
Experimental groups worldwide are working to develop detectors that may allow the observation of neutrinoless doublebeta decay in isotopes such as 76Ge, 136Xe, 130Te, and 82Se. Observation would have far reaching consequences: it would prove that lepton number is violated, demonstrate that neutrinos are Majorana particles, and provide information about the neutrinomass generation mechanism and possibly about baryogenesis. The decay rate, however, depends on matrix elements that cannot be measured. These depend in turn on complex nuclear structure but may also reflect particle and hadronic physics since the decay can proceed through the exchange of either a light Majorana neutrino or exotic stillundiscovered heavy particles, in which case both particle and hadronic physics will be involved. Clearly, in order to draw conclusions about the mechanism and parameters of lepton number violation from either a positive or null experimental result, one must compute the matrix elements with reasonable accuracy.
This fiveweek program brought together experts in nuclear structure, lattice QCD, hadronic physics, BSM physics, and experiment to address outstanding problems associated with the matrix elements that govern neutrinoless doublebeta decay. The program hosted three intense twoday workshops, two on its own and one through the independent workshop 1767W on lattice QCD. The opening workshop brought experimentalists together with theorists to discuss all the issues associated with doublebeta decay. The workshop in week two, a meeting of the Topical DOE Nuclear Theory Collaboration for DoubleBeta Decay and Fundamental Symmetries, featured reports on nearly the entire range of abinitio manybody methods.
Talks and discussions during the program summarized progress along several lines of research.
The major issues and questions included:
What are the broader implications of neutrinoless doublebeta decay searches for new physics scenarios? In particular, several presentations discussed the interplay with other neutrino experiments, cosmology, and highenergy probes of lepton number violation.
Since neutrinoless doublebeta decay involves a number of quite distinct physical scales (the scale of lepton number violation, the scale of weak interactions, the hadronic and nuclear scales), effective field theory (EFT) can be used to organize the calculation of matrix elements. A number of talks discussed the EFT approach to doublebeta decay, both in the context of heavy particle and lightneutrino physics, and the interface of nuclear structure and lattice QCD.
Thus far abinitio nuclearstructure calculations have dealt mainly with the Hamiltonian and spectra. Several talks and discussions focused on plans to compute processes such as twoneutrino double beta decay, single beta decay and neutrinonucleus scattering, all of which are more closely related to neutrinoless doublebeta decay.
One of the largest sources of uncertainty in matrixelement calculations goes by the sobriquet "the renormalization of gA," i.e. a tendency to overpredict βdecay and two neutrino doublebeta decay matrix elements. In modern language any quenching must be due to either neglected correlations or manybody currents. The big question is then: does whatever is responsible for this "gA quenching" also quench neutrinoless doublebeta decay?
At the program preliminary results were presented indicating that twobody currents do not quench gA in light nuclei, but may in heavier ones. Ab initio nuclear theory and chiral EFT will soon give us the ability to provide a complete answer to the big question, at least in light nuclei.
Matrixelement calculations must be tested and benchmarked. Concrete suggestions were put forward for comparing calculations of matrix elements results fro single and doublebeta decay in light nuclei (that do not actually undergo observable double beta decay).
Finally, it was gratifying for the organizers to see that a number of scientists who had only thought briefly about doublebeta decay or were just beginning (or considering) research projects on the subject came away with much clearer ideas about how to proceed, and with a thorough education. They also saw collaborations form between experts in heavy nuclei and in light nuclei, and between manybody and hadronic theorists, to address (for example) the "quenching of gA" and possible manybody enhancement of subleading operators in chiral effective field theory. The interaction and collaborations fostered by this INT program should bear fruit in the years to come.
