Measurement of the Neutron Radius of 208Pb Through Parity-Violation in Electron Scattering

Abstract: We report the first measurement of the parity-violating asymmetry A_PV in the elastic scattering of polarized electrons from 208Pb. A_PV is sensitive to the radius of the neutron distribution (Rn). The result A_PV = 0.656 \pm 0.060 (stat) \pm 0.014 (syst) ppm corresponds to a difference between the radii of the neutron and proton distributions Rn - Rp = 0.33 +0.16 -0.18 fm and provides the first electroweak observation of the neutron skin which is expected in a heavy, neutron-rich nucleus.

• The paper demonstrates a novel electroweak method by measuring a parity-violating asymmetry of 0.656 ppm to determine a neutron skin thickness of 0.33 fm in 208Pb.
• It employs precise polarized electron scattering to establish the neutron-proton radius difference, offering a model-free approach to probing nuclear structure.
• The results have significant implications for refining nuclear models and understanding the equation of state in neutron-rich matter relevant to astrophysical phenomena.

Measurement of the Neutron Radius of $^{208}$Pb Through Parity-Violation in Electron Scattering

This paper presents a significant experimental achievement in nuclear physics: the first measurement of the parity-violating asymmetry ($A_{PV}$) in the elastic scattering of polarized electrons from $^{208}$Pb. By leveraging the electroweak interaction, this experiment provides a new methodology to probe the neutron distribution within the nucleus. Specifically, the work focuses on accurately determining the neutron radius ($R_n$) in relation to the proton radius ($R_p$) in $^{208}$Pb, offering valuable insights into the neutron skin thickness in heavy, neutron-rich nuclei.

The authors report $\APV = 0.656 \pm 0.060\ {\rm (stat)}\pm0.014\ {\rm (syst)}$ parts per million (ppm). This measurement allows the calculation of the neutron-proton radius difference $R_n-R_p$ as 0.33 fm, with an uncertainty range of $+0.16$ to $-0.18$ fm. These numerical results are crucial as they offer the first electroweak observation of the neutron skin. The evidence obtained supports theoretical predictions regarding the presence of a neutron skin in heavy nuclei that are rich in neutrons, such as $^{208}$Pb.

The practical implications of this research are substantial. Understanding neutron skin thickness is vital for nuclear models that describe the structure of atomic nuclei and the equation of state (EOS) for neutron-rich matter, which has implications for both terrestrial experiments and astrophysical phenomena, such as neutron stars. This experimental approach, utilizing parity violation, complements hadronic probes, providing an independent and model-free determination of neutron distributions.

Theoretically, the findings of this paper may catalyze further research into both nuclear structure and fundamental symmetries in nature. As the measurement methodologies evolve and improve, additional data on parity-violating asymmetries in other heavy nuclei could refine the understanding of neutron skins and offer stringent tests for nuclear theory models, including mean-field theories and ab initio methods.

Future developments would likely involve extending the parity-violating asymmetry measurements to a wider range of isotopes, thereby testing models of nuclear matter under varying neutron-to-proton ratios. Such explorations hold potential for elucidating the isovector component of the nuclear force, influencing both nuclear physics and the understanding of asymmetry energy in the nuclear EOS.

In summary, this paper makes a valuable contribution to nuclear physics by utilizing a novel electroweak method to measure the neutron radius in heavy nuclei, opening avenues for deeper understanding and subsequent research in nuclear structure and astrophysical phenomena.