Focus section on double beta decay

Abstract

Double beta decay, the conversion of a nucleus changing the atomic number Z by two units while leaving the atomic mass A constant, is an extremely rare nuclear decay. It can occur via the emission of two electrons or two positrons, double electron capture or a mixed mode depending on the available Q-value. Furthermore, the neutrino-less mode is of major importance for neutrino physics. This mode violates the lepton number by two units and its observation would point towards physics beyond the Standard Model. The standard interpretation is via the exchange of a light Majorana neutrino, the mass of which could be measured via a half-life determination of double beta decay. There exist 11 isotopes for the double electron mode with a Q-value above 2 MeV, and for those considered for experimental searches, the neutrino-accompanied mode has been measured in the last decade. In recent years various new developments have occurred in almost all aspects related to double beta decay. With all the mixing angles of the PMNS leptonic mixing matrix measured, the regions of interest are well determined. Furthermore, with the launch of the LHC the existence of possible TeV particles contributing to neutrino-less double beta decays can be explored, complementing the searches in underground experiments. Nuclide pairs with the potential of degenerate nuclear states which would lead to a resonance enhancement of neutrino-less double electron capture were intensively searched for with Penning traps and caused a revival of this process. Last but not least, massive progress has been made in the involved conversion factors from the experimentally accessible half-life to the quantity of interest, the neutrino mass. New phase space factors were calculated and new approaches like the interacting boson model or energy density functional approaches joined the nuclear shell model and QRPA calculations to determine the involved nuclear transitions matrix elements. This is accompanied by a massive experimental program to improve the input parameters for these calculations using charge-exchange reactions, nucleon transfer reactions and muon capture. Furthermore, Penning traps have measured almost all relevant Q-values to sub-keV precision. All these exciting new developments over the last few years triggered the idea of a focus section in Journal of Physics G: Nuclear and Particle Physics on double beta decay during a workshop on the topic held in Dresden in July 2010. In addition to the research topics mentioned before, this focus section also covers the implication of double beta decay for current particle physics and theories beyond the Standard Model.