First Measurement of the ${}^3\mathrm{He}{(}^3\mathrm{He},2\mathrm{}\mathit{p}{)}^4\mathrm{He}$ Cross Section down to the Lower Edge of the Solar Gamow Peak

We give the LUNA results on the $\sigma \left(E\right)$ cross section measurement of a key reaction of the proton-proton chain strongly affecting the calculated neutrino luminosity from the Sun: ${}^3\mathrm{He}{(}^3\mathrm{He},2p{)}^4\mathrm{He}$. Because of the cosmic ray suppression provided by the Gran Sasso underground laboratory, it has been possible to measure $\sigma \left(E\right)$ throughout the energy range in which this reaction occurs in the Sun, i.e., down to $16.5\mathrm{keV}$ center of mass energy. The data clearly show the cross section increase due to the electron screening effect but they do not exhibit any evidence for a narrow resonance suggested to explain the ${}^8\mathrm{B}$ and ${}^7\mathrm{Be}$ solar neutrino flux reduction.