Physics potential of searching for $0\nu\beta\beta$ decays in JUNO

In the past few decades, numerous searches have been made for the neutrinoless double-beta decay (0$\nu\beta\beta$) process, aiming to establish whether neutrinos are their own antiparticles (Majorana neutrinos), but no 0$\nu\beta\beta$ decay signal has yet been observed. A number of new experiments are proposed but they ultimately suffer from a common problem: the sensitivity may not increase indefinitely with the target mass. We have performed a detailed analysis of the physics potential by using the Jiangmen Underground Neutrino Observatory (JUNO) to improve the sensitivity to 0$\nu\beta\beta$ up to a few meV, a major step forward with respect to the experiments currently being planned. JUNO is a 20 kton low-background liquid scintillator (LS) detector with 3\%/$\sqrt{E \text{(MeV)}}$ energy resolution, now under construction. It is feasible to build a balloon filled with enriched xenon gas (with $^{136}$Xe up to 80\%) dissolved in LS, inserted into the central region of the JUNO LS. The energy resolution is $\sim$1.9\% at the $Q$-value of $^{136}$Xe 0$\nu\beta\beta$ decay. Ultra-low background is the key for 0$\nu\beta\beta$ decay searches. Detailed studies of background rates from intrinsic 2$\nu\beta\beta$ and $^{8}$B solar neutrinos, natural radioactivity, and cosmogenic radionuclides (including light isotopes and $^{137}$Xe) were performed and several muon veto schemes were developed. We find that JUNO has the potential to reach a sensitivity (at 90\% C. L.) to $T^{0\nu\beta\beta}_{1/2}$ of $1.8\times10^{28}$ yr ($5.6\times10^{27}$ yr) with $\sim$50 tons (5 tons) of fiducial $^{136}$Xe and 5 years exposure, while in the 50-ton case the corresponding sensitivity to the effective neutrino mass, $m_{\beta\beta}$, could reach (5--12) meV, covering completely the allowed region of inverted neutrino mass ordering.