## Saturday, March 19, 2011

### SuperK at Neutel 11

The most fascinating set of results to come out of Neutel 11 are without a doubt those from the Super-Kamiokande experiment. As indicated, they report perfect CPT invariance between neutrinos and antineutrinos, in conflict with the MINOS results.
I must confess that when I first saw this, my immediate thought was, "My God, how could they do such a bad job!" Obviously, MINOS is correct. But, wait a moment! Now we ask: what exactly is a neutrino and an antineutrino? Super-K is not doing the same experiment as MINOS. Super-K looks for solar, astrophysical and atmospheric neutrinos. Like the numi beam at MINOS, cosmic rays contain pions, which decay into muons or antimuons, along with either neutrinos or mirror neutrinos. But these are not the only neutrinos that Super-K sees. Now an antimuon typically decays in the atmosphere to

$\mu^{+} \rightarrow e^{+} + \nu_{e} + \overline{\nu}_{\mu}$

yielding one electron neutrino and one muon antineutrino. An event is tagged by the Cherenkov cone, which indicates distinct path lengths for the two species. Now what is an antineutrino in this decay? There are no pions around. According to lepton number conservation, we insist on there being one neutrino and one antineutrino. So maybe it really is just the antineutrino! That is, antineutrino, rather than a mirror neutrino as observed by MINOS.

Ah but, you say, MINOS also looks at atmospheric antineutrinos. Yes, and their preliminary atmospheric results appear to agree with those from Super-K.

Does this makes sense? You tell me. At present, everyone seems to doubt the apparently CPT violating MINOS results, as if the Super-K results contradict them. The neutrino experiments are full of confusing, seemingly contradictory results. Perhaps it is time we labelled our particles a little more carefully.

2. Note that the mirror muon neutrinos can oscillate into mirror electron neutrinos. So when Super-K looks for a $\Delta m^2$ for so called antineutrinos, it is looking at a mixture of antineutrinos and mirror neutrinos.
3. Now since most of the muon antineutrinos will be just that ... muon antineutrinos ... while many of the electron antineutrinos are secretly heavy mirror neutrinos, we expect a $\Delta m^2$ slightly lower than the $\Delta m^2$ for ordinary neutrinos. Just as observed: $2.0$ instead of $2.4$.