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Art Mc Donald: Neutrino Oscillations, Past and Present

March 15, 2011

Art McDonald gave the first presentation at NEUTEL 2011 this morning. It was a very thorough introduction to the issue of neutrino oscillations, pointing out the progression of our understanding in neutrino physics of the last seventy years. I offer below a transcript of the most interesting things he mentioned. Mind you, my speed-writing capabilities are limited, so I transcribed only what I understood (sort of!), and what I thought was interesting to report. If the text is not understandable, I apologize in advance.

You are also of course  invited to check McDonald’s slides.

Pontecorvo first proposed neutrino oscillations in 1957, motivated by the initial reports of measurements by Davis with a chlorine detector at a reactor. The reaction considered was back then the one involving an electron neutrino turning into electron antineutrino. But already in 1946 in a classified 1946 Chalk River report the same Pontecorvo had proposed detecting neutrino from reactors and from the sun with that technology. So this may be taken as the start of the research in the field of neutrino oscillations.

In 1962 Maki, Nakagawa and Sakata describe electron and muon neutrinos asthe superposition of two states nu_1 and nu_2. This is the foundation of the neutrino matrix we use today.

In the 1970s there were lots of accelerator-based measurements. Among them Chorus, Nomad, CDHSW. No oscillations were seen. For solar neutrinos, Davis continued at Homestake. Deficits were seen with respect to predicted fluxes.

In the 80’s Kamiokande confirmed the solar neutrino deficit. The MSW effect was described, due to W exchange in matter traversed by electron neutrinos. In atmospheric neutrinos, puzzling results came from the IMB experiment, and from Kamiokande. The ratio of muon neutrinos to electron neutrinos was found to be low by a factor of two with respect to theoretical calculations. No oscillation however was seen by Bugey, Krasnoyarsk, ROVNO, Goesgen, ILL.

In the 1990s no oscillations were seen at 1km from reactors at Palo Verde and CHOOZ. In 1998, finally Superkamiokande saw a disappearance of atmospheric neutrinos as a function of the zenith angle. This was a quite striking and clear signature of the searched effect.

In the early 2000’s SNO could see clearly that electron neutrinos which were oscillating were reappearing as muon and tau neutrinos, clear evidence for flavour change. Kamland showed a disappearance from reactors with same oscillation parameters as those from the Sun. The number of experiments and results associated with neutrino oscillations has expanded greatly since then: Karmen, K2k, MINOS, MiniBoone for mu neutrinos; MINOS, MiniBoone for muon antineutrinos; and Borexino for solar neutrinos.

In the 2010s a dominant mechanism for neutrino flavor change appears to be that of oscillations among three active flavors of massive neutrinos. The formalism involves three active finite mass neutrinos. A MNSP matrix breaks into a set of matrices multiplied together, which describe the physics.

THe MSW effect has made a major impact in what one would observe for the mixing parameters. It describes the extra interaction of solar electron neutrinos with W exchange with electrons in the Sun and in the Earth. The MSW effect can produce energy spectrum distortion and flavour regeneration in the Earth, giving rise to detectable day-night effects.

As SuperKamiokande came in, it was already clear that the first explanation was the correct one. Initial results were really spectacular, observing as a function of zenith angle that muon-like neutrinos were behaving very much like oscillations into tau neutrinos. The difference of squared mass was small, but consistent with previous observations of Kamiokande.

For solar neutrinos, in 2001 and early 2002 SNO, a detector made of heavy water in a deep location to remove cosmic backgrounds, could see cleanly that the ratio of electron neutrino charged-current to neutral-current reactions  was significantly different to what one’d expect for no oscillations; neutral current reactions are sensitive to all neutrino types. The first SNO paper in 2001 obtained a 3.3 sigma variance from the null oscillation hypothesis. The flux of muon and tau neutrinos showed clear indication of oscillations from nu_e to other active neutrinos.

So, as of 2002, the PDG showed a plot with the LSND band, the supeK result, with a clear measurement in the SNO, and a definition of the Large-mixing-angle region for the mixing angle, due to electron neutrinos mixing into others.

In 2004 KamLand was updated, with 182 GW of reactor power from Japan and Korea. 180km average distance, 515 days of exposure. Fascinatingly, the observed measurements were overlapping with the results of the solar neutrinos for the tan^2 theta vs delta m^2 plane.

