Underground measurement of 14N(p, γ)15O astrophysical factor at low energy

In stars, four hydrogen nuclei are converted into a helium nucleus by two competing nuclear fusion processes: the proton-proton chain (p-p) and the carbon-nitrogen-oxygen (CNO) cycle. At temperatures higher than 2-107 K, the CNO cycle dominates the energy production. In particular, its rate is determined by the slowest reaction: N-14(p,gamma)O-15. Direct measurement in a laboratory at the surface of the Earth is hampered by the background due to the cosmic rays. Here we report on an experiment performed with the LUNA (Laboratory for Underground Nuclear Astrophysics) accelerator placed deep underground in the Gran Sasso laboratory (Italy). Thanks to the cosmic ray suppression provided by the mountain shield, we could measure the N-14(p,gamma)O-15 cross section for the first time directly at energies corresponding to stellar temperatures and with unprecedented accuracy. The results are strictly related to carbon stars formation, an independent lower limit on the age of the universe and solar neutrinos flux. The N-13 and O-15 neutrinos coming from the CNO cycle are strictly correlated to the N-14(p,gamma)O-15 S-factor and their flux will play an important role in some future solar neutrino experiment, such as Borexino.