Beam-target double-spin asymmetry in quasielastic electron scattering off the deuteron with CLAS

Abstract

Background: The deuteron plays a pivotal role in nuclear and hadronic physics, as both the simplest bound multi-nucleon system and as an "effective neutron target". Quasi-elastic electron scattering on the deuteron is a benchmark reaction to test our understanding of deuteron structure and the properties and interactions of the two nucleons bound in the deuteron.Purpose: The experimental data presented here can be used to test state-of-the-art models of the deuteron and the two-nucleon interaction in the final state after two-body breakup of the deuteron. Focusing on polarization degrees of freedom, we gain information on spin-momentum correlations in the deuteron ground state (due to the D-state admixture) and on the limits of the Impulse Approximation (IA) picture as it applies to measurements of spin-dependent observables like spin structure functions for bound nucleons. Information on this reaction can also be used to reduce systematic uncertainties on the determination of neutron form factors or deuteron polarization through quasi-elastic polarized electron scattering.Method: We measured the beam-target double spin asymmetry (A || ) for quasi-elastic electron scattering off the deuteron at several beam energies (1.6−1.7 GeV, 2.5 GeV, 4.2 GeV and 5.6−5.8 GeV), using the CEBAF Large Acceptance Spectrometer (CLAS) at the Thomas Jefferson National Accelerator Facility. The deuterons were polarized along (or opposite to) the beam direction. The double spin asymmetries were measured as a function of photon virtuality Q 2 (0.13−3.17 (GeV/c) 2 ), missing momentum (pm = 0.0 − 0.5 GeV/c), and the angle between the (inferred) "spectator" neutron and the momentum transfer direction (θnq). Results:The results are compared with a recent model that includes Final State Interactions (FSI) using a complete parameterization of nucleon-nucleon scattering, as well as a simplified model using the Plane Wave Impulse Approximation (PWIA). We find overall good agreement with both the PWIA and FSI expectations at low to medium missing momenta (pm ≤ 0.25 GeV/c), including the change of the asymmetry due to the contribution of the deuteron D-state at higher momenta. At the highest missing momenta, our data clearly agree better with the calculations including FSI. Conclusions:Final state interactions seem to play a lesser role for polarization observables in deuteron twobody electro-disintegration than for absolute cross sections. Our data, while limited in statistical power, indicate that PWIA models work reasonably well to understand the asymmetries at lower missing momenta. In turn, this information can be used to extract the product of beam and target polarization (P b Pt) from quasi-elastic electrondeuteron scattering, which is useful for measurements of spin observables in electron-neutron inelastic scattering. However, at the highest missing (neutron) momenta, FSI effects become important and must be accounted for.

Nucleon-nucleon correlations, short-lived excitations, and the quarks within

“…The carbon data is still under analysis. Other double polarization experiments were not sensitive to the effects of nucleon modification (Mayer et al, 2017;Mihovilovic et al, 2014;Passchier et al, 2002).…”