Tackling the hypothesis of charge independence and charge symmetry of nuclear forces using the Trojan Horse Method

The difference in proton-proton and neutron-neutron scattering lengths contributes to understanding the charge-symmetry breaking of nuclear forces, yet, the Coulomb-free proton-proton scattering length (a_pp) cannot be measured directly and relies heavily on numerous and distinct theoretical techniques to remove the Coulomb contribution. In the article here highlighted and available online at https://doi.org/10.1038/s42005-023-01221-0 or as a PDF here https://www.nature.com/articles/s42005-023-01221-0.pdf, the authors determine the Coulomb-free p-p scattering length from the half-off-the-energy-shell p-p scattering cross section measured at center-of-mass energies below 1 MeV using the quasi-free p + d→p + p + n reaction. The resulting difference in proton-proton and neutron-neutron scattering lengths suggests a lower charge symmetry breaking of nuclear forces than predicted so far. A model based on universality concepts has been developed to interpret this result in the framework of the short-range physics. This result reveals a field of low-energy nuclear physics that borders on the complexity of elementary particle physics, but which can be described with the great simplicity that characterizes the window of universality.