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Question and Answer – by Mauro Mezzetto

March 16, 2011

After I posted some quick write-up of the SuperKamiokande talk by Jeffrey Wilkes, a reader asked me to clarify an obscure sentence on the disfavouring of oscillations into sterile neutrinos, here. I answered as I could, but also checked with the organizer of our conference, Mauro Mezzetto, who knows the subtleties much better than me. He answered in the thread too, but I think his text is worth electing as a separate post. So here it is below (T.Dorigo)

OK, I know I should have presented the conference time ago and I’m not contributing to the blog as I promised to Tommaso. I have to say I’m too committed to the organization of the workshop, an engagement I grossly underestimated, and the dramatic events in Japan, with consequences to the experiment I’m working, T2K, and to the conference, are not helping in finding spare time. I just realized I followed only 4 talks today, and I missed the SuperKamiokande talk. I’m discovering that the organizers are not allowed, de facto, to follow the conference.

Anyway what is the meaning of these 7 sigmas?

SuperKamiokande (SK) discovered in 1998 that muon neutrinos (numu) disappear. In a three neutrino scheme they could become electron neutrinos (nue) or tau neutrinos (nutau). In the same years Chooz experiment at reactors excluded the possibility of conversions to electron neutrinos. So the only viable option are conversions to tau neutrinos … or to sterile neutrinos if we accept that neutrinos are more than three.

SK measured that this numu disappearance happens at a difference of  masses squared of about 3 milli-ev, and we know that the numu mass cannot be higher than about 2.5 eV.

We are talking of steriles in this range of masses. SK cannot say much about steriles of high masses.

How can SK discriminate  disappearance to nutau from disappearance to steriles? In charged current interactions active neutrinos create a lepton; the largest part of the atmospheric neutrinos don’t have enough energy to create a tau lepton (with a mass of 1.77 GeV) : no way to separate nutaus from steriles with charged current events (steriles neutrino are such because they cannot interact with weak interactions).
The only way to separate the two is by studying neutral currents, where the neutrino scatters and part of its energy is released to the target nucleus generating hadrons (protons, neutrons, pions etc). Neutral currents are a category of events much more difficult to detect. Nutaus can interact with neutral currents (no problems of tau mass since the neutrino just scatters) and steriles can’t.

Data analysis disfavors disappearance in steriles at 7 sigmas.

What is not said in the slides is the fact that one could imagine that the most of the muon neutrinos oscillate into tau neutrinos, but a fraction of them  oscillates to steriles (subdominant oscillations). In the previous analysis the limit on the fraction of nutau that oscillate to steriles was around 20%, at 90% confidence level.
There are more subtleties in the sterile analysis of SK data, they will be addressed by T. Schwetz talk of tomorrow and C. Giunti talk of Thursday.

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