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  Long-Baseline Neutrino Physics and Astrophysics
 (INT Program July 26 - August 27, 2010)

  Reported by Wick Haxton, Boris Kayser, Bill Marciano, and Aldo Serenelli
  Date posted November 1, 2010

The program on Long-Baseline Neutrino Physics and Astrophysics focused on the unresolved questions in neutrino physics and their implications for astrophysics. While we now know that neutrinos are massive and that the mass eigenstates are distinct from the flavor eigenstates, many associated properties are not yet determined. There are two possible mass hierarchies, normal and inverted, as there has been no determination of the sign of the atmospheric mass2 difference. The absolute mass scale is also unknown, constrained only by bounds from tritium beta decay and from cosmology. Thus the quantitative impact of neutrino mass on the evolution of structure in the cosmos remains to be determined. We do not know the particle-antiparticle properties of neutrinos, that is, whether they are Dirac or Majorana particles. In fact, the seesaw mechanism accounts for the anomalous scale of neutrino mass by postulating that neutrinos have both Dirac and Majorana masses, with mD/mR serving as the small parameter that differentiates neutrino masses from those of other standard-model fermions. The third mixing angle θ13 is known to be smaller than the angles mediating solar and atmospheric neutrino oscillations, but its value (including whether it is nonzero) has not been established. This angle will determine whether future long-baseline neutrino experiments have the requisite sensitivity to constrain CP violation. The electromagnetic properties of neutrinos - their magnetic dipole, electric dipole, or anapole moments, or a nonzero charge radius - have not been established. We do not know whether there are neutrinos beyond the three identified light flavors, or whether sterile neutrinos are responsible for possible anomalies seen in recent experiments.

Many of these questions are important to astrophysics and cosmology. The new properties of neutrinos were first recognized in astrophysics: neutrino oscillations were the source of both the solar and atmospheric neutrino anomalies. Despite their relatively small contribution to the mass-energy of the cosmos, neutrinos should have a measurable effect on the growth of large-scale structure, as their transition from relativistic to nonrelativistic matter leads to distinct effects that are depend on both red-shift and scale. The large-scale surveys of the next decade will likely achieve the precision necessary to determine the neutrino mass scale and potentially the hierarchy, even if the scale is near the atmosphere mass2 lower bound. Unique tests of neutrino properties can be made in extreme astrophysical environments. The enormous neutrino densities found shortly after core-bounce in a Type II supernova produces a nonlinear neutrino-neutrino component in the MSW potential that is predicted to induce distinctive flavor swaps. Ultra-high-energy neutrinos provide our only known probe of the high-energy limits of the universe, as other cosmic rays cannot propagate through the microwave background above the GZK cutoff energy.

The Long-Baseline Neutrino Physics and Astrophysics program examined the opportunities for probing neutrino properties through experiments or observations involving long distances and large matter potentials. In the US, Europe, and Japan, plans are being made to utilize intense accelerator sources of neutrinos to probe CP violation and the neutrino hierarchy. The matter effects sensitive to the hierarchy and the CP effects that distinguish neutrino oscillations from those of the corresponding antineutrino both grow approximately linearly with distance. In the US the program to exploit these phenomena could couple DUSEL (the proposed Deep Underground Science and Engineering Laboratory), Project X (FermiLab's proposed high-intensity proton driver), and a massive water Cerenkov or liquid argon detector in the range of 100-300 ktons.

Borexino, which has produced first results, has the potential to map the transition from matter enhanced to vacuum oscillations, and may be able to isolate the CN solar neutrinos (in addition to the 7Be and pep neutrinos), helping to constraint solar composition.

Thus one of the program's keystone activities was the LBNE Science Collaboration's workshop to examine the status of US long-baseline planning. The development of a large water detector raises important technical questions connected with beam optimization and background rejection. In such broad-beam experiments the energy of the initiating neutrino must be reconstructed from final-state observations, despite missing energy from unobserved π0s and nucleons spalled from excited 16O. Important issues include the optimization of beams and the development of near detectors to reduce the severity of such backgrounds, as well as the incorporation of more reliable nuclear theory into event generators, so that the subtractions made to isolate quasi-elastic events are reliable. This latter task is a challenge to nuclear theory, as the neutrino response at the relevant energies (2-4 GeV) is complicated, involving comparable contributions from quasi-elastic scattering and resonance production. Several program talks dealt with the uncertainties due to the initial scattering event (sensitive to the tails of the single-nucleon spectral function and to nucleon correlations), resonance production (many of the vertices are poorly constrained by experiment), propagation in the nuclear medium, and final-state re-absorption of produced mesons.

The program also hosted the DAEdALUS collaboration, which has proposed that a low-energy oscillation program be conducted with the DUSEL water detector. DAEdALUS is predicated on the development and deployment of a set of high-intensity cyclotrons. The ∼1 GeV protons produced by these machines would be stopped in targets, producing stopped-pion neutrino fluxes with well-known spectra. The collaboration envisions such sources arrayed at distances of up to 20 km from DUSEL. In particular, the νe appearance channel could be detected by charged-current interactions with protons, followed by the detection of neutron-capture γs in a gadolinium-doped water detector with adequate PMT coverage. Discussions focused on the complementary relationship between such a short-baseline program and the DUSEL-FermiLab program.

The program hosted two workshops focused on exploiting a DUSEL megadetector in conjunction with either a new FermiLab neutrino beam (peaking at 2-4 GeV) or stopped-pion neutrinos (peaking at 30 MeV) produced from next-generation cyclotrons proposed for construction within 20 km of DUSEL.

The program also covered a range of related topics in neutrino astrophysics. Recent results from Borexino and from the low-energy SNO analysis were discussed. Several talks focused on possibilities for probing solar properties - composition, opacities, etc. - by combining results from helioseismology and future neutrino experiments. The proposed measurement of CN neutrinos by SNO+ is one important possibility. Other talks described the status of multi-D supernova models, their neutrino burst predictions, and the possibility that supernova neutrino observations might constrain otherwise elusive properties of neutrinos. Two "new-physics" possibilities are the neutrino-neutrino contribution to the MSW potential and the level-crossing associated with the atmospheric mass2 difference. Prospects for neutron star tomography through a high-statistics, extended measurement of the supernova neutrino flux were described. In many cases, the megadetectors under consideration for LBNE physics would be the most capable instruments for such astrophysics, were a galactic supernova to occur during a detector's lifetime. Alternatively, such massive detectors might allow experimenters to begin to exploit the continuous flux of relic supernova neutrinos associated with all past supernovae.