Weak charge form factor and radius of208Pb through parity violation in electron scattering

Abstract

We use distorted wave electron scattering calculations to extract the weak charge form factor F W (q), the weak charge radius R W , and the point neutron radius R n of 208 Pb from the Lead Radius Experiment (PREX) parity-violating asymmetry measurement. The form factor is the Fourier transform of the weak charge density at the average momentum transferq = 0.475 fm −1 . We find F W (q) = 0.204 ± 0.028 (exp) ± 0.001 (model). We use the Helm model to infer the weak radius from F W (q). We find R W = 5.826 ± 0.181 (exp) ± 0.027 (model) fm. Here the experimental error includes PREX statistical and systematic errors, while the model error describes the uncertainty in R W from uncertainties in the surface thickness σ of the weak charge density. The weak radius is larger than the charge radius, implying a "weak charge skin" where the surface region is relatively enriched in weak charges compared to (electromagnetic) charges. We extract the point neutron radius R n = 5.751 ± 0.175 (exp) ± 0.026 (model) ± 0.005 (strange) fm from R W . Here there is only a very small error (strange) from possible strange quark contributions. We find R n to be slightly smaller than R W because of the nucleon's size. Finally, we find a neutron skin thickness of R n − R p = 0.302 ± 0.175 (exp) ± 0.026 (model) ± 0.005 (strange) fm, where R p is the point proton radius.

Density dependence of the symmetry energy from neutron skin thickness in finite nuclei

“…It implies a central value for Δr np that lies both within the range between 0.15 and 0.22 fm suggested by the experiments with strong probes [33,84], and within the range between 0.14 and 0.20 fm suggested by recent constraints on the nuclear EOS derived from observed masses and radii of neutron stars [85,86]. While the latter constraints and a majority of studies point toward a fairly thin neutron skin in lead [28], it is to be mentioned that ruling out a thick neutron skin as suggested by the central value of the first run of PREX [65,72] could be premature [87].…”

Effects of the symmetry energy on the isovector properties of neutron-rich nuclei within a Thomas-Fermi approach

“…Even in this case, we found that the intersection between our results and the results compatible with (44) corresponds to values of binding energy very close to the experimental for the specific nuclei. Very recently the Lead Radius Experiment (PREX) at the Jefferson Laboratory has provided the first model-independent evidence for the existence of a neutron-rich skin in 208 Pb [66,67]. The determined neutron skin was R skin = 0.33 +0.…”

Density dependence of the nuclear symmetry energy from measurements of neutron radii in nuclei

“…It implies a central value for Δr np that lies both within the range between 0.15 and 0.22 fm suggested by the experiments with strong probes [73,37], and within the range between 0.14 and 0.20 fm suggested by recent constraints on the nuclear EOS derived from observed masses and radii of neutron stars [74,75]. While the latter constraints and a majority of studies point toward a fairly thin neutron skin in lead [28], it is to be mentioned that ruling out a thick neutron skin as suggested by the central value of the first run of PREX [53,62] could be premature [76]. The 3%-accuracy test value of A pv plotted in Fig.…”

“…With some mild model assumptions, the analysis of this value has allowed to extract Δr np = 0.33 +0.16 −0.18 fm for the NST of 208 Pb [53], and Δr np = 0.302 ± (0.175) exp ± (0.026) model ± (0.005) strange fm, where the last uncertainty arises from the uncertainty in the electric strange quark form factor, in a following study [62]. Both results are much consistent with previous estimates, although the central value is larger than the typical value reported from the existing evidence on Δr np of 208 Pb [28].…”