Strange Quarks in the Proton

The worldwide program of parity violating electron scattering data that constrain the contributions of strange quarks to the proton’s charge and magnetism at large spatial distances (low Q2). The solid ellipse represents a fit to the data shown, incorporating a theoretical prediction for the proton’s axial form factor (GA), which is not yet well-constrained experimentally. The dashed ellipse incorporates more data at shorter spatial distances and removes the theoretical constraint on the axial term.

While the proton is most simply described as a bound state of three quarks (2 up and 1 down), a more complete description includes a sea of gluons and virtual quark/anti-quark pairs arising from interactions between the three quarks. For instance, strange quark/anti-quark pairs are present in this quark sea even though the proton has, on average, no overall strangeness. The effect of this intrinsic strangeness on the charge and magnetism of the proton can be precisely studied by using the weak interaction (Z-boson exchange) as a probe. While the weak force is normally too slight to be detected alongside the dominant electromagnetic force, the weak interaction is required in any process which violates parity symmetry.

Researchers at Jefferson Lab and elsewhere have therefore turned to high precision measurements of the parity-violating electron scattering (PVES) asymmetry in order to study the effects of strange quarks in the proton. PVES has become an essential tool in mapping out the flavor composition of the electromagnetic form factors. Exposing the role of the strange quark with such measurements provides direct information on the underlying dynamics of non-perturbative QCD – a considerable achievement both experimentally and theoretically.

World data at the lowest momentum transfer Q2, which most directly relates to the “static” strange magnetic moment and charge radius, is shown in figure 1 as constraints on the fractional strange quark contributions to the proton form factors. Superimposed are results from global fits of the low Q2 data , which differ in treatment of the theoretically challenging correction term from the anapole moment of the proton. The ellipses represent allowed regions at 95% statistical confidence level. As is evident from these fits, the strange charge radius is very small, while the strange quark contribution to the proton magnetic moment contribution is less than 10%.

(See also Strange Magnetic Moment entry in the Theory section)



References:

A. Acha et al. (HAPPEX Collaboration), Phys. Rev. Lett. 98 (2007) 032301

D. S. Armstrong et al. (G0 Collaboration), Phys. Rev. Lett. 95 (2005) 092001

R.D. Young et al., Phys. Rev. Lett. 97 (2006) 102002

Jianglai Liu et al., Phys. Rev. C 76 (2007) 025202