Solar neutrino problem

The solar neutrino problem concerned a large discrepancy between the flux of <a href="/facts/Solar_neutrino/IETdGDoj">solar neutrinos</a> as predicted from the <a href="/facts/Sun/CadPCVHh">Sun</a>'s <a href="/facts/Luminosity/1tdhpk9l">luminosity</a> and as measured directly. The discrepancy was first observed in the mid-1960s and was resolved around 2002.
The flux of <a href="/facts/Neutrinos/yGWVfSg2">neutrinos</a> at <a href="/facts/Earth/HLLmDfj0">Earth</a> is several tens of billions per square centimetre per second, mostly from the <a href="/facts/Sun%2527s_core/V4wKBVzp">Sun's core</a>. They are nevertheless difficult to detect, because they interact very weakly with matter, traversing the whole <a href="/facts/Earth/HLLmDfj0">Earth</a>. Of the three types (<a href="/facts/Flavour_(particle_physics)/RR7rsuTD">flavors</a>) of neutrinos known in the <a href="/facts/Standard_Model/LhVGf9ow">Standard Model</a> of <a href="/facts/Particle_physics/YgIuyj2t">particle physics</a>, the Sun produces only <a href="/facts/Electron_neutrino/e5hQPWIQ">electron neutrinos</a>. When <a href="/facts/Neutrino_detector/LwtyrVAc">neutrino detectors</a> became sensitive enough to measure the flow of electron neutrinos from the Sun, the number detected was much lower than predicted. In various experiments, the number deficit was between one half and two thirds.
Particle physicists knew that a mechanism, discussed in 1957 by <a href="/facts/Bruno_Pontecorvo/xAHhKjoS">Bruno Pontecorvo</a>, could explain the deficit in electron neutrinos. However, they hesitated to accept it for various reasons, including the fact that it required a modification of the accepted Standard Model. They first pointed at the solar model for adjustment, which was ruled out. Today it is accepted that the neutrinos produced in the Sun are not massless particles as predicted by the Standard Model but rather <a href="/facts/Mixed_quantum_state/7jbxd7Dx">mixed quantum states</a> made up of defined-<a href="/facts/Mass/HHdc8XmF">mass</a> <a href="/facts/Quantum_state/7jbxd7Dx">eigenstates</a> in different (<a href="/facts/Complex_number/2w7mqI7e">complex</a>) proportions. That allows a neutrino produced as a pure electron neutrino to <a href="/facts/Neutrino_oscillation/c1cc5noc">change during propagation</a> into a mixture of electron, muon and tau neutrinos, with a reduced probability of being detected by a detector sensitive to only electron neutrinos.
Several neutrino detectors aiming at different flavors, energies, and traveled distance contributed to our present knowledge of neutrinos. In 2002 and 2015, a total of four researchers related to some of these detectors were awarded the <a href="/facts/Nobel_Prize_in_Physics/o6w1nCbr">Nobel Prize in Physics</a>.

Solar neutrino problem open-in-new

Solar neutrino problem