Experimental neutrino physics is focused on the measurements of mass and other properties of neutrinos that may have profound consequences for understanding the evolution of the universe. Over the last few decades, particle physicists have accumulated experimental evidence on the properties of major constituents of matter, including neutrinos, to explain how neutrinos interact with matter and how these ghostly particles propagate over long distances. We have discovered a phenomenon of neutrino oscillation, a quantum mechanical effect that is possible only if neutrinos carry masses and mix among themselves.
Why is this important? With precise knowledge of neutrino mixing, it is now possible to start using neutrinos as a tool to understand why the universe is built from matter rather than antimatter -- one of the fundamental questions of the universe. The idea is to create beams of neutrinos (i.e. particles) and anti-neutrinos (i.e. anti-particles) and measure its mixing properties with detectors away from particle sources.
Argonne has been a leader in neutrino physics from the development of the twelve foot bubble chamber in the 1970s, and through the creation of NuMI beam for the MINOS experiment. Physicists at Argonne National Laboratory have had important roles in discoveries of neutrino oscillation and measurements of mixing parameters. Examples of recent involvement include MINOS and Double Chooz experiments, and the current NOvA (NuMI Off-Axis νe Appearance) experiment.
Neutrino physics is the dominant motivation for the next round of HEP facilities in the US. After NOvA probes neutrino CP-violation and neutrino mass-hierarchy, we plan to perform the final precise determination of these quantities with the DUNE (Deep Underground Neutrino Experiment) experiment, planned to start after the end of the NOvA operation.
Recent experimental data, which show that neutrinos have a mass, are forcing theorists to revise the Standard Model of particle physics. The discovery of the Higgs boson at the Large Hadron Collider is a great success of particle physics, but the Higgs boson may not be enough to explain the hierarchy of fermion masses and the smallness of neutrino masses. A list of important questions in neutrino physics, not in any particular order, include
- What are precise values of oscillation parameters?
- Do neutrinos violate CP symmetry and if so, by how much?
- What is the hierarchy of neutrino masses?
- Is there a sterile neutrino?
- What are the absolute values of neutrino masses?
- Is the neutrino its own anti-particle?
- Can we detect Big Bang relic neutrinos?
- Is the neutrino a component of dark matter?
Understanding neutrino physics is imperative for finding out the relationships between the neutrinos and other elementary particles, particularly when it relates to their relative masses. Because making direct observations of neutrino masses is so difficult, we must use neutrino oscillation experiments and neutrino-less double-beta decay searches to get a better sense of the characteristics of these mysterious particles.