Improved measurement of the 2νββ half-life of 136Xe with the EXO-200 detector

We report on an improved measurement of the 2νββ half-life of ¹³⁶Xe performed by EXO-200. The use of a large and homogeneous time-projection chamber allows for the precise estimate of the fiducial mass used for the measurement, resulting in a small systematic uncertainty. We also discuss in detail the data-analysis methods used for double-β decay searches with EXO-200, while emphasizing those directly related to the present measurement. The ¹³⁶Xe 2νββ half-life is found to be T1/22ν_β^β = 2.165±0.016(stat)±0.059(sys)×10²¹ yr. This is the most precisely measured half-life of any 2νββ decay to date.