We agree with David Dean (Physics Today, Physics Today 0031-9228 60 11 2007 48 https://doi.org/10.1063/1.2812123 November 2007, page 48 ) that computational advances in solving many-body problems have led to important progress in understanding the structure of nuclei. However, we need to examine two of the five questions highlighted by the author, namely, the nature of the nuclear forces (beyond the well-understood long-range pion exchange) that bind nucleons in nuclei and the structure of neutron stars and dense cold nuclear matter. The answers to those two cannot be obtained within the mean-field approximation without probing the high-momentum component of the nuclear wavefunction. In the mean-field approximation and in the effective field theory approach, that component is hidden in the parameters of the effective interaction. At the same time, according to the most realistic calculations (see reference 1 and references therein), approximately 60% of the kinetic energy of nucleons in medium to heavy nuclei is due to the high-momentum component of the nuclear wavefunction.
In this respect we would like to mention recent significant progress in the investigation of the high-momentum nucleon–nucleon short-range correlations (SRC) that for years remained an elusive feature of the nuclei. The progress was made through the use of high-energy electrons at the Thomas Jefferson National Accelerator Facility and high-energy protons at Brook-haven National Laboratory. 2 Analyses of those data demonstrate that nucleons with momenta exceeding the Fermi momentum are present in medium and heavy nuclei with a probability of approximately 20–25%. The shape of the momentum distributions of the SRC does not depend on atomic number. Experiments also established that large nucleon momenta are balanced predominantly by one nearby nucleon with strong preference (by a factor of nine) for a proton momentum to be balanced by a neutron rather than by a proton [see Physics Today, July 2008, page 15 ]. Several observed properties of the dynamics of high-momentum nucleons in nuclear medium surprisingly resemble the properties deduced from the short-range two-nucleon interaction. 1,3
Further investigations of nuclear phenomena sensitive to the SRC are planned at Jefferson Lab, both at 6 GeV and after the 12-GeV upgrade, a high-priority item in the long-range nuclear physics plan of the US Nuclear Science Advisory Committee. 4 Those studies represent one of the best opportunities for exploring the drops of high-density cold nuclear matter and for understanding quark–gluon degrees of freedom in the short-range nuclear forces.
