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T. Nakaya: The Neutrino Physics Program of HyperKamiokande

March 14, 2013

Hyper-K fits well in the Japanese future strategy. A recommendation was made for two large-scale projects: a linear collider, and a large-scale neutrino and nucleon decay detector. The KEK roadmap draft (Jan 2013) includes the project Hyper-K.

Arxiv:1109.3262 (letter of intent) discusses the physics topics and motivations for Hyper-K. The oscillation probability for muon to electron neutrinos and antineutrinos allows sensitivity to exotic CPV as well.

The neutrino beam from J-PARC to HyperK has small contaminations from the wrong flavours, both in neutrino and in antineutrino modes.The strenght of the water Cherenkov design are the large mass, and the excellent reconstruction and PID performance, especially in the sub-GeV region. A full simulation of the neutrino beam, interaction, detector response and reconstruction has been performed. The event selection is almost the same as that of T2K. Assuming a 7.5MW*y exposure, they expect signal efficiencies of 64%, a background rejection of 1000:1, and a clean signal of electron neutrino appearance: 2000-4000 signal events for each of the neutrino modes.

It needs to be noted that a large theta_13 is good for Hyper-K, because the signal is enhanced. 70% of the possible values of delta_CP are testable easily. The speaker showed a very nice graph of the possible precision in theta_13 versus delta_CP for various values of the parameters. I will paste this important figure here as I have a chance.

How realistic is to beat down the systematic uncertainties down to 5% ? They can use the T2K experience, where the systematics on the beam flux are already down to 5.7%. Upcoming cross section measurements will improve the picture.

Nakaya then discussed the combination of Beam and Atmospheric neutrino results from the single experiment HyperK. Atmospheric neutrinos help remove a degeneracy in the measurement of delta_CP. For the hierarchy, atmospheric neutrinos give some sensitivity flat in delta_CP so they also help throughout the parameter space in increasing the sensitivity to the two possible hierarchy scenarios.

For proton decay, experimental limits are already dominated by Super-K. A similar measurement can be done in Hyper-K, and obtain results for many decay modes, with 10 times better sensitivity thank super-K.

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