# Theory Center Research Highlights

### Strangeness and Parity Violation in Hadrons and Nuclei

**
"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 *Q*^{2} ≤ 0.1 GeV^{2},
which are largest at backward angles, and have a strong
*Q*^{2} dependence at low *Q*^{2}
(see figure).
Two-boson contributions to the weak axial current are found to be
enhanced at low *Q*^{2} 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
*Q*^{2} = 0.1 GeV^{2}. 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 *Q ^{2}* ~ 0.3 GeV

^{2}, 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

*Q*= 0.1 GeV

^{2}^{2}. 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
*G _{E}^{s}(Q^{2}*=0.1 GeV

^{2}) 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

*G*, this result considerably constrains the role of hidden flavor in the structure of the nucleon.

_{M}^{s}