I was reading up on the history of the solar neutrino problem, and as far as I can understand it, neutrinos supposedly oscillate from one form to another, thus explaining why there were only one-third the number of neutrinos detected than were expected, when they began neutrino observations in the 1960's.

The Wikipedia article on the topic ends with this statement:

The convincing evidence for solar neutrino oscillation came in 2001 from the Sudbury Neutrino Observatory (SNO) in Canada. It detected all types of neutrinos coming from the Sun, and was able to distinguish between electron-neutrinos and the other two flavors (but could not distinguish the muon and tau flavours), by uniquely using heavy water as the detection medium. After extensive statistical analysis, it was found that about 35% of the arriving solar neutrinos are electron-neutrinos, with the others being muon- or tau-neutrinos. The total number of detected neutrinos agrees quite well with the earlier predictions from nuclear physics, based on the fusion reactions inside the Sun.

But as far as I can see, none of this or anything else I've read seems to give any proof that solar neutrinos change type while en route to the Earth. It seems that the sun just emits about 1/3 of each of the three types.

Or is it that at the temperature of the solar core only electron neutrinos are emitted, and then they oscillate (randomly?) from that type to the others and back again? I'd welcome a little clarity about this.


2 Answers 2


The thing to understand is how we tag neutrinos for flavor in the first place.

Neutrinos are created and destroyed in reactions that also involve a charged lepton (electron, muon or tau). At vertex level these are $$ W^\pm \to l^\pm + \nu_l $$ and various rotations. The flavor of a neutrino is defined as coincident with that of the charged lepton produced. (I've neglected $Z \to \nu + \bar{\nu}$ reactions here, but these are far off-shell at the energies available in the core of the sun, so they don't contribute.)

The reactions in the sun are fusion reaction that don't have enough excess energy to create a heavy lepton, so the neutrinos must (by definition) be electron-type.

This rule for neutrino flavors can be tested at accelerator where we can generate beams of known neutrino content (because we can count the number and type of hadrons decaying to charged leptons), and when the beam is directed onto a detector very near-by the number of charged-current neutrino interactions of each flavor that we detect agrees with the flavor content of the beam.

But there is more, we can predict the energy spectrum of the solar neutrinos assuming that they are electron-type, and that is the spectrum that we detect. By conservation of energy neutrinos created in concert with heavy leptons (that is, other flavors) would have a different energy spectrum.

Now, the evidence that the theoretical oscillation framework we use is correct is pretty diverse, but some of the very best "one plot" evidence comes from KamLAND, where we plot the flux of electron-type anti-neutrinos from Japanese power reactors as a function of observed energy and compare to the expected $\sin \left( \frac{L}{E} \right)$ behavior (appropriately convoluted to account for the many different distances to reactors). enter image description here (image from http://kamland.lbl.gov/).

Full disclosure: I was a member of the KamLAND collaboration for about 4 years and am named as an author on the paper from which that figure is drawn.

  • $\begingroup$ This is a fascinating answer, but it doesn't say anything about oscillation (unless it does by implication and I missed it). $\endgroup$ Jan 29, 2014 at 23:51
  • $\begingroup$ It says two things about oscillation. (1) It says that it is not the case that the sun generates a mix of flavors. (2) It is the case that our theoretical framework for oscillation is well demonstrated by experiment. $\endgroup$ Jan 30, 2014 at 0:17
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    $\begingroup$ OK, I missed that in the first three readings of your answer. Thanks for all the detail and the chart. $\endgroup$ Jan 30, 2014 at 0:21

http://en.wikipedia.org/wiki/CERN_Neutrinos_to_Gran_Sasso Neutrino flavor oscillation is facilitated by passage through matter. They travel hard by lightspeed, but not faster.

Solar core fusion emits two electron neutrinos/helium output. They scramble flavors during passage to the surface, traveling through our atmosphere (a yard of lead at sea level, mass/area), and through rock. Absence of observed neutrinoless double beta-decay validates neutrinos and anti-neutrinos being distinct Dirac fermions, rather than being identical Majorana fermions.

Here's a poser: Is an electron neutrino an electron without its charge?

  • $\begingroup$ To your poser: that's a good question, and it would seem likely. The WP article you cite is instructive. I wonder why neutrinos would oscillate in response to passage through matter, when supposedly the only interaction they can experience is to directly ram matter -- being chargeless, how could they otherwise interact at all? And what stops a neutrino? Only hadrons? $\endgroup$ Jan 30, 2014 at 0:01
  • $\begingroup$ @Cyberherbalist Neutrino oscillate in vacuum, too, though for the solar case the flavors are fully mixed by the time they leave the sun. The matter effect is due to coherent forward scattering of the electron flavor only (it is worth reading physics.stackexchange.com/questions/89804/…), which adds and additional term to the Hamiltonian. $\endgroup$ Jan 30, 2014 at 4:07
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    $\begingroup$ Electron neutrino has a different mass than electron, so it is not just an electron without charge. Also neutrinos always have left-handed chirality, which is not the case for the electrons. And moreover - electrons and el.neutrions have different weak isospin. So they are distinct particles with different properties. $\endgroup$
    – mpv
    Jan 30, 2014 at 4:10
  • $\begingroup$ @Uncle Al : Do you have a link to some paper discussing the disproval of neutrinos being Majorana fermions? $\endgroup$
    – mpv
    Jan 30, 2014 at 4:17
  • $\begingroup$ @mpv At this point it is premature to claim that neutrinos have been proved to have Dirac nature, but some preliminary data seems to rule out Majorana nature in a theoretically favored mass range. See arxiv.org/abs/1305.0056 . But compare to arxiv.org/abs/1108.4193 . $\endgroup$ Jan 30, 2014 at 5:31

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