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Max Planck Institute for Astrophysics
LBNE Science Collaboration Mini-Workshop
Interim LBNE Physics Report
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Long-Baseline Neutrino Physics and Astrophysics |
July 26 - August 27, 2010
The program will focus on the upcoming opportunities for new physics discoveries using long-baseline neutrinos - neutrinos produced by accelerators, reactors, or astrophysical sources such as the Sun, nearby supernovae, or the Big Bang, that travel long distances through matter or fields prior to being detected. This field is expected to advance rapidly due to the advent of intense neutrino superbeams, the continued development of underground sites providing low-background detection environments, the construction of massive new detectors, and the possibility that the international community will collaborate on a Neutrino Factory. The program will bring together neutrino physicists and astrophysicists, in both theory and experiment, to discuss how to optimize advances in this field.
The program coincides with the start of a second generation of neutrino experiments that will further constraint neutrino masses and mixing, including the value of θ13, the mass hierarchy, the absolute neutrino mass, and the effects of CP violation on neutrino oscillations. Issues raised by these new experiments and the rapid progress in neutrino astrophysics and cosmology include:
- One attractive strategy for breaking various parameter degeneracies involves use of a broad-band neutrino spectrum, so that the effects of oscillations can be extracted from the distinctive pattern imprinted over several oscillation lengths. With, for example, a FermiLab-DUSEL baseline of about 1300 km, this argues for a rather low beam energy of 1-2 GeV. The program will discuss optimization of this energy, and the nuclear physics issues that arise for lower beam energies, due to the rather complicated combination of quasi-elastic scattering and resonance production in target nuclei. The tools available for constraining this response include the use of near and far detectors, neutrino and electron calibration experiments, and the introduction of improved nuclear modeling into event generators.
- What is the relationship between cosmological tests of neutrino properties and direct laboratory tests? Neutrinos have a significant effect on cosmological evolution, contributing about 10% of the energy density at the time of decoupling, and remaining relativistic (thus affecting the growth of large-scale structure) to relatively long times. Thus, anticipating a decade of improvements in precision cosmology due to much larger galaxy red-shift surveys, gravitational lensing studies, and Lyman- absorption and 21-cm emission studies, it appears that cosmology could test the absolute neutrino mass down to its 0.05 eV lower bound and potentially distinguish the normal and inverted hierarchies. But as there are sources of uncertainty in such analyses -- conclusions depend on a cosmological model and on the combined constraints of data sets with different systematics -- the importance of laboratory cross checks is clear.
- What is the relationship between astrophysical tests of neutrino properties and direct laboratory tests? For example, a new aspect of the MSW mechanism connected with the intense neutrino background can alter supernova dynamics (and the neutrino signal), but has no analog in terrestrial experiments. Also, values of θ13 one to two orders of magnitude below Double Chooz/Daya Bay sensitivities can mediate conventional adiabatic MSW inversions in the supernova's mantle. Is there the potential to extract meaningful constraints on new neutrino physics from model-dependent phenomena like supernovae?
- Conversely, is our understanding of flavor physics now sufficiently complete that we can return to the original goal of Ray Davis, to use neutrinos as a unique probe of stellar interiors? One idea of recent interest is the possibility of combining existing and future (e.g., SNO+ measurements of the CN solar neutrinos) to directly measure the metal content of the solar core at an interesting level of precision. This could help us resolve the current conflict between solar interior helioseismology and recent improved analyses of photospheric absorption lines, which lowered estimates of the Sun's metalicity by about 30%. Another question that has been explored recently by several groups is the implications of measuring the diffuse background neutrinos from past supernovae. What is the astrophysical significance of such a measurement, and its connections with other inventories of massive star evolution such as the galaxy's net heavy element abundance, given that statistics will be low even in future megadetectors?
- What are the long-baseline frontiers for terrestrial beams? There are exciting possibilities for high-precision mixing angle and CP violation studies using Neutrino Factory beams and unprecedented magic baselines of around 7500 km.
- What are the long-baseline frontiers in astrophysics? As Pierre Auger appears to see the GZK cutoff in hadronic cosmic rays, neutrinos may be our only probe of the cosmos at asymptotically high energies - and thus the only test of the limits of Nature's high-energy accelerators. Can we detect such super-GZK neutrinos by radio or other means? Can we interpret the results, given that the neutrino-matter interactions are occurring at energies beyond the reach of any terrestrial accelerator? What can we potentially learn about either the astrophysical engines producing such neutrinos, or about truly exotic new physics that might be apparent at unexplored energies?
- As we make progress in understanding the nature of neutrino mass and mixing, what are the implications for theory? We already have unexpected results, such as an atmospheric neutrino mixing angle consistent with maximum mixing. What does the special form of the neutrino mass matrix mean for underlying fundamental theory, through which low-energy neutrino properties might be related to high-energy phenomena such
The organizers will structure the program around these questions and others suggested by the participants.