I always thought it was accepted that neutrinos had antiparticles, my physics dictionary lists one of the products of Beta decay as the electron anti-neutrino 
, but now you mention it I have certainly read in some places that neutrinos are their own antiparticle.
The short answer: In this case, it is a detector issue.
The longer answer: In this case, they are only seeing neutrino-electron elastic scattering, which, at these energies, produces final state (that is, post-interaction) electrons which are traveling in nearly the same direction as the initial incoming neutrino. The problem, then, is to measure that electron’s direction well.
The much longer answer: (Oh, and scr4: I know you already know much of what I’m about to say, but I thought I’d try to be complete…)
Super-K contains water, so the possible targets for the solar electron neutrinos (denoted [symbol]n[/symbol][sub]e[/sub]) are:
- hydrogen nuclei = free protons
- oxygen nuclei = bound protons and neutrons
- free electrons
Further, Super-K gets its information about events from the Cerenkov light produced by charged particles moving faster than the speed of light in water.
Further still, neutrinos can interact in only two ways:
- in neutral current interactions, the neutrino stays a neutrino, but it transfers some momentum and energy to the target;
- in charged current interactions, the neutrino turns into an appropriate charged lepton, and it transfers some momentum and energy to the target.
Putting these things together:
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proton § target: The only interaction that is possible at these approx. 10 MeV energies is the neutral current interaction: [symbol]n[/symbol][sub]e[/sub] + p --> [symbol]n[/symbol][sub]e[/sub] + p. The final state proton will make no Cerenkov light, as it will be far far below the threshold. These events will be ignored.
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oxygen target: Now there are neutrons around, so one might expect that the charged current interaction [symbol]n[/symbol][sub]e[/sub] + n(bound) --> e[sup]-[/sup] + p(free/bound) might be possible. If it were, the final state electron would be readily detected. However, the energies involved here are not high enough to remove the nucleon from the nucleus entirely, so the p(free) final state isn’t available. But, all of the bound proton states within the nucleus are already occupied by the other protons and are thus unavailable for the newly created proton. That means the only final states that are available at all are ones in which the new proton is kicked into an excited nuclear level, but the solar neutrinos don’t have enough energy to do that. Thus, no charged current scattering off oxygen will occur. And, neutral current scattering will (like in the free proton case above) be ignored.
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free electrons: [symbol]n[/symbol][sub]e[/sub] + e[sup]-[/sup] --> [symbol]n[/symbol][sub]e[/sub] + e[sup]-[/sup] happens without difficulty. The final state electron will be above Cerenkov threshold, and because it is so light (0.5 MeV/c[sup]2[/sup] as compared to the energy of the neutrino, ~10 MeV), its outgoing direction will be almost exactly along the original neutrino’s direction with little smearing.
Thus, the bulk of the angular resolution issues for solar neutrinos in Super-K comes from the reconstruction of the outgoing electron direction.
However…
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For neutrinos of slightly higher energy, or for antineutrinos, the nucleons in a water target would dominate the physics. For these cases, the outgoing electron or positron or what-have-you will come out of the interaction in a wide ranges of (lab) angles and energies. That is, the scattering itself introduces a large unknown. (One can try to measure the recoiling nucleon’s energy, but that’s another post entirely…)
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I spend my days with 100 MeV to 1000 MeV muon-type neutrinos hitting an (approximately) CH[sub]2[/sub] target. Our dominant interesting interaction is [symbol]n[/symbol][sub][symbol]m[/symbol][/sub] + n(bound) --> [symbol]m[/symbol][sup]-[/sup] + p(free). We can easily measure the final state [symbol]m[/symbol]'s direction to within 4 degrees (maybe 2 degrees for the highest energy ones), but we can’t turn that into a neutrino direction because A) like above, the scattering can send the muon in all sorts of directions, and B) the target neutron is not at rest! It is swimming inside a nucleus, and the instantaneous momenta of these target neutrons can be hundreds of MeV/c. Fortunately, we don’t care about this – our neutrinos come from a beamline whose direction we know pretty darn well.
Chronos’ dire warning having been issued too late …
Everyone agrees that neutrinos are examples of spinors. The problem is that there are different types of spinors and it’s difficult to argue that one type is more natural than another. One possibility is that they’re Majorana spinors, in which case they would have an antiparticle that’d be the same as the particle. This would have certain, fairly recondite, experimental consequences - the issue of double-beta decay of nuclei being the classic one - and so people argue about the experimental results.
(Useless obscure historical fact about Majorana: he was an Italian physicist who appears to have commited suicide by throwing himself off a ferry, but no body was ever found.)