Some DAMA/LXe results on 2β decay

2001 - Preliminary new and improved limits on 136Xe 2β decay modes (running time 6843.8 h, ~6.5 kg LXe enriched in 136Xe at 68.8%):

Note that the competitive limits for the processes with two ν have cautiously been obtained without applying any background subtraction. In particular, the limit on the half-life of the 2β2ν(0+ → 0+) process in 136Xe is in the range of the theoretical estimate by A.Staudt et al., Europhys. Lett. 13 (1) (1990), 31; moreover, theoretical estimates on half-life of 2β decay processes suffer of the large uncertainties typically associated to the calculations of the nuclear matrix elements. The transition probability of the 2β2ν(0+ → 2+) process is obviously depressed with respect to this latter one.

Decay ModeT1/2 (y)
(90% C.L.)
2β0ν(0+ → 0+) > 7.0 x 1023
2β0ν (0+ → 2+) > 4.2 x 1023
2β 0νM (0+ → 0+) > 8.9 x 1022
2β 2ν (0+ → 0+) > 1.1 x 1022
2β 2ν (0+ → 2+) > 1.0 x 1022
< mν>
(eV)
< gM>
(10-5)
Suhonen et al. < 1.8 < 5.9
Engel et al. < 2.2 < 7.1
Tomoda < 1.5 < 4.8
Muto et al. < 1.8 < 5.7

From the results obtained on the half life for the decay channels 2β0ν (0+ → 0+) and 2β 0νM (0+ → 0+) upper limits on the effective neutrino mass, < mν >, and on the effective coupling constant Majoron-neutrino, < gM >, can be derived. These limits depend on the evaluation of the nuclear matrix elements, therefore they have been calculated (90% C.L.) for various theoretical models.