Conclusion and outlook.  Double beta decay  Comparison with predictions for

Interpretation of the data on $\beta\beta 0\nu$ and $\beta\beta \chi^0$.

The fair agreement between measured and calculated half lives in $\beta\beta 2\nu$ decay suggests that the same models can reasonably predict the matrix elements for the other decay modes. Table  lists the measured $\beta\beta 0\nu$ half life limits from several experiments. For ¹²⁸Te, it was assumed that the geochemically determined half life is a lower limit for that of $\beta\beta 0\nu$ decay. The corresponding neutrino mass limits, using the QRPA calculation of Caltech as well as from two other groups, are shown. The spread between the limits for a given half life is largest for ⁷⁶Ge, and relatively small for the other, heavier, nuclei. This spread can be considered a measure of the uncertainty associated with the $\beta\beta 0\nu$ matrix elements. The last column gives the limits obtained with the shell model matrix elements of Strasbourg-Madrid. One sees that the ⁷⁶Ge data give the most constraining limit. In all, one may conclude that the various experiments give:  

Table: Measured lower limits for the half-life of various $\beta\beta \chi^0$ transitions. Deduced upper limits on the effective coupling constant $\langle g_{eff}\rangle$. 

 

	[yr] (90 % CL)	$\langle g_{eff}\rangle$ Caltech
direct	> 7.91 > 5 > 1.4	< (3.9 - 4.4) < (2.5 - 3.1) < (1.2 -1.8)
geochem. 128Te	> 2.5 - 7.7	< (2 - 5)

 

 

Table  shows some measured half life limits on $\beta\beta \chi^0$ decay and the corresponding limits on $\langle g_{eff}\rangle$ obtained from the more conservative Caltech matrix elements. Here the geochemical ¹²⁸Te data are the most constraining. Of the direct searches the ¹³⁶Xe experiment gives the most stringent limit. In any event, one may conclude that the effective coupling constant is bound by: .