a reply to: Arbitrageur
Yeah the solar neutrino problem is an interesting one and a good shout out.
As you said, the early experiments that looked for solar neutrinos came out with roughly 1/4 to 1/3 of the expected flux. Theorists scratched their
heads, did some more numbers and said "Hey, you guys did something wrong, there is no way the sun would work if we reduce the energy production
The experimentalists looked at their experiment and double checked and added more data to the findings and nope, couldn't solve it.
So it was thought maybe the method was a bit wrong or there was some effect not being accounted for. So they performed the experiment in i believe 3
different locations using 3 different detector materials and methods.
Still the numbers didnt work out. So physicists sat back and thought, hmmm what could be happening? At the same time, one of the detectors which is
directionally sensitive, and sensitive to the flavour of neutrinos had another interesting result... Neutrinos originating from the atmosphere from
cosmic ray interactions had different fluxes depending on if they had originated from directly above or from the atmosphere on the opposite side of
A Theorist then dug out some very old notes from a dead theorist (i think the work was done in the 1930s) which said basically that if neutrinos have
mass, it is possible that the the mass and flavour eigenstates do not have to propagate at the same rate. THUS a neutrino born as one flavour, can
spontaneously flavour change during flight. For this to work the mass needs to be very low, and the differences between the states needs to be small
So the experimentalists went to work. Now the detector in Japan that was directional and can separate a couple of flavours of neutrinos from each
other wouldn't actually be able to tell if electron neutrinos from the sun had oscillated into mu or tau neutrinos since such a neutrino would be of
such low energy it could not undergo a lepton production process when interacting with the detector. Still they ran the numbers and yes, it started to
look leasable that all the experiments performed where seeing what they expected too, all the experiments were consistent with oscillation (very
The neutrinos from the atmosphere, theory matched that well also (same theory is used for all oscillation, not just the ones from the sun)
But the story doesn't end there, Scientists figured out that if they are able to use a hybrid detector, a detector that has a component of it that
will give a very specific signal should an electron neutrino interact ( SNO ) it meant that they could compare the Electron neutrino flux, with the
total flux and make corrections for the theoretical interaction cross sections which would be suppressed in the scattering of electrons by mu and tau
This experiment was ultimately successful and was operated in 3 different phases with different detector configurations and materials in order to make
sure it wasn't weird effect specific to the detector.
Their numbers? Well the first low stats result predicted the number of neutrinos coming from the sun was almost exactly the same as the theoretical
prediction. This was done using blind analysis also, in which scientists work on analysis without actually looking at the data set. Once they opened
the box and applied their analysis... boom result without searing for it or bias removal of backgrounds.
The current state of play? Well all parameters of the Neutrino mixing are entering the precision stage, there are a few areas of interest still left
to go and the current leading experiment (T2K - where i did my PhD) is in its final stages before upgrades are to be performed. The state of the art
right now is to search for CP violation processes in the neutrino sector since CP violation is a theoretical possibility. So the method to do that is
to change from neutrino to anti-neutrino mode (this is a neutrino beam experiment, the switch is quite simple) and checking that the oscillation
parameters for neutrinos are identical to those of anti-neutrinos.
The search carries on