Parity violating measurements of neutron densities - Jefferson Lab

Analysis II: Weak form factor
• Compare Rw to well measured Rch=5.503 fm implies weak charge skin
Rw-Rch=0.32±0.18(exp)±0.03(model) fm. Experimental milestone: direct
evidence that weak charge density is more extended than E+M charge
density.
• Rch slightly larger than point proton radius, Rch 2 =Rp 2 ++N/Z
(neglecting small spin-orbit and meson exchange currents).
• Proton size =0.769 fm 2 , neutron =-0.116 fm 2 --> Rp=5.45 fm.
• (Qw/qnN)Rw 2 =Rn 2 +(qpZ/qnN)Rch 2 ++Z/N-(A/qnN)
• Total weak charge Qw=Nqn+Zqp=-118.55. Radiative corrected neutron
weak charge is qn=-0.9878, proton qp=0.0721.
• Nucleon strangeness radius ≤±0.04 fm 2 from HAPPEX, G0, A4...
• From Rw deduce Rn=5.751 ±0.175(exp) ±0.026(model) ±0.005(strange) fm.
• Neutron skin: Rn-Rp=0.302 ±0.175 ±0.026 fm, consistent with Analysis I,
Rn-Rp=0.33 +0.16 -0.18 fm, within limitations of Helm model (≈ ± model error).
-- Phys. Rev. C 85, 032501(R) (2012)