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Theory Center Research Highlights

Strangeness and Parity Violation in Hadrons and Nuclei

Two-boson exchange corrections Δρ (in %) to the (hadronic) vector part of the parity-violating asymmetry, as a function of Q2 for fixed scattering angles θ=10, 60, 110 and 150o.

"Effect of two-boson exchange on parity-violating e p scattering"
J.A. Tjon, W. Melnitchouk
Phys. Rev. Lett. 100, 082003 (2007)

We compute the corrections from two-photon and γ-Z exchange in parity-violating elastic electron-proton scattering, used to extract the strange form factors of the proton. We use a hadronic formalism that successfully reconciled the earlier discrepancy in the proton's electron to magnetic form factor ratio, suitably extended to the weak sector. Implementing realistic electroweak form factors, we find effects of the order 2-3% at Q2 ≤ 0.1 GeV2, which are largest at backward angles, and have a strong Q2 dependence at low Q2 (see figure). Two-boson contributions to the weak axial current are found to be enhanced at low Q2 and for forward angles. We provide corrections at kinematics relevant for recent and upcoming parity-violating experiments.

Determination of the magnetic and electric strange form factors at Q2 = 0.1 GeV2. Contours depict the 68% and 95% confidence intervals for the determination of the two form factors. The red ellipse displays the theory results of Leinweber et al. [Phys. Rev. Lett. 94, 212001 (2005)].

"Extracting nucleon strange and anapole form factors from world data"
R.D. Young, J. Roche, R.D. Carlini, A.W. Thomas
Phys. Rev. Lett. 97, 102002 (2006)

Using the complete world set of parity violating electron scattering data up to Q2 ~ 0.3 GeV2, we extract the current best determination of the strange electric and magnetic form factors of the proton, as well as the weak axial form factors of the proton and neutron at Q2 = 0.1 GeV2. The results are consistent with state of the art calculations of all four form factors, with the latter including the anapole contribution.

"Strange electric form factor of the proton"
D.B. Leinweber, S. Boinepalli, A.W. Thomas, P. Wang, A.G. Williams, R.D. Young, J.M. Zanotti, J.B. Zhang
Phys. Rev. Lett. 97, 022001 (2006)

By combining the constraints of charge symmetry with new chiral extrapolation techniques and recent low mass lattice QCD simulations of the individual quark contributions to the electric charge radii of the baryon octet we obtain an accurate determination of the strange electric charge radius of the proton. While this analysis provides a value for GEs(Q2=0.1 GeV2) in agreement with the best current data, the theoretical error is comparable with that expected from future HAPPEx results from JLab. Together with the earlier determination of GMs, this result considerably constrains the role of hidden flavor in the structure of the nucleon.