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Nusag Recommendations On Recommendations to the Department of Energy and the National Science Foundation on a U.S. Program of Reactor- and Accelerator-based Neutrino Oscillation Experiments Report to the High Energy Physics Advisory Panel and the Nuclear Science Advisory Committee Submitted by the Neutrino Scientific Assessment Group February 28, 2006 1 Contents 1 An experimental program in neutrino oscillations.............................................. 3 2 The science of neutrino oscillation ..................................................................... 4 2.1 The goals..................................................................................................... 4 2.2 What we have learned so far....................................................................... 5 2.3 The importance of the open questions ........................................................ 6 2.4 How the questions can be answered ........................................................... 8 3 NuSAG and the process leading to this report.................................................. 11 4 The experiments................................................................................................ 13 4.1 Accelerator experiments ........................................................................... 13 4.2 Reactor experiments.................................................................................. 18 5 Conclusions....................................................................................................... 24 5.1 Global Conclusions................................................................................... 24 5.2 Accelerator................................................................................................ 25 5.3 Reactor ...................................................................................................... 26 6 Recommendations for the U.S. program in neutrino oscillations..................... 27 6.1 General recommendations ........................................................................ 27 6.2 Recommendations on accelerator-based experiments .............................. 27 6.3 Recommendations on reactor experiments ............................................... 28 Appendix A: The NuSAG Charge............................................................................ 29 Appendix B: Members of the DOE/NSF Neutrino Scientific Assessment Group (NuSAG)........................................................................................................................... 31 Appendix C: Agenda of the Open NuSAG Meeting ................................................ 32 References.................................................................................................................. 33 Sections 5, Conclusions, and 6, Recommendations, can be read as a summary of the report. 2 1 An experimental program in neutrino oscillations During the past decade, we have found compelling evidence that neutrinos can oscillate between one “flavor” and another, which implies that they have nonzero masses. The Standard Model (SM) of particle physics is constructed with massless neutrinos. Though a more fundamental theory is desired, the SM, which has been under experimental challenge since its full formulation in the 1970’s, has proved to be consistent with all experimental data — until massive neutrinos. Thus, neutrino experiments are the only demonstrated experimental window on physics beyond the Standard Model. Neutrino oscillation also implies that neutrinos mix, just as quarks do. As with the mixing of quarks, the mixing of three or more flavors of neutrino can generate a violation of the combined symmetry of the exchange of particles with antiparticles (Charge conjugation) and mirror inversion (Parity). This phenomenon, called “CP violation,” is a key requirement of theories that try to generate the imbalance of matter over antimatter observed in the current universe from an initially symmetric state. In neutrino oscillation, the presence and magnitude of CP violation are controlled by two as-yet-unmeasured parameters. The discovery of neutrino mass and mixing has raised a number of intriguing questions. Especially interesting are several qualitative questions about the nature of the neutrinos and their relations to the rest of physics and to astrophysics-cosmology. The new questions, and how they may be answered through future experiments, are discussed in Section 2. A global effort to pursue the answers has come up with a well-developed experimental program that, subject only to the constraints imposed by the magnitudes of the unknown parameters, can completely fill out our knowledge of three-neutrino mixing. There is a striking convergence in this program: access to all the relevant physics, including CP violation, comes from studying ν μ → νe,ν μ → ν e, and ν e → Not ν e flavor transitions at or near the peak of an expected oscillating contribution with a wavelength equal to that of the observed oscillation of atmospheric neutrinos. The proposed program assumes that Nature contains only the three known neutrino types. With one unconfirmed exception, the LSND experiment, all present observations are consistent with this assumption. However, we must be alert to surprises. Confirmation of the LSND oscillation evidence would require re-evaluation of the interpretation and priority of the proposed experiments. The LSND result is currently being tested, and it is anticipated that resolution of this issue will take place early enough to allow such re-evaluation if needed. The United States has been a leader of the field of neutrino oscillations from the beginning, discovering the neutrino flavors themselves and the first evidence for solar neutrino disappearance, later confirmed as neutrino oscillations. U.S. physicists have played important roles in the milestone experiments Super-K, KamLAND, and K2K in Japan and SNO in Canada. It is likely that the next important neutrino oscillation results will come from two current U.S.-based experiments, MiniBooNE and MINOS, both at Fermilab. This leadership represents a wealth of experience and a substantial investment, which together position the U.S. program to continue its leading role. 3 NuSAG, the Neutrino Scientific Assessment Group, has been charged by the High Energy Physics Advisory Panel and the Nuclear Science Advisory Committee of the Department of Energy and National Science Foundation “to make recommendations on the specific experiments that should form part of the broad U.S. neutrino science program.” One of the charges, dealing with a program in neutrino-less double beta decay, was the subject of NuSAG’s first report.1 The experiments before NuSAG under the remaining two charges, both of which are addressed in this report, are proposals for U.S. participation in the next phase of the worldwide program in neutrino oscillations. This program includes both accelerator- and reactor-based experiments, and its goal is the complete exploration of three-neutrino mixing. In the next section, we describe the physics of neutrino oscillations and the specific aims of the worldwide program. Section 3 describes the charge to NuSAG in more detail and reviews the procedures NuSAG followed to arrive at the recommendations in this report. The experiments NuSAG considered are discussed in Section 4. NuSAG’s conclusions are reported in Section 5, and our recommendations for a U.S. program in neutrino oscillations are presented in the final section. NuSAG strongly endorses the scientific goals of the worldwide program and recommends that the U.S. mount key experiments necessary to advance this science. 2 The science of neutrino oscillation 2.1 The goals Through future experiments on neutrino oscillation, we would like to answer the following questions: 1. What is the approximate size of the small mixing angle θ13? With what kinds of underlying theories is it compatible? 2. Is the atmospheric mixing angle maximal? If not, is the heaviest neutrino more ντ than νμ , as naively expected, or more νμ than ντ ? 3. Does the neutrino mass spectrum resemble the charged lepton and quark spectra, or is it an inverted version of those other spectra? 4. Do neutrinos violate CP? In the past few years, dramatic insights into the nature of neutrinos have been gained from experiments with naturally-occurring neutrinos. Answering these new questions will require experiments with man-made neutrinos, from accelerators and reactors. The other critical questions of neutrino physics, whether neutrinos are their own antiparticles and the absolute scale of neutrino mass, are not addressed by oscillation experiments, but by neutrino-less double beta decay. As already mentioned, the latter process was the topic of NuSAG’s first report. In this section, we first briefly review what has been learned so far about the neutrinos, then discuss the importance of the open questions to be addressed by oscillation experiments, and then consider how these questions can be answered. 4 2.2 What we have learned so far Neutrinos come in three “flavors”: νe, νμ, and ντ. Each of these is coupled only to the charged lepton of the same flavor: νe to the e, νμ to the μ, and ντ to the τ. If there are additional neutrino flavors, they must be very massive or have non-Standard-Model couplings. Neutrino oscillation is the remarkable morphing of a neutrino of one flavor into that of another. That this occurs implies that neutrinos have nonzero masses and mix. That they mix means that each neutrino of definite
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