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Vernon Barger: perspectives on neutrino physics May 22, 2008

Posted by dorigo in cosmology, news, physics, science.
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Yesterday morning Barger gave the first talk at PPC 2008, discussing the status and the perspectives of research in physics and cosmology with neutrinos. I offer my notes of his talk below.

Neutrinos mix among each other and have mass. There is a matrix connecting flavor and mass eigenstates, and the matrix is parametrized by three angles and a phase. To these can be addded two majorana phases for the neutrinoless double beta decay: \phi_2 and \phi_3. Practically speaking these are unmeasurable.

What do we know about these parameters ? We are at the 10th year anniversary of the groundbreaking discovery of SuperKamiokande. This was then confirmed by other neutrino experiments: MACRO, K2K, MINOS. The new result by MINOS has a 6.5 sigma significance in the low energy region. This allows to measure the mass difference precisely. It is found that (\Delta m^2)_{12}  = 2.4 \times 10^{-3} eV squared at \sin^2 \theta_{23}=1.00. The mixing angle is maximal, but we do not really know because there is a substantial error on it.

Solar neutrino oscillations are a mystery that existed for years. The flux of solar neutrinos was calculated by Bahcall, and there was a deficit. The deficit has an energy structure, as measured by the Gallium, Chlorine, and SuperK and SNO experiments by looking in neutrinos coming from different reactions -because of the different energy thresholds of the detectors: pp interactions, Beryllium 7, and Boron 8 neutrinos.
The interpretation of the data, which evolved over time, is now that the solar mixing angle is quite large, and what happens is that the high energy neutrinos sent here are created in the center of the sun, but they make a transition in matter, an adiabatic transition to a state \nu_2 which travels to the earth. This happens to the matter-dominated higher energy neutrinos. The vacuum dominated ones at lower energy have a different phenomenology.

There is a new result from Borexino, they measured neutrinos from the Beryllium line, and they reported a result consistent with others. Borexino is going to measure with 2% accuracy the deficit, and if KamLand has enough purity they can also go down to about 5% accuracy.

Kamland data provides a beautiful confirmation of the solution of the solar neutrino problem: solar parameters are measured precisely. M^2_{21} vs \tan^2 \theta_{12}. The survival probability as a function of distance versus neutrino energy has a beautiful oscilation. The result only assumes CPT invariance. The angle \theta_{12} is 34.4° with 1° accuracy, and (\Delta_m^2)_{12} = 7.6 \times 10^{-5} eV squared.

There is one remaining angle to determine, \theta_{13}. From reactor experiments you expect that the probability for electron neutrino disappearance is zero in short baseline experiments. Chooz had a limit at \theta_{13} < 11.5 degrees. There are experiments that have sensitivities on \sin^2 \theta_{13} of 0.02 (Double CHooz, DB, Reno). The angle is crucial because iti s a gateway to CP violation. If the angle is zero CP violation is not accessible in this sector.

What do we expect theoretically on \theta_{13} ? There are a number of models to interpret the data. Predictions cluster around 0.01. Half of the models will be tested with the predicted accuracies of planned experiments.

There is a model called tri-bimaximal mixing: a composition analogous to the quark model basis of neutral pions, eta and eta’ mesons. An intriguing matrix: it could be a new symmetry of nature, possibly softly broken with a slightly non-zero value of the angle \theta_x. Or, it could well be an accident.

So, we need to find what \theta_{13} is. It is zero in the Tri-binaximal mixing. We then need to measure hte mass hierarchy: is it normal ( the third state much heavier than the other two) or inverted (the lightest much lighter than the others) ? Also, is CP violated ?

Neutrinoless double-beta decay can tell us if the neutrinos are Majorana particles. In the inverted hierarchy, you measure the average mass versus the sum. There is a lot of experimental activity going on here.

Cosmic microwave has put a limit on sum of masses at 0.6 eV. By doing 21cm tomography one can measure neutrino masses with precision of 0.007 eV. If this is realizable, it could individually measure the masses of these fascinating particles.

Barger then mentioned the idea of mapping the universe with neutrinos: the idea is that active galactic nuclei (AGN) produce hadronic interactions with pions decaying to neutrinos, and there is a whole range of experiments looking at this. You could study the neutrinos coming from AGN and their flavor composition.
Another late-breaking development is that Auger has shown a correlation of ultra high-energy cosmic rays with AGNs in the sky: the cosmic rays seem to arrive directly from the nuclei of active galaxies. Auger found a higher correlation with sources at 100 Mpc, but falling off at higher distances. Cosmic rays are already probing the AGN, and this is very good news for neutrino experiments.

Then he discussed neutrinos from the sun: annihilation of weakly interacting massive particles (WIMPS), dark matter candidates, can give neutrinos from WW, ZZ, ttbar production. The idea is that the sun captured these particles gravitationally during its history, and the particles annihilate in the center of the sun, letting neutrinos escape with high energy. The ICECUBE discovery potential is high if the spin-dependent cross section for WIMP interaction in the sun is sufficiently high.

In conclusion, we have a rich panorama of experiments that all make use of neutrinos as probes of exotic phenomena as well as processes which we have to measure better to gain understanding of fundamental physics as well as gather information about the universe.