Fermilab Steering Group Report

Chapter 4
Physics at the
Intensity Frontier

Neutrino science

An upgrade to the Fermilab proton complex could greatly enhance the laboratory's current world-class program of neutrino science by strengthening Fermilab's flagship program of long-baseline neutrino-oscillation experiments. It would provide for a next-generation experiment to discover CP violation in the leptonic sector, and consequently to explore leptogenesis as the source of matter-antimatter asymmetry in the evolution of the universe. The upgrade would also provide an opportunity for new, smaller-scale experiments using intense neutrino beams generated by 8 GeV and 800 GeV protons that would complement the long-baseline program.

Sensitivity to mass hierarchy
Ability of NOνA and NOνA plus T2K experiments to resolve mass hierarchy at 95 percent confi dence level.

Sensitivity to mass hierarchy
Ability to resolve mass hierarchy at 95 percent confidence level (dotted lines) and 3σ (solid lines) of potential future Fermilab experiments. See details in Appendix C.

Long-baseline neutrino oscillations

The Neutrino Scientific Assessment Group, convened by HEPAP and the Nuclear Science Advisory Committee, and a study group originally commissioned by Fermilab and Brookhaven National Laboratory have recently studied and documented the physics opportunities of long-baseline neutrino experiments. As laid out by NuSAG, this accelerator- based program has as its primary goals to complete our understanding of neutrino mixing and oscillations, in particular to determine the ordering and splitting of the neutrino mass states, to measure the mixing angles and to determine whether there is CP violation in neutrino mixing1. The study of CP violation in neutrino oscillations is especially compelling because CP violation in the leptonic sector may help explain the very fundamental problem of the matter-antimatter asymmetry of the universe through the process known as leptogenesis. Together, the Japanese T2K experiment and NOνA will begin to explore CP violation in neutrinos. Discovering the ordering of the neutrino mass states—the mass hierarchy—will help determine whether neutrino mass is related to the unification of the forces and whether neutrino oscillations violate CP symmetry. It may be key to interpreting the outcome of neutrinoless double-beta-decay experiments. Provided that neutrino mixing is large enough, the ability of the NOνA experiment to determine the ordering of the neutrino mass states makes the U.S. long-baseline neutrino program unique in the world.

Experiments to address these neutrino science goals will require both powerful beams and very large detectors, with the product of beam power and detector mass more than an order of magnitude larger than NOνA-generation experiments. Such "Phase II" experiments will require intense muon neutrino beams, regardless of detector technology and regard- less of whether the detector has an off-axis or wide-band beam energy configuration. The discovery potential of these experiments will greatly benefit from higher proton beam power (thus higher neutrino flux) and greater flexibility of beam energy than is presently planned.

The proposed Project X would provide 2 MW or more in the range of proton energy between 50 GeV and 120 GeV. Compared to the NuMI proton plan for NOνA, it would supply approximately a factor of seven higher power at 50 GeV and a factor of three higher power at 120 GeV. Project X's flexible beam energy and higher beam power, combined with the longer baselines of Phase II oscillation experiments such as those at the proposed Deep Underground Science and Engineering Laboratory, 1300 km from Fermilab, would confer impressive sensitivity to the neutrino mass hierarchy and CP violation.

A detector for a Phase II neutrino oscillation experiment, if located in the National Science Foundation's DUSEL, would also be a world-class detector for proton decay, addressing the question Do all the forces become one? This detector could also perform high-statistics studies of atmospheric neutrinos and carry out astrophysical searches including detection of relic-supernova neutrinos and neutrino bursts from supernovae in our galaxy and nearby.

Sensitivity to CP violation
Sensitivity to CP violation at 3σ confidence level of potential future Fermilab experiments. See details in Appendix C.

Proton beam power
Beam power versus beam energy for possible proton facilities at Fermilab.

The physics reach and competitiveness of the near-term NOνA experiment would also improve with SNuMI, an upgrade of NuMI that would increase 120 GeV proton power to 1.2 MW. (SNuMI's beam power with 50 GeV protons would be approximately 400 kW.) SNuMI would support a neutrino program that would be both competitive and complementary to the T2K program based on the Japanese Proton Accelerator Research Complex. The SNuMI beam power is roughly 60 percent higher than that planned for Phase I of the J-PARC facility, and would remain competitive at least through the latter half of the next decade, depending on upgrades undertaken at J-PARC. Project X would markedly increase NOνA's sensitivity to the mass hierarchy and, with a Phase II experiment, would likely exceed the capabilities of the J-PARC facility (see Appendix C).

Neutrino physics with 8 GeV and 800 GeV protons

The Booster neutrino beam generated by 8 GeV protons offers opportunities for neutrino studies beyond the existing experiments MiniBooNE and SciBooNE. In addition, experiments using high-energy neutrinos produced in a Tevatron fixed-target neutrino beam line would become possible if the Main Injector can provide sufficient 50-120 GeV protons both to feed the long-baseline neutrino program and to generate 800 GeV protons in the Tevatron. Some possible future experiments (see Appendix D) include:

Using 800 GeV protons
  • an experiment to precisely measure the weak mixing angle.
Using 8 GeV protons
  • an experiment to study low-energy neutrino interactions for neutrino-oscillation experiments such as MiniBooNE, NOνA and T2K, and to develop liquid-argon detector technology,
  • an experiment to measure the strange quark contribution to the nucleon "spin."

The ability to conduct these experiments depends on the flexibility of the accelerator complex. Beam power at 8 GeV is currently available for Booster neutrino experiments, because NuMI cannot use all Booster pulses for the long-baseline neutrino program. This situation will continue with the NOνA program. The SNuMI design, however, will be capable of using all Booster pulses for running NOνA at higher intensity, leaving none for neutrino experiments at the Booster. Alternatively, the Booster neutrino beam can run simultaneously with SNuMI at a tax of approximately 15 to 20 percent on the NuMI beam. Project X, on the other hand, could deliver substantial 8 GeV beam power (an order of magnitude more than is currently available) to experiments without a tax on NuMI. An experiment with an 800 GeV proton beam would impose approximately a five percent tax on NuMI for both Project X and SNuMI. Proton-source upgrades, particularly Project X, make possible a stronger neutrino-science program.

1 The long-baseline program could also unveil exotic effects due to sterile neutrinos, extra dimensions and dark energy.