The standard model of particle physics, developed around 1973, is a theory describing three of the four known fundamental interactions between the particles that make up all matter. The theory is a description of the electromagnetic, weak, and strong force interactions, mathematically represented in a consistent framework combining special relativity and quantum field theory. Nearly all experimental tests of the three forces described by the standard model have agreed with the model's predictions. However, despite the immense success of the standard model, the theory is less than satisfactory in two aspects. The first is the lack of inclusion of the fourth known interaction, the gravitational force described by general relativity. The second more intrinsic limitation of the standard model is that it contains many parameters that are not derived from first principles. Weak interaction physics research has a distinct role in confronting the grand challenge of moving beyond the standard model of particle physics and addressing these two limitations of the theory. The PNNL program, capitalizing on expertise in ultra-low background radio-assay and ultra-pure materials, will expand the opportunities for future measurements of ultra-rare processes studied by the weak interaction field of physics.
The standard model of particle physics is under pressure from three directions studied by weak interaction physics. These three are the discovery of massive neutrinos, the mystery of cosmological dark matter, and the theoretical extension to super-symmetric models of particle physics. Each of these three research areas has the potential to help answer the grand challenge of taking us beyond the standard model, and each of the three research areas is addressed by experiments in weak interaction physics. Furthermore, each of these three topic areas will contribute in a distinct way.
In the last couple decades, the standard model has been challenged experimentally by the discovery of neutrino oscillations of solar, atmospheric, accelerator and reactor neutrinos. The neutrino oscillation behavior, the propensity to alter their preferred interaction process, proved neutrinos are massive particles. With the realization that neutrinos are massive, there is an increased interest in investigating their intrinsic properties. Understanding the neutrino mass generation mechanism, the absolute neutrino mass scale and the neutrino mass spectrum are some of the main focuses of future neutrino experiments. Lepton number is conserved in neutrino interactions in the standard model because neutrinos are assumed to be massless, and there is no chirally right-handed neutrino state. The guiding principles for extending the standard model are the conservation of electroweak isospin and renormalizability, which do not preclude each neutrino mass eigenstate to be identical to its own anti-particle, called a Majorana particle.