On Thursday, the EXO-200 experiment released a preprint on a search for exotic double beta decay modes involving no neutrinos but with emission of one or more “Majorons,” which are bosons related to the violation of lepton number required by these decay modes.
EXO-200 is an experiment at the Waste Isolation Pilot Plant (WIPP) near Carlsbad, NM. It consists of a time projection chamber (TPC) using highly enriched liquid xenon as the target material. The xenon is enriched with a higher fraction of a particular isotope that is expected to undergo double beta decay – which happens when regular beta decay is disallowed due to energy constraints but double beta decay, where two electrons (or positrons) are emitted at once is allowed. The main goal of EXO is to search for neutrinoless double beta decay, where the two neutrinos created along with the two electrons basically cancel one another. The existence of neutrinoless double beta decay requires that neutrinos be Majorana fermions, where they are their own antiparticles, instead of Dirac fermions like all the other known fermions. This paper looks at more complicated decay mode than the standard one of just two electrons and a nucleus in the final state.
The paper sets lower limits on the lifetime of xenon-136 using different models of neutrinoless double beta decays with Majorons in the final state. It also sets equivalent upper limits on the Majoron-electron coupling for some of these models.