The double beta decay is a second-order process in the weak nuclear force, where two neutrons in the nucleus are converted into two protons, two electrons, and two neutrinos. The 0νββ decay, if observed, would imply that the neutrinos are Majorana particles (i.e., their antiparticles are themselves) and have mass.
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However, I need to point out that there seems to be a typo in your request. The commonly studied isotopes for 0νββ decay are not CO3 but rather nuclei like Germanium-76 (76Ge), Selenium-82 (82Se), Molybdenum-100 (100Mo), and Tellurium-130 (130Te), among others. co3 0nu