![]() Today it is accepted that the neutrinos produced in the Sun are not massless particles as predicted by the Standard Model but rather mixed quantum states made up of defined- mass eigenstates in different ( complex) proportions. They first pointed at the solar model for adjustment, which was ruled out. However, they hesitated to accept it for various reasons, including the fact that it required a modification of the accepted Standard Model. Particle physicists knew that a mechanism, discussed back in 1957 by Bruno Pontecorvo, could explain the deficit in electron neutrinos. In various experiments, the number deficit was between one half and two thirds. When neutrino detectors became sensitive enough to measure the flow of electron neutrinos from the Sun, the number detected was much lower than predicted. Of the three types ( flavors) of neutrinos known in the Standard Model of particle physics, the Sun produces only electron neutrinos. They are nevertheless hard to detect, because they interact very weakly with matter, traversing the whole Earth. The flux of neutrinos at Earth is several tens of billions per square centimetre per second, mostly from the Sun's core. The discrepancy was first observed in the mid-1960s and was resolved around 2002. The solar neutrino problem concerned a large discrepancy between the flux of solar neutrinos as predicted from the Sun's luminosity and as measured directly. Issue in astrophysics regarding discrepancy between the Sun's luminosity and neutrinos
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