the outgoing electrons provides the means to discriminate different underlying mechanisms for the neutrinoless double β decay by measuring the decay half-life and the electron angular and energy distributions.

Triple beta decay of 150Nd to the ground and excited states of 150Eu and quadruple beta decay of 150Nd to the excited states of 150Gd have been studied using a 400 cm3 low-background HPGe detector. A half-life limit for the quadruple beta decay to the 01+ state of 150Gd was found to be T1/2(0v + 4v) > 8.7 × 1020 yr (90% C.L.). For other (0v+4v) transitions to the excited states limits for the first time have been obtained at the level of (6.1 − 9.5) × 1020 yr (90% C.L.). We report here also the results for the first search for triple beta decay to the ground and excited final states of 150Eu, T1/2(0v + 3v) > (0.04 − 4.8) × 1020 yr (90% C.L.).

Triple beta decay of 150 Nd to the ground and excited states of 150 Eu and quadruple beta decay of 150 Nd to the excited states of 150 Gd have been studied using a 400 cm 3 low-background HPGe detector. A half-life limit for the quadruple beta decay to the 0 1 + state of 150 Gd was found to be T 1/2 (0 v + 4 v ) > 8.7 × 10 20 yr (90% C.L.). For other (0 v +4 v ) transitions to the excited states limits for the first time have been obtained at the level of (6.1 − 9.5) × 10 20 yr (90% C.L.). We report here also the results for the first search for triple beta decay to the ground and excited final states of 150 Eu, T 1/2 (0 v + 3 v ) > (0.04 − 4.8) × 10 20 yr (90% C.L.).

Limits on double-beta processes in 74Se and 120Te have been obtained using a 400 cm3 HPGe detector and an external source consisting of natural Se or TeO2 powder. In particular, limit with a 90% C.L. on the 74Se half-life of 5.5 × 1018 y for ECEC(0ν) transitions to the 2+2 excited state in 74Ge (1204.2 keV) has been obtained. This transition is discussed in association with a possible enhancement of the decay rate, in this case by several orders of magnitude, because the ECEC(0ν) process is nearly degenerate with an excited state in the daughter nuclide. For other β+ EC and ECEC transitions (to the ground and excited states) obtained limits are on the level ∼ 1018 – 1019 y.

Double beta decay of 150Nd and 148Nd to the excited states of daughter nuclei have been studied using a 400 cm3 low-background HPGe detector and an external source consisting of 3046 g of natural Nd2O3 powder. The half-life for the two-neutrino double beta decay of 150Nd to the excited 0+1 state in 150Sm is measured to be T1/2 [1.33+0.36−0.23(stat)+0.27−0.13(syst)] · 1020 y. For other (0ν + 2ν) transitions to the 2+1, 2+2, 2+3, and 0+2 levels in 150Sm, limits are obtained at the level of ~ (2 − 8) · 1020 y. In the case of 148Nd only limits for the (0ν + 2ν) transitions to the 2+1, 0+1, and 2+2 excited states in 148Sm were obtained and are at the level of ~ (4 − 8) · 1020 y.

Triple beta decay of \\(^{150}\\)Nd to the ground and excited states of \\(^{150}\\)Eu and quadruple beta decay of \\(^{150}\\)Nd to the excited states of \\(^{150}\\)Gd have been studied using a 400 cm\\(^3\\) low-background HPGe detector and an external source consisting of 3046 g of natural Nd\\(_2\\)O\\(_3\\) powder. A half-life limit for the quadruple beta decay to the 0\\(_1^+\\) state of \\(^{150}\\)Gd was found to be \\(T_{1/2} (0\\nu+4\\nu) > 8.7\\times10^{20}\\) yr (90\\% C.L.). For other (0\\(\\nu\\)+4\\(\\nu\\)) transitions to the 2\\(_1^+\\), 3\\(_1^-\\), 4\\(^+_1\\), 2\\(_2^+\\) and 2\\(^+_3\\) excited states limits for the first time have been obtained at the level of \\((6.1-9.5)\\times10^{20}\\) yr (90\\% C.L.). We report here also the results for the first search for triple beta decay to the ground and excited final states of \\(^{150}\\)Eu. We find no evidence of this decay and set lower limits on the half-life in the range \\(T_{1/2} (0\\nu+3\\nu) > (0.04-4.8)\\times10^{20}\\) yr (90\\% C.L.).

We consider the energy spectrum of emitted electrons in \\(\\beta\\)-decay. Exact Coulomb Dirac wave functions describing the \\(\\beta\\)-electron in the Coulomb field of the daughter nucleus are used. Further, the improved wave functions which include the screening of the Coulomb field due to the atomic electron cloud are also used. Thus, the interaction between the \\(\\beta\\)-electron and the field due to the daughter atom is treated within a nonperturbative approach. Are shown the modifications due to the screening on the \\(\\beta\\) spectra and shown that those effects are very important. In addition, are addressed the contributions to the \\(\\beta\\) spectra due to the exchange terms and shown that the corresponding effects can be substantial. Higher orders arising from the multipole expansion are considered. A comparison of the theoretical results obtained in this work has been made with recent experimental data and a very good agreement was observed.