Pycnonuclear reactions taking place in dense astrophysical plasmas at very low temperatures are enhanced due to electron screening of the Coulomb barrier even by many orders of magnitude. Study of the d+d reactions at very low energies in metallic environments, being a very good model for the strongly coupled plasma, enables us to determine the strength of this effect in the terrestrial laboratories. First experiments performed under high vacuum (HV) conditions showed that the experimentally determined screening energies corresponding to the reduction of the Coulomb barrier height were larger than the theoretical values calculated in terms of the dielectric function theory by at least a factor of two.
Since contamination of the target surface plays a crucial role in the screening experiments we performed a series of new experiments dealing with atomically clean targets under ultra-high vacuum (UHV) conditions. The resulting screening energies turned to be significantly larger than the previous experimental values, stepping up the discrepancy to the theoretical data. As the origin of the so-called enhanced screening effect observed in nuclear reactions taking place in metals still remains unexplained we discuss here two alternative scenarios. First, we analyse a strong long-range correlation between conduction electrons as a solid-state effect. On the other hand, we examine the interplay between a strong plasma screening and a narrow resonance placed close to reaction threshold, which leads to the target material dependence of the reaction cross section.