JLab highlight: Strange Quarks in the Proton

26. JLab highlight:Strange Quarks in the Proton 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%.

27. JLab highlight:Quark-Hadron Duality in structure functions One of the principal challenges of QCD is to bridge the small- and large-scale behavior of the strong nuclear interactions. At short distances, perturbative QCD is very successful in describing nucleon structure in terms of quarks and gluons. At large distances, the effects of confinement impose a more efficient description in terms of collective hadron degrees of freedom. Despite this apparent dichotomy, an intriguing connection has been observed between the low- and high-energy data on nucleon structure functions, which is referred to as "quark-hadron duality." Based on unpolarized structure functions it was found that quark-hadron duality occurs at much lower momentum transfers, in more observables, and in far less limited regions of energy than expected. These results allow the first studies to be made of the flavor dependence of quark-hadron duality and provide vital clues to the long-standing challenge of QCD to describe nuclear forces at large distances.

28. JLab highlight:Strong coupling constant at low Q2 The strength of the strong force is set by the value of its coupling αs. At small distances, much smaller than a fermi (1 fermi = 10-15m, about the size of a proton), αs is small and the strong force can be studied with the standard methods of perturbation theory. This discovery by David J. Gross, H. David Politzer and Frank Wilczek was acknowledged by the 2004 Nobel Prize in Physics. However, at large distances (greater than about a half fermi) the strength of the force becomes large and the perturbative calculations predict that αs becomes infinite. On the other hand, it is not clear if this result can be trusted, since perturbative calculations work only for small αs. Several nonperturbative theoretical approaches have conjectured that at large distances, the coupling should "freeze" at a constant value. (A few others have suggested that it may even vanish.) No direct experimental tests of these speculations have been made until recently, however. In the nonperturbative domain, one can define "effective" strong couplings, which absorb all nonperturbative effects (as well as higher-order perturbative effects) in their definition. Because nonperturbative effects depend on the studied physical process, effective couplings are process dependent. However, QCD using these effective couplings retains its predictive power, because the couplings may in some cases be related to each other by the theory The effective strong coupling extracted from JLab structure function data, as well as from other processes, is shown in Figure 1 as a function of the distance d. It suggests that αs_eff obtained from the Bjorken sum rule tends to a constant (or "freezes") at large d. Such behavior is an essential ingredient in applying the AdS/CFT (Anti de Sitter Space/Conformal Field Theory) correspondence to the strong force, since the lack of Q2-dependence of αs_eff means that the theory of the strong force is a conformal field theory. The application of this correspondence, established in the context of superstring theories, opens promising opportunities for calculations in the nonperturbative regime of the strong force