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Was reading some papers and review articles on accelerator based neutrino experiments and this came up a few times. Most of what I could find mentions "shrinkage in neutrino energy spectra" and reducing the effects of electron neutrino impurities in a muon neutrino beam, but remains vauge.
For example, NOvA in the US is mentioned to be located 14 milliradians/12 km away from the beam direction.
I was just wondering what use to these experiments this has?
Off-axis beams, depending on the neutrino energy and the production source, can offer fluxes of neutrinos with a much more narrow range of energies. This is useful because the incoming neutrino energy is unknown on an event by event basis, and the neutrino flux is often a large source of systematic uncertainty in cross section and oscillation analysis.
The physics behind this is summarized in this excellent paper https://arxiv.org/pdf/hep-ex/0111033.pdf by Kirk McDonald. Basically for $\theta$ measured as the angle between the daughter neutrinos momentum and the direction of the parent pion's momentum, there exists a maximum neutrino energy possible. This implies that "many different pion energies contribute to the this neutrino energy, which enhances the neutrino spectrum at this angle-energy combination, θ ≈ (30-50 MeV)/Eν".
One can see this in the plot below from the paper which shows the relative neutrino flux as a function of neutrino energy and off axis angle.
It depends on the particular experiment , and depends on the kinematics of decays. Example here in this link
Also the use of off-axis neutrinos has been considered for NuMI and CNGS as a future option . The off-axis neutrino beams are of interest in case intense low-energy beams are required, e.g. for νµ → νe searches. The highintensity at small angles relative to the direction of the parent π is a peculiarity of the π → ν kinematics relating Eν to the decay angle in the laboratory frame. Of course, this set-up also requires new detector positions. Such a beam is foreseen for nu-JHF from the beginning
