NOvA

The NOνA (NuMI Off-Axis ν_e Appearance) experiment is a particle physics experiment designed to detect neutrinos in Fermilab's NuMI (Neutrinos at the Main Injector) beam. Intended to be the successor to MINOS, NOνA consists of two detectors, one at Fermilab (the near detector), and one in northern Minnesota (the far detector). Neutrinos from NuMI pass through 810 km of Earth to reach the far detector. NOνA's main goal is to observe the oscillation of muon neutrinos to electron neutrinos. The primary physics goals of NOvA are:

Neutrino oscillation is parameterized by the PMNS matrix and the mass squared differences between the neutrino mass eigenstates. Assuming that three flavors of neutrinos participate in neutrino mixing, there are six variables that affect neutrino oscillation: the three angles θ₁₂, θ₂₃, and θ₁₃, a CP-violating phase δ, and any two of the three mass squared differences. There is currently no compelling theoretical reason to expect any particular value of, or relationship between, these parameters.

θ₂₃ and θ₁₂ have been measured to be non-zero by several experiments but the most sensitive search for non-zero θ₁₃ by the Chooz collaboration yielded only an upper limit. In 2012, θ₁₃ was measured at Daya Bay to be non-zero to a statistical significance of 5.2 σ. The following year, T2K discovered the transition ${\displaystyle \nu _{\mu }\rightarrow \nu _{e}}$ excluding the non-appearance hypothesis with a significance of 7.3 σ. No measurement of δ has been made. The absolute values of two mass squared differences are known, but because one is very small compared to the other, the ordering of the masses has not been determined.

NOνA is an order of magnitude more sensitive to θ₁₃ than the previous generation of experiments, such as MINOS. It will measure it by searching for the transition ${\displaystyle \nu _{\mu }\rightarrow \nu _{e}}$ in the Fermilab NuMI beam. If a non-zero value of θ₁₃ is resolvable by NOνA, it will be possible to obtain measurements of δ and the mass ordering by also observing ${\displaystyle {\bar {\nu }}_{\mu }\rightarrow {\bar {\nu }}_{e}.}$ The parameter δ can be measured because it modifies the probabilities of oscillation differently for neutrinos and anti-neutrinos. The mass ordering, similarly, can be determined because the neutrinos pass through the Earth, which, through the MSW effect, modifies the probabilities of oscillation differently for neutrinos and anti-neutrinos.

The neutrino masses and mixing angles are, to the best of our knowledge, fundamental constants of the universe. Measuring them is a basic requirement for our understanding of physics. Knowing the value of the CP violating parameter δ will help us understand why the universe has a matter-antimatter asymmetry. Also, according to the Seesaw mechanism theory, the very small masses of neutrinos may be related to very large masses of particles that we do not yet have the technology to study directly. Neutrino measurements are then an indirect way of studying physics at extremely high energies.

In our current theory of physics, there is no reason why the neutrino mixing angles should have any particular values. And yet, of the three neutrino mixing angles, only θ₁₂ has been resolved as being neither maximal or minimal. If the measurements of NOνA and other future experiments continue to show θ₂₃ as maximal and θ₁₃ as minimal, it may suggest some as yet unknown symmetry of nature.

NOνA can potentially resolve the mass hierarchy because it operates at a relatively high energy. Of the experiments currently running it has the broadest scope for making this measurement unambiguously with least dependence on the value of δ. Many future experiments that seek to make precision measurements of neutrino properties will rely on NOνA's measurement to know how to configure their apparatus for greatest accuracy, and how to interpret their results.

An experiment similar to NOνA is T2K, a neutrino beam experiment in Japan similar to NOνA. Like NOνA, it is intended to measure θ₁₃ and δ. It will have a 295 km baseline and will use lower energy neutrinos than NOνA, about 0.6 GeV. Since matter effects are less pronounced both at lower energies and shorter baselines, it is unable to resolve the mass ordering for the majority of possible values of δ.