Astroparticle Physics

6.2 Neutrino Astronomy 97A reconstructed image of the Sun in the light of neutrinosis shown in Fig. 6.20.Many proposals have been made to solve the solar neutrinoproblem. The first thing for elementary particle physicistsis to doubt the correctness of the standard solar model.The flux of 8 B neutrinos varies with the central temperatureof the Sun like ∼ T 18 . A reduction by only 5% of the centralsolar temperature would bring the Kamiokande experimentalready in agreement with the now reduced expectation.However, solar astrophysicists consider even a somewhatlower central temperature of the Sun rather improbable.The theoretical calculation of the solar neutrino flux usesthe cross sections for the reactions (6.27) up to (6.34). Anoverestimate of the reaction cross sections would also leadto a too high expectation for the neutrino flux. A variation ofthese cross sections in a range which is considered realisticby nuclear physicists is insufficient to explain the discrepancybetween the experimental data and expectation.If neutrinos had a mass, they could also possess a magneticmoment. If their spin is rotated while propagating fromthe solar interior to the detector at Earth, one would not beable to measure these neutrinos because the detectors are insensitiveto neutrinos of wrong helicity.Finally, solar neutrinos could decay on their way fromSun to Earth into particles which might be invisible to theneutrino detectors.A drastic assumption would be that the solar fire hasgone out. In the light of neutrinos this would become practicallyimmediately evident (more precisely: in 8 minutes).The energy transport from the solar interior to the surface,however, requires a time of several 100 000 years so that theSun would continue to shine for this period even though thenuclear fusion at its center has come to an end.Since all mentioned explanations are considered ratherunlikely, it is attractive to interpret a deficit of solar neutrinosalso by oscillations like in the case of atmospheric neutrinos.In addition to the vacuum oscillations described by(6.23), solar neutrinos can also be transformed by so-calledmatter oscillations. The flux of electron neutrinos and itsoscillation property can be modified by neutrino–electronscattering when the solar neutrino flux from the interior ofthe Sun encounters collisions with the abundant number ofsolar electrons. Flavour oscillations can even be magnifiedin a resonance-like fashion by matter effects so that certainFig. 6.20Reconstructed image of the Sun inthe light of solar neutrinos. Due tothe limited spatial and angularresolution of Super-Kamiokande,the image of the Sun appears largerthan it really is {12}cross sectionmagnetic momentof neutrinos?neutrino decay?extinct solar fire?neutrino oscillations?matter oscillations