Sensitivity of the NEXT-100 detector to neutrinoless double beta decay

A high pressure xenon gas time projection chamber with electroluminescent amplification (EL HPGXe TPC) searching for the neutrinoless double beta ($0\nu\beta\beta$) decay offers: excellent energy resolution ($0.5-0.7\%$ FWHM at the $Q_{\beta\beta}$), by amplifying the ionization signal with electroluminescent light, and tracking capabilities, as demonstrated by the NEXT collaboration using two kg-scale prototypes. The NEXT collaboration is building an EL HPGXe TPC capable of holding 100 kg (NEXT-100) of xenon isotopically enriched in ${{}^{136}\rm Xe}$. The installation and commissioning of the NEXT-100 detector at the Laboratorio Subterr\'aneo de Canfranc (LSC) is planned for 2018. The current estimated background level for the NEXT-100 detector is of $4\times10^{-4}$ counts/keV-kg-yr or less in the energy region of interest. Assuming an energy resolution of 0.75$\%$ FWHM at the $Q_{\beta\beta}$ and a $0\nu\beta\beta$ signal efficiency of about 28$\%$, this gives an expected sensitivity (at 90$\%$ CL) to the $0\nu\beta\beta$ decay half life of $T^{0\nu}_{1/2}>6.0\times10^{25}$ yr for an exposure of 275 kg yr. A first phase of the NEXT experiment, called NEW, is currently being commissioned at the LSC. The NEW detector is a scale 1:2 in size (1:10 in mass) of the NEXT-100 detector using the same materials and photosensors and will be used to perform a characterization of the $0\nu\beta\beta$ backgrounds and a measurement of the standard double beta decay with neutrinos (${2\nu\beta\beta}$). An 8 sigma significance for the ${2\nu\beta\beta}$ signal in the NEW detector has been estimated for a 100-day run.