DAMA/LIBRA results on possible Charge-Non-Conserving (CNC) processes in 127I (full text)

2012 - A new search for charge-non-conserving (CNC) electron capture with excitation of the 417.9 keV nuclear level in the 127I has been carried out by using the coincidence technique in an exposure of 0.87 ton x yr.
The conservation of the electric charge is one of the fundamental laws of standard quantum electrodynamics based on the underlying principle of gauge invariance. Nevertheless, the possibility that charge conservation may be broken in future unified gauge theories and the implications of such a violation have been extensively discussed in the literature. The symmetry breaking can be either explicit or spontaneous; moreover, modern extra-dimension theories offer new challenges for the searches of CNC processes. CNC electron porcess considered:
(A,Z) + e- -> (A,Z)* + νe
Each CNC-EC decay produces an excited level and the relaxation of the atomic shells. Considering only 127I K-shell electrons, a total energy of 33.2 keV is released in the process. The γ quanta emitted in the nucleus de-excitation may escape from the source = detector and then interact with one of the surrounding detectors, giving events in coincidence with multiplicity 2 (2 detectors fire). The search for events with multiplicity 2 in the peculiar energy interval offers both a peculiar signature for this process and a significant reduction of the background.
No excess is found in the double coincidences, thus a new limit on the mean life is derived:
τ >1.2 x 1024 yr (90 % C.L.).
This is about one order of magnitude larger than those previously available for CNC electron capture involving nuclear level excitations of 127I and of the same order of magnitude than those achieved for analogous processes in 129Xe.
Further devoted data taking is foreseen.