Thennquasifreendbreakup cross section: Discrepancies with theory and implications for the1

Abstract

Large discrepancies between quasi-free neutron-neutron (nn) cross section data from neutron-deuteron (nd) breakup and theoretical predictions based on standard nucleon-nucleon (NN) and three-nucleon (3N) forces are pointed out. The nn 1 S 0 interaction is shown to be dominant in that configuration and has to be increased to bring theory and data into agreement. Using the next-to-leading order (NLO) 1 S 0 interaction of chiral perturbation theory (χPT) we demonstrate that the nn QFS cross section only slightly depends on changes of the nn scattering length but is very sensitive to variations of the effective range parameter.In order to account for the reported discrepancies one must decrease the nn effective range parameter by about ≈ 12% from its value implied by charge symmetry and charge independence of nuclear forces.

Neutron-neutron quasifree scattering inndbreakup at 10 MeV

“…A pulsed neutron beam with a period of 400 ns and pulse width of about 2 ns was used. An uncollimated beam of 10 MeV neutrons produced via the 2 H(d, n) 3 He reaction bombarded a cylindrical deuterated polyethylene target ( = 28.3 mm and h = 36.4 mm). The target was oriented such that its central axis was perpendicular to the incident neutron beam axis.…”

“…This scaling results in a sign change of the nn scattering length, suggesting the existence of a weakly bound di-neutron state. The discrepancy can also be resolved by decreasing the effective range parameter by approximately 10%, implying greater charge-symmetry-and charge-independence-breaking than is expected [3]. Because of the serious implications of this discrepancy, we have undertaken an independent measurement of this cross section.…”

“…The measurements are performed at incident neutron beam energies below 20 MeV. The neutron beam is produced via the 2 H(d, n) 3 He reaction. The target is a deuterated plastic cylinder.…”

“…The theoretical values can be brought into agreement with these results by increasing the strength of the 1 S 0 nn potential matrix element by roughly 10%. However, this modification of the nn effective range parameter and/or the 1 S 0 scattering length causes substantial charge-symmetry breaking in the nucleon-nucleon force and suggests the possibility of a weakly bound di-neutron state [3]. We are conducting new measurements of the cross section for nn QFS in nd breakup.…”

Determination of Energy of n n-Singlet Virtual State in $$\varvec{d+{{}^{2}}}\hbox {H}\varvec{\rightarrow p+p+n+n}$$ d + 2

“…At the same time, the theory describes well the cross sections obtained for quasifree np scattering at these energies. Since, at low energies, the quasifree-scattering cross section is dominated by the singlet 1 S0 potential component, it was concluded by H.Witala and W Glöckle [3] that this very component was underestimated and this was a reason for the discrepancy between the experiment and theory. An increase of about 8% in this component allows to reach agreement between the theory and experiment for quasifree np and nn scattering.…”