Executive Summary

1 Executive Summary

On Dec. 1, 2001, Associate Laboratory Director Tom Kirk appointed a BNL based neutrino physics study group. Its charge was to examine future forefront neutrino oscillation experiments that could be carried out using traditional ν_µ (anti-ν_µ) beams of exceptional intensity (super beams) from an upgraded AGS. The study, as reported in this document, addressed detector distances, sizes and technologies as well as novel ideas for cost effective beam lines and AGS upgrade paths. Most important, it focused on the physics discovery and study potential in its assessment of various options.

Given the success of solar and atmospheric neutrino studies in discovering neutrino oscillations and measuring some mixing and mass parameters, it became clear that the next generation accelerator based neutrino oscillation program must be very ambitious. In addition to improving measurements of already approximately known Δ m_ij² = m_i² - m_j² and the large mixing angles θ₂₃ and θ₁₂, the next major effort should be capable of determining the as yet unknown mixing angle θ₁₃, the mass hierarchy of neutrinos and the phase δ_CP. Together these will provide a measure of CP violation in the lepton sector via the Jarlskog invariant

J_CP =

sin2 θ₁₂ sin2 θ₂₃ sin2 θ₁₃cosθ₁₃ sinδ

Indeed, CP violation is properly viewed as the Holy Grail of neutrino oscillations, since it may be closely connected with the matter-antimatter asymmetry of the universe.

In order to cover a significant region of the allowed θ₁₃ parameter space (sin² 2 θ₁₃ ≤ 0.2, 0≤ δ ≤ 2π), to allow for the determination of the mass ordering to the three neutrinos and the possible observation of CP violation a very large detector of approximately 500 kton, a long baseline (≥2000 km) and an intense proton source of 1 megawatt are necessary. For that reason, our studies concentrated primarily on a water Cherenkov detector where the required technology is mature and capable of achieving the required large tonnage. The technical performance of the water detectors has also been fully demonstrated in the relevant event energy ranges. Similarly, a relatively simple cost effective AGS upgrade that primarily increases the repetition rate was examined. Such a large water Cherenkov detector could also be used to search for proton decay, supernova neutrinos, n anti-n oscillations, etc. It could also be used to significantly improve measurements of atmospheric neutrino oscillations. Indeed, an extremely attractive picture that emerged from our studies was a very large multi-physics water Cherenkov detector with outstanding discovery potential in many frontier areas of physics as well as a robust guaranteed program of detailed studies and precise measurements.

In this report, we describe our vision of the very long baseline neutrino oscillation experimental component of that program. It assumes that a 500 kton or larger water Cherenkov detector will be built somewhere in the USA perhaps as a major component of a National Underground Lab and its distance from BNL will be considerable, e.g.. BNL-Homestake (2540 km) or BNL-WIPP (2900 km). To have a sufficient number of detected neutrino events at that distance, a 1 MW AGS proton source (currently the AGS has 0.14 MW of power) is envisioned with targetry focusing and a decay tunnel capable of providing an intense wide band neutrino beam (at 0 degree production) with good support in the 0.5 ≤ E_ν≤ 7 GeV energy range.

The experimental specifications described above were originally chosen with the idea of measuring the CP violating parameter δ via ν_µ→ ν_e oscillations. However, during the course of our studies, it became clear that such an effort has a much richer and more diverse physics program. Indeed, in the scenario we have studied in detail (BNL-Homestake), two measurements, ν_µ disappearance oscillations detected via muon events and ν_µ→ ν_e appearance oscillations via electron events together provide a wealth of information.

During the initial research program, a run of 5 × 10⁷ sec (probably distributed over 5 years), the ν_µ disappearance study will resolve several oscillation maxima and minima (thus firmly establishing oscillations) and measure Δ m₃₂² to 1% or better and sin² 2 θ₂₃ to 1% or better, significant improvements over existing or planned measurements. In the ν_µ→ ν_e appearance mode, the ν_e + n → e^- + p quasi-elastic events over the 0.5 GeV range will allow the following investigations to be completed:

Search for and measurement of sin² θ₁₃ to below 0.005 via matter enhanced oscillations.
Determine the sign of Δ m₃₁², i.e. whether m₃ is the largest or smallest of the 3 neutrino masses, also via matter enhancement or suppression effects in the 3-7 GeV region.
Measure sinδ (and cosδ) to about ± 25% thus determining J_cp and the δ quadrant.
Measure Δ m₂₁² and θ₁₂ from the ν_µ→ ν_e oscillations of low energy 0.5-1.0 GeV neutrinos with about the same sensitivity as Kamland, but in an appearance rather than disappearance mode.

The above program is extremely rich, covering essentially all the parameters of 3 generation neutrino mixing as currently envisioned. It is also robust, offering important measurements even if some parameters whose values we have assumed in our calculations change significantly. Together with the search for proton decay and study of cosmic neutrinos, our accelerator based long baseline neutrino oscillation program represents a major step forward in the advancement of science. Beyond the first research period, one could envision further accelerator and beam upgrades, antineutrino runs, or additional beams from other accelerator facilities. Indeed, the large detector that forms the centerpiece of this effort should be expected to function for half a century or more expanding our knowledge of all the above noted research areas.

This report will show that the bold program envisioned above is technically feasible and economically attractive. We show that the existence of the AGS machine at BNL with its straightforward and economical upgrade to the needed 1 MW power level, taken together with the needed very long baseline available for at least two appropriate detector sites, makes this approach to a practical facility the best one for the next-generation U.S. neutrino physics program. The identified physics goals are compelling and not covered by less ambitious alternatives. Nevertheless, its realization will require strong commitment and vision. The high payoff is worth the effort.