1 + m_2 +m_3 < 0.230 {\rm ~eV}~(95\% {\rm CL})
\label{eq:cosmo}
\end{equation}

Figure \ref{fig.Planck} shows the implications of such a measurement, when combined with the current limits from KamLAND-Zen and EXO. As it can be seen, the current sensitivity is not enough to explore significantly the inverse hierarchy, while Planck data exclude most of the so-called degenerate hierarchy. It follows that the next generation of \ensuremath{\beta\beta0\nu}\ experiments must aim for extraordinary sensitivities to the effective neutrino mass. In particular, we will show that a sensitivity of 20 meV in \ensuremath{m_{\beta\beta}}\ is within the reach of a ton-scale HPXe detector. However, with luck, a discovery could be made before, if \ensuremath{m_{\beta\beta}}\ is near 100 meV. 

 \begin{table}
\centering
\caption{Experimental parameters of the three xenon-based double beta decay experiments: (a) total mass of \ensuremath{{}^{136}\rm Xe}, $M$; (b) enrichment fraction $f$; (c) signal detection efficiency, $\varepsilon$; (d) energy resolution, $\delta E$, at the $Q$ value of \ensuremath{{}^{136}\rm Xe}; and (e) background rate, $b$, in the region of interest around \ensuremath{Q_{\beta\beta}}\ expressed in \ensuremath{\rm counts/(keV \cdot kg \cdot y)}\ (shortened as ckky)  \cite{GomezCadenas:2013ue}. } 
\label{tab:ExpParams}
\vspace{0.5cm}
\begin{tabular}{lccccc}
\hline
Experiment & $M$ (kg) & $f$ & $\varepsilon$ & $\delta E$ (\% FWHM) & $b$ ($10^{-3}$~ckky) \\
\hline
EXO-200 		& 110 & 81 & 52 & 3.9  & 1.5 \\
KamLAND-Zen & 330 & 91 & 62 & 9.9 & 1.0  \\
NEXT-100 	& 100 & 91 & 30  & 0.7  & 0.5 \\
\hline
\end{tabular}
\end{table}

\begin{figure}
\centering
\includegraphics[width=0.7\textwidth]{img/SensitivityXeReference.pdf}
\caption{Sensitivity of the three xenon experiments as a function of the running time, assuming the parameters described 
in Table \ref{tab:ExpParams}. We consider a run of 8 years for EXO-200 and KamLAND-Zen (2012 to 2020) and a run of 5 years for NEXT (2015 to 2020).} \label{fig.exoNext}
\end{figure}

In order to gain a feeling of the potential of the NEXT technology it is interesting to compare the experimental parameters of the three xenon experiments, which are collected in Table~\ref{tab:ExpParams}. The parameters of EXO-200 and KamLAND-Zen are those published by the collaborations \cite{Auger:2012ar,Gando:2012zm}. The resolution in NEXT corresponds to the most conservative result obtained by the NEXT prototypes  \cite{Alvarez:2012xda}, and the predicted background rate and efficiency comes from the full background model of the collaboration \cite{Alvarez:2012haa, MartinAlbo:2013ve}, assuming a conservative background level for the dominant source of background (the energy--plane PMTs). Notice that the background rate of all the experiments is very good. The HPXe technology offers less efficiency than the other two but a much better resolution. 

Figure \ref{fig.exoNext} shows the expected performance of the three experiments, assuming
the parameters described 
in Table \ref{tab:ExpParams} and the central value of the nuclear matrix elements described in \cite{GomezCadenas:2010gs}.
We consider a run of five years for NEXT (2015 to 2020) and a longer run of eight years for EXO-200 and KamLAND-Zen (2012 to 2020). A total dead-time of 10\% a year for all experiments is assumed.  It follows that all the three experiment will have a chance of making a discovery if \ensuremath{m_{\beta\beta}}\ is in the range of 100 meV. The fact that the experiments are based in different experimental techniques, with different systematic errors, makes their 
simultaneous running even more attractive. The combination of the three can reach a sensitivity of about 65 meV \cite{GomezCadenas:2013ue}.  Notice that, in spite of its late start, NEXT sensitivity can surpass that of the other xenon experiments.

