Abstract
A search for instability of nucleons bound in $^{136}$Xe nuclei is reported with 223 kg$\cdot$yr exposure of $^{136}$Xe in the EXO-200 experiment. Lifetime limits of 3.3$\times 10^{23}$ and 1.9$\times 10^{23}$ yrs are established for nucleon decay to $^{133}$Sb and $^{133}$Te, respectively. These are the most stringent to date, exceeding the prior decay limits by a factor of 9 and 7, respectively.
Highlights
Current experimental data are consistent with baryon number (B) and lepton number (L) conservation
It is possible that there are symmetries suppressing the simple baryon ΔB 1⁄4 1 and lepton number ΔL 1⁄4 2 violating processes despite the fact that the corresponding baryon number violating scale is relatively low. An example of such a symmetry was proposed in [8] where it was shown that the standard model (SM) Lagrangian, extended to include small neutrino masses, has an anomaly-free Z6 discrete gauge symmetry
This symmetry forbids the welltested ΔB 1⁄4 1 and ΔB 1⁄4 2 decay processes but allows the ΔB 1⁄4 3 triple-nucleon decay. This mode of nucleon decay, which may occur through dimension 15 operators, is tested in this work at the Λ ∼ 102 GeV energy scale [9]
Summary
Current experimental data are consistent with baryon number (B) and lepton number (L) conservation. Violation of total lepton number by two units could be related to the dimension 5 operator, the so-called Weinberg operator, (llΛHLH) (where l is the left-handed lepton doublet, H is the Higgs doublet, and ΛL is the cutoff scale associated with lepton number violation) This is the lowest dimension operator that can produce neutrinoless double-beta decay. It is possible that there are symmetries suppressing the simple baryon ΔB 1⁄4 1 and lepton number ΔL 1⁄4 2 violating processes despite the fact that the corresponding baryon number violating scale is relatively low An example of such a symmetry was proposed in [8] where it was shown that the SM Lagrangian, extended to include small neutrino masses, has an anomaly-free Z6 discrete gauge symmetry. This mode of nucleon decay, which may occur through dimension 15 operators, is tested in this work at the Λ ∼ 102 GeV energy scale [9]
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