Borexino added significantly to this picture. A nice experiment in Gran Sasso with liquid scintillator looking cleanly at solar neutrinos: both Berillium 7 and Boron 8 neutrinos. Also pp neutrinos were studied by all solar neutrino experiments. In the energy spectrum, the survival probability for electron neutrinos was observed to fit well with expectations from MSW. Since then all of these experiments have been proceeding. SNO has improved its accuracy with a re-analysis. KamLAND has a beautiful oscillation pattern that improves the accuracy of delta m^2. SuperKamiokande in terms of atmospheric neutrinos showed indication of nu_mu to nu_tau. Their full data showed that the hypotheses of oscillations from nu_mu to nu_e and from nu_mu to nu_sterile are incompatible with these data to a large degree.

Continuing the discussion of further experiments, McDonald discussed the K2K experiment, which had a nice result confirming what one could observe with SuperKamiokande. Then MINOS: another accelerator experiment, longer baseline. Two detectors, near and far, studied the oscillation pattern of muon neutrinos. The decay was excluded at over 6-sigma significance. It appeared clearly in the nu_mu disappearance that one knew well the parameters delta m^2 and sin^2 (2 theta).
Then McDonald discussed two reactor experiments: Chooz and Palo Verde showed no oscillations early on, and that was a limitation of the value of theta_13. Minos now measures that the sin^2(2 theta_13) is small. There is an ambiguity, since you start to get matter interactions, and CP violation comes in to the determination of theta_13, such that for normal or invertex hierarchy you get two different limits, sin^2 (2theta_13)<0.12 or <0.20 respectively.

OPERA is continuing to search for accelerator muon neutrinos going into tau neutrinos, in the attempt to confirm by direct observation the SuperKamiokande result. They saw one event, and they are continuing to search.

In summary, the result for three active neutrino types, is a very complicated plot. For theta_13, limits now are due to various experiments, sin^2 theta_13<0.031 comes from a global fit to all data. In the future, we expect to increase the sensitivity to that parameter, going down to the 10^-2 in the next few years. Precision reactor experiments are studying it with two detectors at a few hundred meters and a few km to sit at the maximum for the maximum effect.

Results for muon neutrino to electron neutrino oscillations are very complicated. The objective is nowadays to show whether theta_13 is large enough to have sensitivity to the CP-odd effect. There will be in addition a much greater sensitivity for theta_23, by looking at muon neutrino disappearance. It will be interesting to know whetehr that effect is maximal or not.

MINOS, when it started to run, observed an oscillation pattern where the best fit for the muon neutrinos (as opposed to “any” neutrinos) showed that the delta_m^2 wrt sin^2 (2 theta) had a different result from muon neutrinos and muon antineutrinos. This is continuing. McDonald said it will be interesting to hear it from the MINOS talk later on, and the one by Jeff Wilkes from SuperKamiokande.

As for MiniBoone, its first objective was to test LSND result. Reason for emphasizing the region between 200 and 475 MeV was that this is where the finite effect was observed by LSND. The first running was not with antineutrinos, but rather with neutrinos. In order to make the measurement you need to calculate potential backgrounds that are significant. Above 475 MeV they were confident about their estimates, and they saw no effect there, but at lower energy they saw some excess. Now they have moved on to any-neutrino running, and they see a small effect in the 475-1250 MeV region, a 1.5 sigma effect. It is very interesting to keep an eye on it.

ICARUS is now online running at the Gran Sasso, as Carlo Rubbia will report. There is the possibility to test LSND-MiniBoone physics. Can also observe tau neutrinos similar to what Opera does, and oscillations of muon to electron neutrinos to study the LSND effect.

Finally, McDonald looked to the future a bit, mentioning a couple of things. One was the question of the energy plot of the survival probability, whcih is interesting and deserves further study. Mass-varying neutrinos?

Another issue is the recent reassessment of the reactor antineutrino anomaly: an interesting paper (which will be reported on Wednesday): a reassessment of neutrino fluxes from nuclear interactions. This is an increase of 3%, so this shows indication of neutrino oscillations from past experiments. With this renormalization, taking the average at measurements with a baseline shorter than 100 meters, the flux is 0.937+-0.027 of expectation. While authors are very careful to explain that other explanations than oscillations are possible, this needs to be investigated further. It might be a very important contribution to the understanding of reactor neutrino physics. It might be a suppression due to sterile neutrinos with sin^2 (2theta_14)=0.17 and delta m^2 >1.5 eV^2.

Concluding his talk, McDonald categorized what we know and do not in neutrino physics according to a classification due to Donald Rumsfeld: there are known knowns: mixing angles. Known unknowns: 2-3 hierarchy, absolute mass, whether neutrinos are Majorana or Dirac, what is theta_13… And semi-known previously unknowns: sterile neutrinos, CPT violation ? Finally, there are the unknown unknowns: who knows ? there is the real fun!

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