About this Research Topic
Double beta decay (DBD) is a rare nuclear process of great interest due to its potential to provide information about physics beyond the Standard Model (BSM). There are several possible DBD modes that can be classified according to the number and kind of leptons released in the decay, which can be shared in two categories: i) the two neutrino DBD (2νββ) modes, where two anti-neutrinos/neutrinos are emitted besides the two electrons/positrons. These decays occur with lepton number conservation (LNC) and are allowed within the original formulation of the SM; ii) the neutrinoless DBD (0νββ), that occur without emission of any neutrinos, and hence with LN violation, which is predicted by theories more general than the SM. The discovery of any 0νββ decay mode could give answers to fundamental issues about possible violation of the Lorentz and CP symmetries in the weak sector, LNC, or about neutrino properties such as i) are neutrinos Dirac- or Majorana-like particles? ii) neutrino absolute masses; iii) what is the correct hierarchy of the neutrino masses? iv) are there sterile neutrinos?, etc. Therefore, there are strong reasons for DBD to be one of the most investigated processes nowadays.
Theoretically, from the DBD study one expects the precise computation of the nuclear matrix elements (NMEs) and phase space factors (PSFs) entering the DBD half-lives formulas, for different decay modes and transitions to final ground or excited states. An accurate computation of these quantities would result in reliable predictions of DBD half-lives and constrains of the BSM parameters appearing in various possible mechanisms that may contribute to the 0νββ decay.The precise computation of the NMEs involved in DBD is a long-standing problem, which is not yet resolved. The existing nuclear methods, used by different groups, still give NME values that differ to much from each other. Precise PSF values also contribute to the accuracy of the DBD calculations and their recent computation performed with improved methods revealed differences as compared with the values obtained previously. Another interesting topic is the contribution of different possible mechanisms to 0νββ decay. From the DBD study, some limits have been extracted of the ratio between the mass, right-left and right-right terms in Lagrangean. On the other hand, the actual increased luminosity at LHC allows the search of same sign dilepton channels in proton proton collisions and in meson and baryon rare decays. These channels are also mediated by heavy mass neutrinos and are activated by the exchange of virtual right-handed W-bosons. Hence, it is interesting to make a correspondence between DBD and LHC investigations in order to improve the constrains on the relative strengths of these terms.
From the experimental side, the first DBD experiments were performed since late 1940s. Until present, significant progress has been made in increasing the sensitivity of such experiments with the goal of searching the 0ν β β decay mode. Now, for three nuclei (76 Ge, 136Xe and 130Te) a sensitivity at the level of 1025-1026 years for the half-lives was reached, which corresponds to a sensitivity in the (Majorana) neutrino effective mass of (0.1-0.3 eV). Further, the next goal is to reach the region (0.015-0.05) eV, predicted in the scheme with inverse hierarchy of the neutrino masses, which would correspond to a sensitivity of ~ 1027-1028 years. To achieve this sensitivity, it will be necessary to build up detectors with hundreds of kilograms of enriched isotopes. Currently, the main directions of experimental research are the following: HPGe detectors made of germanium enriched with the 76 Ge isotope; low-temperature bolometers using TeO2 crystals; low-temperature scintillation bolometers; TPC on liquid and gaseous xenon; detectors on loaded liquid scintillators; tracking detectors, allowing to restore the full picture of 2b-events.
We intend this Research topic to be a consistent collection of articles covering the DBD topic. There will be review articles on the main theoretical and experimental DBD topics as NME computation with different methods, PSF computation, possible mechanisms for 0νββ, BSM physics in connection with the DBD, connection between DBD and LHC searches, as well as on the main DBD experimental directions and experiments. It is assumed that these will be quite complete reviews, including a historical part, the current state and the prospects of a particular subject. In addition, we also expect contributions reporting interesting recent results on different subjects of the DBD area.
Theoretically, from the DBD study one expects the precise computation of the nuclear matrix elements (NMEs) and phase space factors (PSFs) entering the DBD half-lives formulas, for different decay modes and transitions to final ground or excited states. An accurate computation of these quantities would result in reliable predictions of DBD half-lives and constrains of the BSM parameters appearing in various possible mechanisms that may contribute to the 0νββ decay.The precise computation of the NMEs involved in DBD is a long-standing problem, which is not yet resolved. The existing nuclear methods, used by different groups, still give NME values that differ to much from each other. Precise PSF values also contribute to the accuracy of the DBD calculations and their recent computation performed with improved methods revealed differences as compared with the values obtained previously. Another interesting topic is the contribution of different possible mechanisms to 0νββ decay. From the DBD study, some limits have been extracted of the ratio between the mass, right-left and right-right terms in Lagrangean. On the other hand, the actual increased luminosity at LHC allows the search of same sign dilepton channels in proton proton collisions and in meson and baryon rare decays. These channels are also mediated by heavy mass neutrinos and are activated by the exchange of virtual right-handed W-bosons. Hence, it is interesting to make a correspondence between DBD and LHC investigations in order to improve the constrains on the relative strengths of these terms.
From the experimental side, the first DBD experiments were performed since late 1940s. Until present, significant progress has been made in increasing the sensitivity of such experiments with the goal of searching the 0ν β β decay mode. Now, for three nuclei (76 Ge, 136Xe and 130Te) a sensitivity at the level of 1025-1026 years for the half-lives was reached, which corresponds to a sensitivity in the (Majorana) neutrino effective mass of (0.1-0.3 eV). Further, the next goal is to reach the region (0.015-0.05) eV, predicted in the scheme with inverse hierarchy of the neutrino masses, which would correspond to a sensitivity of ~ 1027-1028 years. To achieve this sensitivity, it will be necessary to build up detectors with hundreds of kilograms of enriched isotopes. Currently, the main directions of experimental research are the following: HPGe detectors made of germanium enriched with the 76 Ge isotope; low-temperature bolometers using TeO2 crystals; low-temperature scintillation bolometers; TPC on liquid and gaseous xenon; detectors on loaded liquid scintillators; tracking detectors, allowing to restore the full picture of 2b-events.
We intend this Research topic to be a consistent collection of articles covering the DBD topic. There will be review articles on the main theoretical and experimental DBD topics as NME computation with different methods, PSF computation, possible mechanisms for 0νββ, BSM physics in connection with the DBD, connection between DBD and LHC searches, as well as on the main DBD experimental directions and experiments. It is assumed that these will be quite complete reviews, including a historical part, the current state and the prospects of a particular subject. In addition, we also expect contributions reporting interesting recent results on different subjects of the DBD area.
Keywords: Double beta decay, BSM, neutrino properties, DBD experiments, beyond SM physics
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