Bulletin of the American Physical Society
APS April Meeting 2011
Volume 56, Number 4
Saturday–Tuesday, April 30–May 3 2011; Anaheim, California
Session Y4: QCD in Nuclear Physics |
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Sponsoring Units: GHP DNP Chair: Doug Higinbotham, Thomas Jefferson National Accelerator Facility Room: Garden 4 |
Tuesday, May 3, 2011 1:30PM - 2:06PM |
Y4.00001: Recent Results and Future Measurements of Elastic Nucleon Form Factors Invited Speaker: The electromagnetic form factors of the nucleon provide experimental access to the underlying charge and magnetic moment distributions from quarks. These form factors provide excellent testing grounds for QCD and QCD-inspired models and are fundamentally important in understanding non-perturbative QCD. By studying them, they provide insight into the underlying mechanisms relevant to the generation of nucleon structure. Recently, these form factors have generated significant attention both experimentally and theoretically, and over the last year in particular, the electric form factors of the proton and neutron have both been measured to unprecedentedly high momentum transfer. An overview of newly published measurements at both low and high momentum transfer will be presented and compared with previous measurements and modern models. Assuming nucleon charge symmetry, these also allow for a quark flavor separation to be done to moderate momentum transfer for the first time. Finally, an overview will be presented of future approved measurements which push the limits of what is currently experimentally accessible. [Preview Abstract] |
Tuesday, May 3, 2011 2:06PM - 2:42PM |
Y4.00002: Abelian anomaly and neutral pion production Invited Speaker: The process $\gamma^\ast \gamma \to \pi^0$ is fascinating because in order to explain the associated transition form factor within the Standard Model on the full domain of momentum transfer, one must combine, using a single internally-consistent framework, an explanation of the essentially nonperturbative Abelian anomaly with the features of perturbative QCD. The case for attempting this has received a significant boost with the publication of data from the BaBar Collaboration [Phys.\ Rev.\ D {\bf 80}, 052002 (2009)] because, while they agree with earlier experiments on their common domain of squared-momentum-transfer [CELLO - Z.\ Phys.\ C {\bf 49}, 401 (1991); CLEO - Phys.\ Rev.\ D {\bf 57}, 33 (1998)], the BaBar data are unexpectedly far \emph{above} the prediction of perturbative QCD at larger values of $Q^2$. I will elucidate the sensitivity of the $\gamma^\ast \gamma \to \pi^0$ transition form factor, $G_{\gamma^\ast \gamma \pi}(Q^2)$, to the pointwise behaviour of the interaction between quarks; and use existing Dyson-Schwinger equation calculations of this and the kindred $\gamma^\ast \gamma^\ast \to \pi^0$ form factor to characterize the $Q^2$-dependence of $G_{\gamma^\ast \gamma \pi} (Q^2)$. It will become apparent that in fully-self-consistent treatments of pion: static properties; and elastic and transition form factors, the asymptotic limit of the product $Q^2 G_{\gamma^\ast\gamma \pi^0}(Q^2)$, which is determined \emph{a priori} by the interaction employed, is not exceeded at any finite value of spacelike momentum transfer: the product is a monotonically-increasing concave function. Studies exist which interpret the BaBar data as an indication that the pion's distribution amplitude, $\phi_\pi(x)$, deviates dramatically from its QCD asymptotic form, indeed, that $\phi_\pi(x)=\,$constant, or is at least flat and nonvanishing at $x=0,1$. I will explain that such a distribution amplitude characterises an essentially-pointlike pion; and show that, when used in a fully-consistent treatment, it produces results for pion elastic and transition form factors that are in striking disagreement with experiment. A bound-state pion with a pointlike component will produce the hardest possible form factors; i.e., form factors which become constant at large-$Q^2$. On the other hand, QCD-based studies produce soft pions, a valence-quark distribution amplitude for the pion that vanishes as $\sim (1-x)^2$ for $x\sim 1$, and results that agree well with the bulk of existing data. It can thus be argued that the large-$Q^2$ BaBar data is inconsistent with QCD and also inconsistent with a vector current-current contact interaction; and hence that the large- $Q^2$ data reported by BaBar is not a true representation of the $\gamma^\ast\gamma \to \pi^0$ transition form factor. [Preview Abstract] |
Tuesday, May 3, 2011 2:42PM - 3:18PM |
Y4.00003: Nucleon Spin Structure Invited Speaker: Since discovering the nucleon was a composite structure, much has been learned about how its properties (charge, momentum, etc.) arise from the partons within. There has been significant recent activity to understand both the helicity and transverse spin structure. The Relativistic Heavy Ion Collider (RHIC) at Brookhaven National Laboratory allows unique access to study the spin structure through polarized proton-proton collisions Results from polarized DIS measurements over the past 20 years have shown that the quark spin only contributes $\sim25-30$\% to the nucleon spin, indicating that the remainder comes from the gluon spin and parton orbital angular momentum. Measurements of the double helicity asymmetry $A_{LL}$ in polarized proton collisions at RHIC can access the gluon spin contribution, $\Delta G$. Recent result from the STAR and PHENIX Collaborations have been used in a global analysis of polarized data, and were shown to significantly constrain $\Delta G$. New measurements at RHIC using the parity violating spin asymmetry in W boson production access the flavor dependent sea quark spin contributions. First results show good agreement with expectations based on DIS results. The first large data set is being taken this year. RHIC is also able to study the transverse spin structure of the proton, a field that has seen significant growth in recent years. In this talk, we will present our current understanding of the nucleon spin structure, and the role RHIC has played and will continue to play in expanding it. [Preview Abstract] |
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