\subsection{Towards the ton scale}

\begin{table}
\centering
\caption{Expected experimental parameters of the three xenon-based double beta decay technologies in a possible ton-scale experiment: (a) signal detection efficiency, $\varepsilon$; (b) energy resolution, $\delta E$, at the $Q$ value of \ensuremath{{}^{136}\rm Xe}; and (c) background rate, $b$, in the region of interest around \ensuremath{Q_{\beta\beta}}\ expressed in \ensuremath{\rm counts/(keV \cdot kg \cdot y)}. } \label{tab:FutureParams}
\vspace{0.5cm}
\begin{tabular}{lccc}
\hline
Experiment &  $\varepsilon$ & $\delta E$ (\% FWHM) & $b$ ($10^{-3}$~ckky) \\
\hline
LXe		& 38 & 3.2  & 0.1 \\
XeSci       & 42 & 6.5 & 0.1  \\
HPXe	& 30  & 0.5  & 0.1 \\
\hline
\label{tab:para}
\end{tabular}
\end{table}


\begin{figure}[!htb]
\centering
\includegraphics[width=0.7\textwidth]{img/SensitivityXeFuture.pdf}
\caption{Sensitivity of the three technologies experiments as a function of the total exposure, assuming the parameters described 
in Table \ref{tab:FutureParams} \cite{GomezCadenas:2013ue}.} \label{fig.NEXTW}
\end{figure}

To cover the full range allowed by the inverse hierarchy, one needs masses in the range of one ton of isotope. Xenon experiments have the potential to deploy those large masses (for instance, to enrich $^{76}$Ge is 10 times more expensive than $^{136}$Xe). This characteristic, together with the fact that one can build large xenon-based TPCs or calorimeters, make them a preferred choice for the next-to-next generation of experiments.

Table~\ref{tab:FutureParams} summarises a projection \cite{GomezCadenas:2013ue} of the experimental parameters for the three technologies, while Figure \ref{fig.NEXTW} shows the expected performance of xenon experiments assuming
the parameters described 
in Table \ref{tab:FutureParams}, up to a total exposure of 10 ton$\cdot$year. At the maximum exposure, the LXe and XeSci detectors reach a draw at 40 meV, while the HPXe detector reaches 25 meV. 

To summarise, the NEXT experiment has an enormous interest for \ensuremath{\beta\beta0\nu}\ searches not only due to its large physics potential --- that is the ability to discover that neutrinos are Majorana particles --- but also as a springboard to the next-to-next generation of very challenging, ton-based experiments. 

\section{Introduction}
\input{src2/Intro.tex}
 This paper is organised as follows. Section \ref{sec:physics} describes the physics of Majorana neutrinos and \ensuremath{\beta\beta0\nu}\ searches, reviews xenon experiments, and discusses their discovery potential. The NEXT detector is described with some detail in Section
\ref{sec:next}, while Section \ref{sec:bkgm} gives details of the NEXT background model. Section
\ref{sec:EL} describes our two electroluminescence prototypes, NEXT-DEMO and NEXT-DBDM. Finally, conclusions are presented in Section \ref{sec:conclu}.

\section{The physics of NEXT}
\label{sec:physics}
\input{src2/Physics.tex}

\section{The NEXT detector}
\label{sec:next}
\input{src2/NextDetector.tex}
\section{NEXT background model}
\label{sec:bkgm}
\input{src2/NextBKGM.tex}
\section{The NEXT EL  prototypes}
\label{sec:EL}
\input{src2/NextDemo.tex}
\input{src2/NextDBDM.tex}

\section{Conclusions}
\label{sec:conclu}

In this paper, the current status and