dimanche 2 octobre 2016

Solar neutrinos: Oscillations or (almost) No-oscillations (?)

Neutrino oscillations disentangled from adiabatic flavor conversion : always mind your terminology!
Next Tuesday will be announced the Nobel prize in physics 2016. That makes two days left to think one more time about the interesting physics from the previous year, learning some lessons from the past:
The Nobel prize in physics 2015 has been awarded "... for the discovery of neutrino oscillations which show that neutrinos have mass". While SuperKamiokande (SK), indeed, has discovered oscillations, {the} Sudbury Neutrino Observatory (SNO) observed effect of the adiabatic (almost non-oscillatory) flavor conversion of neutrinos in the matter of the Sun. Oscillations are irrelevant for solar neutrinos apart from small electron neutrino regeneration inside the Earth. Both oscillations and adiabatic conversion do not imply masses uniquely and further studies were required to show that non-zero neutrino masses are behind the SNO results. Phenomena of oscillations (phase effect) and adiabatic conversion (the Mikheïev-Smirnov-Wolfenstein (MSW) effect driven by the change of mixing in matter) are described in pedagogical way.

In {the figure above} we show graphic representations of the neutrino oscillations and adiabatic conversion which are based on analogy with the electron spin precession in the magnetic field. Neutrino polarization vector in flavor space (“spin”) is moving in the flavor space around the “eigenstate axis” (magnetic field) whose direction is determined by the mixing angle 2θm. Oscillations are equivalent to the precession of the neutrino polarization vector around fixed axis, Fig. a. Oscillation probability is determined by projection of the neutrino vector on the axis z. The direction up of the neutrino vector corresponds to the νe, direction down – to νa. Adiabatic conversion is driven by rotation of the cone itself, i.e. change of direction of the magnetic field (cone axis) according to change of the mixing angle, Fig. b. Due to adiabaticity the cone opening angle does not change and therefore the neutrino vector follow rotation of axis.
Oscillations do not need the mass. Recall that it was the subject of the classical Wolfenstein’s paper [9] to show that oscillations can proceed for massless neutrinos. This requires, however, introduction of the non-standard interactions of neutrinos which lead to non-diagonal potentials in the flavor basis and therefore produce mixing. 
In oscillations we test the dispersion relations, that is, the relations between the energy and momentum, and not masses immediately. Oscillations are induced because of difference of dispersion of neutrino components that compose a mixed state... 
It is consistency of results of many experiments in wide energy ranges and different environments: vacuum, matter with different density profiles that makes explanation of data without mass almost impossible. In this connection one may wonder which type of experiment/measurement can uniquely identify the true mass? Let us mention three possibilities:
• Kinematical measurements: distortion of the beta decay spectrum near the end point. Notice that similar effect can be produced if a degenerate sea of neutrinos exists which blocks neutrino emission near the end point.
• Detection of neutrinoless double beta decay which is the test of the Majorana neutrino mass. Here complications are related to possible contributions to the decay from new L-violating interactions.
• Cosmology is sensitive to the sum of neutrino masses, and in future it will be sensitive to even individual masses. Here the problem is with degeneracy of neutrino mass and cosmological parameters.
In January 1986 at the Moriond workshop A. Messiah (he gave the talk [16]) asked me: “why do you call effect that happens in the Sun the resonance oscillations? It has nothing to do with oscillations, I will call it the MSW effect”. My reply was “yes, I agree, we simply did know how to call it. I will explain and correct this in my future talks and publications”. Messiah’s answer was surprising: “No way..., now this confusion will stay forever”. That time I could not believe him. I have published series of papers, delivered review talks, lectures in which I was trying to explain, fix terminology, etc.. All this has been described in details in the talk at Nobel symposium [17], and for recent review see [8]. 
Ideally terminology should reflect and follow our understanding of the subject. Deeper understanding may require a change or modification of terminology. At the same time changing terminology is very delicate thing and can be done with great care. 
In conclusion, the answer to the question in the title of the paper is 
“Solar neutrinos: Almost No-oscillations”.
The SNO experiment has discovered effect of the adiabatic flavor conversion (the MSW effect). Oscillations (effect of the phase) are irrelevant. Evolution of the solar neutrinos can be considered as independent (incoherent) propagation of the produced eigenstates in matter. Flavors of these eigenstates (described by mixing angle) change according to density change. At high energies (SNO) the adiabatic conversion is close to the non-oscillatory transition which corresponds to production of single eigenstate. Oscillations with small depth occur in the matter of the Earth.
A. Yu. Smirnov (Submitted on 8 Sep 2016)

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