NOνA
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NOνA (NuMI Off-Axis νe Appearance) is a proposed 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 will consist of two detectors, one at Fermilab (the near detector), and one in northern Minnesota (the far detector). Neutrinos from NuMI will pass through 810km of Earth to reach the far detector. NOνA's main goal is to observe the oscillation of muon neutrinos to electron neutrinos. By observing how many neutrinos change from one type to the other, NOνA hopes to accomplish three things:
- Measurement of the mixing angle θ13
- Measurement of the CP-violating phase δ
- Determination of the neutrino mass hierarchy
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[edit] Physics goals
[edit] Primary goals
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 free 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.
θ23 and θ12 have been measured to be non-zero by Super-Kamiokande and SNO, but the best measurement of θ13, by CHOOZ, is only an upper limit. 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 will be an order of magnitude more sensitive to θ13 than any previous detector. It will measure it by searching for the transition 
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 
δ can be measured because it modifies the probabilites of oscillation in opposite ways for neutrinos and anti-neutrinos. The mass ordering, similarly, can be determined because of the fact that the neutrinos pass through the Earth, which, through the MSW effect, modifies the probabilites of oscillation differently for neutrinos and anti-neutrinos.[1]
[edit] Importance
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.[1]
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.[1]
[edit] Relationship to other future experiments
NOνA can potentially resolve the mass hierarchy because it has a very long baseline. It is the only experiment likely to run in the near future that has this ability. Many future experiments that seek to make precision measurements of neutrino properties will rely on NOνA's measurement to know how to interpret their results. One such experiment 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 295km baseline and will use lower energy neutrinos than NOνA, about 0.8GeV. Since matter effects are less pronounced both at lower energies and shorter baselines, it will be unable to resolve the mass ordering.[2]
Neutrinoless double beta decay experiments will also benefit from knowing the mass ordering, since the mass hierarchy affects the theoretical lifetimes of this process.[1]
Reactor experiments also have the ability to measure θ13. While they cannot measure δ or the mass ordering, their measurement of the mixing angle is not dependent on knowledge of these parameters. One such proposed experiment is Daya Bay, to be located at the Daya Bay reactor in Hong Kong, wich will use a 2km baseline optimized for observation of the first θ13-controlled oscillation maximum.[3]
[edit] Secondary goals
In addition to its primary physics goals, NOνA will be able to improve upon the measurements of the already measured oscillation parameters. NOνA, like MINOS, is well suited to detecting muon neutrinos and so will be able to able to refine our knowledge of θ23.
The NOνA near detector will be used to conduct measurements of neutrino interaction cross sections which are currently not known to a high degree of precision. Its measurements in this area will complement other similar upcoming experiments, such as MINERνA, which also uses the NuMI beam.[4]
Since it is capable of detecting neutrinos from galactic supernovas, NOνA will form part of the Supernova Early Warning System. Supernova data from NOνA can be correlated with that from SuperKamiokande to study the matter effects on the oscillation of these neutrinos.[1]
[edit] Design
To accomplish its physics goals, NOνA needs to be efficient at detecting electron neutrinos, which are expected to appear in the NuMI beam (originally made only of muon neutrinos) as the result of neutrino oscillations.
The proposed design is a pair of finely grained liquid scintillator detectors. The near detector will be at Fermilab and will sample the unoscillated beam. The far detector will be in northern Minnesota. The far detector will consist of about 600,000 4cm × 6cm × 16m cells filled with liquid scintillator. Each cell will have a loop of bare fiber optic cable to collect the scintillation light, both ends of which lead to an avalanche photodiode for readout. The near detector will have the same general design, but will only be about 1/200 as massive.
Previous neutrino experiments, such as MINOS, have reduced backgrounds from cosmic rays by being underground. However, NOνA will be on the surface and will rely on precise timing information and a well-defined beam energy to reduce backgrounds. It will be situated 810km from the origin of the NuMI beam and 14 milliradians (12km) west of the beam's central axis. In this position, it will sample a beam that has a much narrower energy distribution than if it were centrally located, further reducing the effect of backgrounds.[1]
[edit] References
- ^ a b c d e f "NOνA Proposal to Build a 30 Kiloton Off-Axis Detector to Study Neutrino Oscillations in the Fermilab NuMI Beamline", D.S. Ayres, et al. (NOνA Collaboration), arXiv:hep-ex/0503053
- ^ "Letter of Intent: Neutrino Oscillation Experiment at JHF", The T2K Collaboration, 21 Jan 2003.
- ^ "Daya Bay Neutrino Experiment", J. Cao, arXiv:hep-ex/0509041
- ^ "MINERvA: a dedicated neutrino scattering experiment at NuMI", K. McFarland (MINERνA collaboration), arXiv:physics/0605088
