PI: Francesca Cavanna
WG members: M Aliotta, D Bemmerer, C Bruno, A Caciolli, P Corvisiero, L. Csedreki, R Depalo, F Ferraro, E Fiore, A Formicola, G Gervino, A Guglielmetti, C Gustavino, I Kochanek, L E Marcucci, V Mossa, V Paticchio, K. Stoeckel, O Straniero,D Trezzi, S Zavatarelli, G. Zorzi.
Astrophysical Motivation:

Determination of the primordial deuterium abundance

Consequence on the ΛCDM cosmological parameters

Neutrino physics^{}
The primordial abundance of light nuclei can be calculated using cosmology, nuclear physics and particle physics unified in the socalled Big Bang Nucleosynthesis (BBN) theory. Moreover, the same primordial abundances can be independently derived starting from spectroscopic observations of ancient astrophysical objects, like quasars or metal poor stars. Usually, data from observations are characterized by higher uncertainties with respect to BBN calculations, except for deuterium. In this case, the uncertainty on the nuclear cross section of the processes involved in deuterium production and destruction are too high. The main contribution comes from the ^{2}H(p,γ)^{3}He reaction, the nuclear cross section of which is known at the 610% level in the BBN energy range (about 30300 keV).
Thanks to the low background of the underground laboratory “Laboratori Nazionali del Gran Sasso” (LNGS) and to the experience accumulated in about 25 years of scientific activity, LUNA (Laboratory for Underground Nuclear Astrophysics) plans to directly measure this reaction in the BBN energy range with a 34% accuracy. Thus, aims of this project are:

the measurement of this reaction using two different (BGO and HpGe) detectors and a windowless gas target,

the consequent data analysis and calculation of the primordial ^{2}H abundance,

the study of possible cosmological outcomes.
Experimental techniques to reduce all possible systematic uncertainties will be developed. Moreover, thanks to the new “peak shape analysis method”, the differential cross section will be provided, even at low energy (< 250 keV). Such differential cross section is also important for theoretical nuclear physics, in particular for “abinitio” or χEFT calculations. To obtain the differential cross section, a low systematic uncertainty measurement of the HpGe detector efficiency along the extended target is required.
Two are the possible outcomes of our measurement: an agreement or disagreement between BBN calculations and astronomical observations. In the former case, thanks to the high accuracy achieved in this project, it will be possible to use astronomical data together with the new cross section value to provide strong constraints on cosmological parameters such as the baryon energy density Ω_{b}. In the latter case, assuming astronomical observations free from unknown systematics, possible solutions to the discrepancy stay within nonstandard cosmology or new physics.