1.1 Standard Solar Model

This contribution describes the science and the technology of the BOREXINO experimentfor sub-MeV solar neutrinos at the undergroundGran Sasso laboratory. The paper shortly summarizes the material presented in reference 1. 1 Solar neutrinos and the solar neutrino problem 1.1 Standard Solar Model Our Sun shines because of fusion reactions similar to those envisioned for terrestrial fusion reactors. The temperature in the core of the Sun is suuciently high (about 15 million of degrees) to initiate hydrogen burning. The most important solar process, called pp cycle, is the fusion of four protons to form a helium nucleus, two positrons and two neutrinos. This chain is responsible for 98% of the energy produced, the residual 2% being produced by the CNO cycle. The amount of energy released in the pp cycle is 26.7 MeV, about 2% of which is carried away by neutrinos. Fig. 1 shows the solar neutrino spectrum predicted by the so-called standard solar model (SSM) 2. The model must reproduce the parameters of the Sun like for example its luminosity (L Sun = 3:8 10 26 W), its radius (R Sun = 7 10 8 m) and its mass (2 10 33 g). The energy generated in the core of the Sun (r 0:25R Sun) by nuclear fusion reactions is radiatively transported through the inner part and (by convection) in the outer part (r 0:7R Sun) of the Sun. The Sun emits around 2 10 38 neutrino per second with energy up to 18.8 MeV; they leave the Sun core and reach the Earth in about 8 min. 1.2 Solar neutrino experiments and the solar neutrino problem The standard solar model and the standard model of particle physics combined make a deenite prediction about the outcome of a solar neutrino experiment on Earth. On one side hand, the SSM predicts the neutrino ux and spectral composition, while the standard model of particle physics deenes the neutrino to be a massles, chargeless, spin 1/2 Dirac particle that propagates unaltered from the Sun to the Earth. 1 Figure 1. Flux of pp Solar Neutrinos as predicted by the standard solar model (based on astro-ph/9805135). Experimental thresholds of a few experiments are indicated: Home-stake (Chlorine), Gallex, Sage and GNO (Gallium) and SuperKamiokande. The CNO component is not shown. Solar neutrinos have been measured by ve experiments: Homestake, Kamiokande, Superkamiokande, Gallex and Sage 3. GNO has recently added to the list and SNO …