Proton spin structure and generalized polarizabilities in the strong quantum chromodynamics regime

Abstract

The strong interaction is not well understood at low energy, or for interactions with low momentum transfer Q 2 , but one of the clearest insights we have comes from Chiral Perturbation Theory (χPT). This effective treatment gives testable predictions for the nucleonic generalized polarizabilitiesfundamental quantities describing the nucleon's response to an external field. We have measured the proton's generalized spin polarizabilities in the region where χPT is expected to be valid. Our results include the first ever data for the transverse-longitudinal spin polarizability δLT , and also extend the coverage of the polarizability d2 to very low Q 2 for the first time. These results were extracted from moments of the structure function g2, a quantity which characterizes the internal spin structure of the proton. Our experiment ran at Jefferson Lab using a polarized electron beam and a polarized solid ammonia (NH3) target. The δLT polarizability has remained a challenging quantity for χPT to reproduce, despite its reduced sensitivity to higher resonance contributions; recent competing calculations still disagree with each other and also diverge from the measured neutron data at very low Q 2 . Our proton results provide discriminating power between existing calculations, and will help provide a better understanding of this strong QCD regime.

Unbound Low-Energy Nucleons As Semiclassical Quantum Networks

“…These quantities are depicted in Figure 4. Substituting into Equation (32) and applying Equation (36) yields…”

“…Nucleon modeling via lattice QCD may be conceptually correct, but is extremely computationally expensive [24], and it is not known how bare current quarks transition into dressed constituent quarks [25,26]. The precision of chiral effective field theory (EFT) [27] has generally been good, but recent discrepancies have emerged at the lowest energies [28][29][30][31][32][33]. This paper offers an alternative approach to modeling unbound ground-state nucleons that seamlessly merges with chiral EFT [27] and lattice QCD [24,34] at higher energies.…”