Astroparticle Physics

1.3 Start of the Satellite Era 13COBE also found spatial asymmetries of the 2.7 Kelvinblackbody radiation at a level of T /T ≈ 10 −5 .Thisimpliesthat the early universe had a lumpy structure, whichcan be considered as a seed for galaxy formation.In parallel with the advance of cosmology, the famoustwo-neutrino experiment of Lederman, Schwartz, and Steinbergerin 1962 (Nobel Prize 1988) represented an importantstep for the advancement of astroparticle physics. This experimentdemonstrated that the neutrino emitted in nuclearbeta decay is not identical with the neutrino occurring inpion decay (ν µ ̸= ν e ). At present, three generations of neutrinosare known (ν e , ν µ ,andν τ ). The direct observationof the tau neutrino was established only relatively recently(July 2000) by the DONUT 8 experiment.The observation of solar neutrinos by the Davis experimentin 1967 marked the beginning of the disciplineof neutrino astronomy (Nobel Prize for R. Davis 2002).In fact, Davis measured a deficit in the flux of solar neutrinos,which was confirmed by subsequent experiments,GALLEX 9 ,SAGE 10 , and Kamiokande (Nobel Prize for M.Koshiba 2002). It is considered unlikely that a lack of understandingof solar physics is responsible for the solar neutrinoproblem. In 1958 Pontecorvo highlighted the possibilityof neutrino oscillations. Such oscillations (ν e → ν µ )are presently generally accepted as explanation of the solarneutrino deficit. This would imply that neutrinos havea very small non-vanishing mass. In the framework of theelectroweak theory (Glashow, Salam, Weinberg 1967; NobelPrize 1979) that unifies electromagnetic and weak interactions,a non-zero neutrino mass was not foreseen. Theintroduction of quarks as fundamental constituents of matter(Gell-Mann and Zweig 1964, Nobel Prize for Gell-Mann1969), and their description by the theory of quantum chromodynamicsextended the electroweak theory to the StandardModel of elementary particles (Veltman, t’Hooft; NobelPrize 1999).In this model, the masses of elementary particles cannotbe calculated a priori. Therefore, small non-zero neutrinomasses should not represent a real problem for the standardmodel, especially since it contains 18 free parameters that8 DONUT – Direct Observation of NU Tau (ν τ )9 GALLEX – German–Italian GALLium EXperiment10 SAGE – Soviet American Gallium ExperimentFig. 1.14Penzias and Wilson in front of theirhorn antenna used for measuring ofthe blackbody radiation {5}neutrino oscillationelectroweak theoryquarksStandard Model