Lattice Qcd for Neutrinoless Double Beta Decay: Short Range Operator Contributions

The Standard Model (SM), the fundamental theory of particle physics, very success- fully describes the world around us, and with a only a few tantalizing exceptions, all the experiments we have performed to understand the fundamental laws of nature. However, the SM accounts for only 4-5% of the matter and energy in the universe, with approxi- mately 25% composed of dark matter (DM) and the remaining 70% composed of the more mysterious dark energy. Further, the SM content of the universe is composed of an excess of matter over anti-matter of about 1 part per billion. Despite being a small excess, it is orders of magnitude larger than can be explained by the SM alone. These observations strongly suggest there is new physics Beyond the Standard Model (BSM). One of the most exciting prospects for searching for physics BSM is 0νββ. Detecting 0νββ is one of the current top scientific priorities of The Nuclear Science Advisory Committee and a new initiative, a ton-scale 0νββ experiment, is described in their Long Range Plan for Nuclear Science [1]. There are many experiments designed world wide to search for evidence of physics BSM, however, the ton-scale search for 0νββ in large nuclei is the most prominent new nuclear physics (NP) experiment. 0νββ, if allowed, is an extremely rare nuclear decay, which violates one of the fundamental symmetries of the Standard Model (SM). There- fore, if observed, 0νββ may provide a possible explanation for the observed abundance of matter over anti-matter in the universe as this lepton number violation could be converted to baryon number violation very early in the universe. 0νββ would happen in a process where two neutrons decay simultaneously into two protons and two electrons but without the emission of any neutrinos. If the neutrinos are their own antiparticles (Majorana-like), the most plausible case, a neutrino emitted from one of the beta-decaying neutrons can be absorbed by the other neutron. This interaction would happen at short distance scales. Thus, a series of calculations based on Quantum ChromoDynamics (QCD), the fundamen- tal theory of nuclear strong interactions, will be required to interpret the results of 0νββ experiments, along with many other NP experiments. However, the only way to perform such calculations to the required accuracy is by using a numerical technique known as lattice QCD (LQCD)