mass dominates [$0\nu\beta\beta$ ]{}decay or not.

Up to now, [$0\nu\beta\beta$ ]{}decay has not been observed. The best half-life limits on [$0\nu\beta\beta$ ]{}decay come from experiments on two isotopes: $^{76}$Ge and $^{136}$Xe. The Heidelberg-Moscow collaboration gives $T^{0\nu\beta\beta}_{1/2}(^{76}{\rm Ge}) \ge 1.9
\cdot 10^{25}$ yr [@KlapdorKleingrothaus:2000sn],[^6] while the recent results from EXO-200 and KamLAND-ZEN quote $T^{0\nu\beta\beta}_{1/2}(^{136}{\rm Xe}) \ge 1.6 \cdot 10^{25}$ yr [@Auger:2012ar] and $T^{0\nu\beta\beta}_{1/2}(^{136}{\rm Xe}) \ge 1.9 \cdot 10^{25}$ yr [@Collaboration:2012zm], both at the 90 % CL. There is, however, reasonable hope that the half-lifes in excess of $10^{26}$ yr will be probed within the next few years, since a number of next generation [$0\nu\beta\beta$ ]{}experiments are under construction or already taking data. For recent reviews and a list of experimental references, see for example [@Barabash:1209.4241; @GomezCadenas:2011it]. Moreover, proposals for ton-scale next-to-next generation [$0\nu\beta\beta$ ]{}experiments claim that even sensitivities in excess $T^{0\nu\beta\beta}_{1/2} \sim 10^{27}$ yr can be reached for $^{136}$Xe [@KamLANDZen:2012aa; @MacLallen:2012aa] and $^{76}$Ge [@Abt:2004yk; @Guiseppe:2011me]. For a brief summary of the long-term prospects for [$0\nu\beta\beta$ ]{}experiments, see for example [@Barabash:2011fs].

The interpretation of these half-life limits in terms of particle physics parameters requires assumptions, such as which contribution dominates the [$0\nu\beta\beta$ ]{}decay amplitude. If neutrinos have Majorana masses, [$0\nu\beta\beta$ ]{}decay can 