Neutrino Astrophysics: Solar neutrino problem

The neutrino detectors discussed in the last post are all well and good, but there's a bit of a problem. It's substantial enough to have earned itself a catchy name: the solar neutrino problem. See, the theory of electroweak interactions and astrophysical models of the Sun makes fairly precise predictions about the fluxes of neutrinos that should be measured by these various detectors. But when the detectors go and look for these neutrinos, they find substantially fewer - around a third to a half as many as theorists predicted. This is true all across the board, from the chemical to the Cerenkov detectors, and caused physicists quite the headache.

From Bahcall, Solar Neutrinos.

Initially, it seemed like this was bad news for us. Neutrinos come to us straight from the center of the sun, as opposed to photons, which take thousands to tens of thousands of years to bounce their way out of the center, so it seemed possible that the lack of neutrinos meant that against all odds, the sun's fusion was dying!

Luckily, SNO came to the rescue. As you can see at the right of the image above, the electron-neutrino reaction monitored by SNO, like all the other detectors, finds less than 30% as many neutrinos as expected. But, when we look at the other reaction, the one insensitive to neutrino flavor, we find around 90% of the expected flux, within uncertainties. So it seems that somehow the neutrinos produced in the center of the Sun, which are all electron neutrinos, somehow morph into other types as they travel to us. This phenomenon is called neutrino oscillations, and will be discussed briefly in the near future.

This is the third post in a series on neutrino astrophysics. Other neutrino-related posts can be found here.