Towards a background-free neutrinoless double beta decay experiment based on a fluorescent bicolor sensor

Searching for neutrinoless double beta decays ($\beta\beta0\nu$) is the only practical way to establish if the neutrinos are their own antiparticles. Due to the smallness of neutrino masses, the lifetime of $\beta\beta0\nu$ is expected to be at least ten orders of magnitude smaller than the noise associated with the natural radioactive chains. A positive identification of $\beta\beta0\nu$ decays requires, ultimately, finding a signal that cannot be mimicked by radioactive backgrounds. This signal could be the observation of the daughter atom in the decay, since no known background processes induce a Z+2 transformation. In particular, the $\beta\beta0\nu$ decay of Xe-136 could be established by detecting the doubly ionised daughter atom, Ba$^{2+}$. Such a detection could be achieved via a sensor made of a monolayer of molecular indicators. The Ba$^{2+}$ would be captured by one of the molecules in the sensor, and the presence of the single chelated indicator would be subsequently revealed by a strong fluorescent response from repeated interrogation with a laser system. Here we describe a fluorescent bicolor indicator that binds strongly to Ba$^{2+}$ and shines very brightly, shifting its emission colour from green to blue when chelated in dry medium, thus allowing the unambiguous identification of single barium atoms in the sensor, and permitting a positive identification of the $\beta\beta0\nu$ decay of Xe-136 in a gas chamber, that could led to a background-free experiment.