Narrow Results

The dielectron yield in pp and dp reactions at a beam kinetic energy of 1.25 GeV/u has been measured using the HADES spectrometer at GSI. In proton-proton reactions the most abundant source above the π⁰ Dalitz region is expected to be Δ Dalitz decay. The obsearved large difference in dielectron production in pp and dp systems suggests that dielectron production in the dp system is dominated by a np process. In order to separate Δ Dalitz decays and np bremsstrahlung we compare the dielectron yield observed in pp with the one of dp reactions measured at the same beam energy.

Preliminary results on charged pion production in np collisions at an incident beam energy of 1.25 GeV obtained with HADES are presented. The np reactions were studied in dp collisions at 1.25 GeV/u using a Forward Wall hodoscope, aimed at registering spectator protons. The separation of np → ppπ^-, np → npπ⁺π^- and np → dπ⁺π^- channels is demonstrated.

By analyzing recent microscopic many-body calculations of few-nucleon systems and complex nuclei performed by different groups in terms of realistic nucleon–nucleon (NN) interactions, it is shown that NN short-range correlations (SRCs) have a universal character, in that the correlation hole that they produce in nuclei appears to be almost A-independent and similar to the correlation hole in the deuteron. The correlation hole creates high-momentum components, missing in a mean-field (MF) description and exhibiting several scaling properties and a peculiar spin–isospin structure. In particular, the momentum distribution of a pair of nucleons in spin–isospin state (ST) = (10), depending upon the pair relative (k_rel) and center-of-mass (c.m.) (K_c.m.) momenta, as well as upon the angle Θ between them, exhibits a remarkable property: in the region k_rel≳2 fm^-1 and K_c.m.≲1 fm^-1, the relative and c.m. motions are decoupled and the two-nucleon momentum distribution factorizes into the deuteron momentum distribution and an A-dependent momentum distribution describing the c.m. motion of the pair in the medium. The impact of these and other properties of one- and two-nucleon momentum distributions on various nuclear phenomena, on ab initio calculations in terms of low-momentum interactions, as well as on ongoing experimental investigations of SRCs, are briefly commented.