Complete event-by-event $alpha$/$gamma(beta)$ separation in a full-size TeO$_2$ CUORE bolometer by Neganov-Luke-magnified light detection

In the present work, we describe the results obtained with a large ($approx 133$ cm$^3$) TeO$_2$ bolometer, with a view to a search for neutrinoless double-beta decay ($0 ubetabeta$) of $^{130}$Te. We demonstrate an efficient $alpha$ particle discrimination (99.9%) with a high acceptance of the $0 ubetabeta$ signal (about 96%), expected at $approx 2.5$ MeV. This unprecedented result was possible thanks to the superior performance (10 eV rms baseline noise) of a Neganov-Luke-assisted germanium bolometer used to detect a tiny (70 eV) light signal from the TeO$_2$ detector, dominated by $gamma$($beta$)-induced Cherenkov radiation but exhibiting also a clear scintillation component. The obtained results represent a major breakthrough towards the TeO$_2$-based version of CUORE Upgrade with Particle IDentification (CUPID), a ton-scale cryogenic $0 ubetabeta$ experiment proposed as a follow-up to the CUORE project with particle identification. The CUORE experiment began recently a search for neutrinoless double-beta decay of $^{130}$Te with an array of 988 125-cm$^3$ TeO$_2$ bolometers. The lack of $alpha$ discrimination in CUORE makes $alpha$ decays at the detector surface the dominant background component, at the level of $approx 0.01$ counts/(keV kg y) in the region of interest. We show here, for the first time with a CUORE-size bolometer and using the same technology as CUORE for the readout of both heat and light signals, that surface $alpha$ background can be fully rejected.