| 1 | \subsection{Solar neutrinos}
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| 2 |
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| 3 | Water \v{C}herenkov detectors have measured the high energy tail
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| 4 | of the solar $^{8}$B neutrino flux using electron-neutrino
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| 5 | elastic scattering \cite{Smy:2002rz}.
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| 6 | Since such detectors could record the time of an interaction and reconstruct
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| 7 | the energy and direction of the recoiling electron, unique information
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| 8 | of the spectrum and time variation of the solar neutrino flux was extracted.
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| 9 | This provided further insights into the ``solar neutrino problem'',
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| 10 | the deficit of the neutrino flux (measured by several experiments)
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| 11 | with respect to the flux expected by the standard solar models.
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| 12 | It also constrained the neutrino flavor oscillation solutions in a fairly
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| 13 | model-independent way.
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| 14 |
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| 15 | The recoiling electrons from solar neutrino interactions are low in energy
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| 16 | and produce few \v{C}herenkov photons. However, if at least 20 \% of the
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| 17 | detection surface is photo-sensitive then solar neutrinos above 10 MeV
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| 18 | could be detected even with a modest photo-sensor efficiency.
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| 19 | A detector with larger size than any existing Water \v{C}erenkov has
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| 20 | the potential to measure spectrum and time-variation of the high-energy
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| 21 | solar neutrino flux more precisely, if systematic uncertainties can be
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| 22 | kept small.
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| 23 | For example, Super-Kamiokande's measurements obtained from 1258 days
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| 24 | of data could be repeated in about half a year (the seasonal flux variation
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| 25 | measurement requires of course a full year). In particular, a first
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| 26 | measurement of the flux of the rare hep neutrinos may be possible.
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| 27 | Elastic neutrino-electron scattering is strongly forward peaked.
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| 28 | To separate the solar neutrino signal from background events, this
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| 29 | directional correlation is exploited. Angular resolution is limited
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| 30 | by multiple scattering. The reconstruction algorithm first reconstructs
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| 31 | the vertex from the PMT times and then the direction assuming a single
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| 32 | Cherenkov cone originating from the reconstructed vertex.
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| 33 | Reconstructing 7 MeV events in a 400 kton
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| 34 | fiducial volume water \v{C}erenkov (UNO,MEMPHYS,...) seems not to be a problem.
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| 35 |
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| 36 | This means we are able to make improvements in solar neutrino detection
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| 37 | with a megaton-scale \v{C}erenkov
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| 38 | detector: even if it is not the main goal of such a detector
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| 39 | it could be an excellent by-product.
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| 40 |
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