NOνA physics goals

Neutrinos are produced and detected via what is known as the weak interaction. Neutrinos have a charge with respect to the weak force, also known as a "flavor", which is labeled e, μ, and τ. Through a quantum mechanical process called superposition, the neutrino states of define flavor are made by combining the neutrinos states that have definite mass. Mathematically, this is expressed as:

The matrix is specified in terms of the sines (s) and cosines (c) of three angles (θ₁₂, θ₂₃, and θ₁₃) and a single phase, δ. The terms containing the phase δ change sign under the exchange of matter and anti-matter and are said to violate CP symmetry.

The fact that the flavor states are composed from a superposition of mass eigenstates leads to a phenomenon called neutrino oscillations. As neutrinos propagate through space, their flavor content changes as a result of a change in the relative phases of the mass states from which they are composed. Thus, a muon neutrino beam produced at Fermilab, may contain electron and tau neutrinos after the beam has been allowed to propagate several kilometers. The effect is illustrated in the figure below.

The flavor content of the neutrino beam used by NOνA. At the source, the neutrino beam is composed almost entirely of muon neutrinos (green). As the beam propagates away from the source, oscillations increase the tau (blue) and electron (red) content of the beam. By roughly 1000 km the beam is entirely composed of electron and tau neutrinos. The relatively pure muon neutrino beam is recovered at 2000 km, and the process repeats. NOνA is located at L=810 km, near the peak in the electron neutrino oscillation.

Much is known already about the neutrino states and their mixing. Studies of neutrinos produced in the Sun and on the Earth at nuclear reactors, tell us that θ₁₂ = 35^o and m₂²-m₁² = +80 μeV². Studies of neutrinos produced in the Earth's atmosphere and at particle accelerators (like the MINOS experiment at Fermilab) tell us that θ₂₃ = 45^o and |m₃²-m₂²| = 2.4 meV². The remaining unmeasured angle, θ₁₃, is the main target of the NOνA experiment. A glance at the matrix above will reveal that if this remaining angle is not zero, then there is a roll for the CP-violating phase δ to play in the neutrino mixing matrix, and hence, there is a possibility that neutrino mixing violates matter/anti-matter symmetry. This is an exciting possibility as it may be connected to the overall matter/anti-matter symmetry which exists in the universe today. Another unknown in this picture is the ordering of the neutrino mass states. It is possible that m₃ is the lightest of the heaviest neutrino state. This question, referred to as the mass hierarchy, can be addressed by NOνA and is key to understanding future neutrino oscillation results and an important feature that distinguishes theoretical models of neutrino mass and mixing.