The Cryogenic Underground Observatory for Rare Events (CUORE) at Laboratori Nazionali del Gran Sasso of INFN in Italy is an experiment searching for neutrinoless double beta ($0\nu \beta \beta$) decay. Its main goal is to investigate this decay in ${}^{130}\mathrm{Te}$, but its ton-scale mass and low background make CUORE sensitive to other rare processes as well. In this Letter, we present our first results on the search for $0\nu \beta \beta$ decay of ${}^{128}\mathrm{Te}$, the Te isotope with the second highest natural isotopic abundance. We find no evidence for this decay, and using a Bayesian analysis we set a lower limit on the ${}^{128}\mathrm{Te}$ $0\nu \beta \beta$ decay half-life of $T_{1/2}>3.6\times {10}^{24}\mathrm{yr}$ (90% CI). This represents the most stringent limit on the half-life of this isotope, improving by over a factor of 30 the previous direct search results, and exceeding those from geochemical experiments for the first time.

• Received 6 May 2022
• Revised 22 July 2022
• Accepted 3 October 2022

DOI:https://doi.org/10.1103/PhysRevLett.129.222501