Appendix D: Neutrino science with 8 GeV and 800 GeV protons
This section lists some experiments with neutrino beams that could be carried out at proton facilities. Possible long-baseline programs for neutrino oscillation and CP violation are not discussed here.
Neutrino-science experiments with 8 GeV protons
The excess of low-energy electron-neutrino-like events recently observed by MiniBooNE could arise either from new physics, not compatible with simple two-flavor oscillations, or from a new kind of background that is of importance for oscillation experiments operating in this energy range. An experiment dubbed microBooNE with excellent low-energy sensitivity provided by a liquid argon time projection chamber is proposed to study individual final states producing events in the region of excess. This experiment would also be an extremely valuable step in demonstrating the effectiveness of LArTPCs for sensitive discrimination of backgrounds to neutrino interactions. If the experiment is sited in the MINOS surface building, it would be exposed to the Booster neutrino beam to accomplish microBooNE. It would also be exposed to a far off-axis NuMI beam, providing useful study of low-energy neutrinos, although it may be desirable to have a LAr detector down in the NuMI tunnel to act as a NOνA near detector. Both detector sites would produce useful neutrino scattering measurements relevant for oscillation physics, as well as scattering measurements of relevance for nuclear physics. Smaller scale LAr experiments like this one can provide very useful experience toward potential long-baseline detectors.
The strange quark contribution to nucleon spin (Δs) can be extracted from neutral current elastic scattering in the Booster neutrino beam with higher precision and less model dependence than in deep-inelastic scattering measurements. In addition to providing the strange quark piece of the proton spin puzzle, the Δs measurement has cosmological implications, as NC-elastic interactions dominate in core-collapse supernovae. At present, Δs results from polarized, inclusive, lepton deep-inelastic scattering and from semi-inclusive leptonic deep-inelastic scattering are not consistent with each other. Although, given additional run time beyond that currently approved, the SciBooNE experiment could better measure the ratio of NC-elastic scattering to charged-current scattering events, a fully sensitive experiment might require detector upgrades to SciBooNE. Required sensitivity is currently being studied.
Neutrino-nucleus cross-sections in the low energy (tens of MeV) regime for a number of nuclear targets pertinent to the process of supernova core collapse can be studied using a neutrino beam generated from stopped pions produced by very intense proton beams of 1-2 GeV, and an experiment similar to NuSNS at the Spallation Neutron Source. In addition, coherent elastic scattering of neutrinos off nuclear targets could possibly be measured, providing a precision test of the Standard Model not possible at the SNS because of neutron backgrounds.
Neutrino-science experiments with 800 GeV protons
Exciting experiments using high-energy neutrinos produced in a Tevatron fixed-target neutrino beam line could be performed if sufficient 120 GeV protons from the Main Injector are available to feed both the long-baseline neutrino program and the Tevatron. For example, a precision measurement of the weak mixing angle θW using muon-neutrino scattering on electrons performed with a high-energy neutrino beam could probe Beyond the Standard Model physics in a way complementary to other electroweak measurements. Tension that presently exists in global electroweak fits perhaps hints at beyond Standard Model effects. Only measurements of the invisible width of the Z in electron-positron collisions probe the Standard Model in the same way. Such a measurement of θW could be performed by an experiment dubbed NuSOnG that would utilize a new spectrometer in a pure muon-neutrino or muon-antineutrino beam generated by 800 GeV protons from the Tevatron with a sign-selected quadrupole train. A measurement of sin2θW in the scattering of neutrinos off electrons to 0.7 percent could be produced with 2×1020 protons on target. Such an experiment could not be performed by any other neutrino beam at Fermilab, CERN or J-PARC.
Upgrade to the Fermilab proton facility
During the era of NOνA operations, neutrino experiments in Booster or Tevatron neutrino lines cannot be supported without compromising NOνA physics, unless upgrades are made to the Fermilab proton accelerator complex. The SNuMI upgrade would increase the sensitivity and physics reach of the NOνA program. It would also increase the competitiveness of NOνA with its contemporary neutrino-oscillation experiments. However, SNuMI could not simultaneously provide adequate 8 GeV beam power for experiments such as microBooNE or SciBooNE with upgrades. A precision electroweak neutrino experiment, such as NuSOnG, would require about 5 percent of the SNuMI 120 GeV beam power. Project X, on the other hand, would provide ample proton beam power to provide both a greater than three-fold increase in 120 GeV beam power for NOνA and future long-baseline experiments and more than ample 8 GeV beam power for neutrino experiments. The 120 GeV beam power available with Project X would also allow operation of a Tevatron fixed-target neutrino line without noticeable impact on the long-baseline neutrino program. Thus, Project X would enable a program of neutrino experiments that would not otherwise be feasible, while greatly enhancing the physics reach of long-baseline neutrino-oscillation experiments.