Improved limit on neutrinoless double beta decay of ~from AMoRE-I

AMoRE searches for the signature of neutrinoless double beta decay of ¹⁰⁰Mo with a 100 kg sample of enriched ¹⁰⁰Mo. Scintillating molybdate crystals coupled with a metallic magnetic calorimeter operate at milli-Kelvin temperatures to measure the energy of electrons emitted in the decay. As a demonstration of the full-scale AMoRE, we conducted AMoRE-I, a pre-experiment with 18 molybdate crystals, at the Yangyang Underground Laboratory for over two years. The exposure was 8.02 kg·year (or 3.89 kg_¹⁰⁰Mo·year) and the total background rate near the Q-value was 0.025 ± 0.002 counts/keV/kg/year. We observed no indication of 0νββ decay and report a new lower limit of the half-life of ¹⁰⁰Mo 0νββ decay as T0ν1/2>3.0×10²⁴~years at 90% confidence level. The effective Majorana mass limit range is mββ<(210--610) meV using nuclear matrix elements estimated in the framework of different models, including the recent shell model calculations.