Determination of the low Q2 evolution of the Bjorken integral

We report on an experimental determination of the Q2-dependence of the Bjorken sum using data from Jefferson Lab Hall A and Hall B in the range 0.16 < Q2 < 1.1 GeV2. A twist analysis is performed. Overall, the higher twist corrections are found to be small due to a cancellation between the twist 4 and 6 terms.Comment: Contribution to the GDH04 symposium proceeding

Jefferson Lab's results on the Q^2-evolution of moments of spin structure functions

We present the recent JLab measurements on moments of spin structure functions at intermediate and low Q^2. The Bjorken sum and Burkhardt-Cottingham sum on the neutron are presented. The later appears to hold. Higher moments (generalized spin polarizabilities and d_2^n) are shown and compared to chiral perturbation theory and lattice QCD respectively.Comment: 4 pages, 2 figures, to appear in the DIS2005 Proceedings (AIP

Self-interacting scalar fields at high-temperature

We study two self-interacting scalar field theories in their high-temperature limit using path integrals on a lattice. We first discuss the formalism and recover known potentials to validate the method. We then discuss how these theories can model, in the high-temperature limit, the strong interaction and General Relativity. For the strong interaction, the model recovers the known phenomenology of the nearly static regime of heavy quarkonia. The model also exposes a possible origin for the emergence of the confinement scale from the approximately conformal Lagrangian. Aside from such possible insights, the main purpose of addressing the strong interaction here --given that more sophisticated approaches already exist-- is mostly to further verify the pertinence of the model in the more complex case of General Relativity for which non-perturbative methods are not as developed. The results have important implications on the nature of Dark Matter. In particular, non-perturbative effects naturally provide flat rotation curves for disk galaxies, without need for non-baryonic matter, and explain as well other observations involving Dark Matter such as cluster dynamics or the dark mass of elliptical galaxies.Comment: 33 pages, 19 figures. Version published in Eur. Phys. J.

Measurement of the Q2-evolution of the Bjorken integral at low Q2

We report on the extraction of the Q2-dependence of the Bjorken sum between 0.16 < Q2 < 1.1 GeV2. A twist analysis performed on these data shows that the higher twist corrections are small due to a cancellation between the twist-4 and 6 terms. The extraction of an effective strong coupling constant is discussed.Comment: Contribution to the proceedings of the 16th International Spin Physics Symposium, spin2004 (Trieste

Comment on "Does gravitational confinement sustain flat galactic rotation curves without dark matter?''

We comment on the methods and the conclusion of Ref. [1], "Does gravitational confinement sustain flat galactic rotation curves without dark matter?" The article employs two methods to investigate whether non-perturbative corrections from General Relativity are important for galactic rotation curves, and concludes that they are not. This contradicts a series of articles [2-4] that had determined that such corrections are large. We comment here that Ref. [1] use approximations known to exclude the specific mechanism studied in [2-4] and therefore is not testing the finding of Refs. [2-4].Comment: 5 Page

Nucleon Spin Sum Rules and Spin Polarizabilities at low Q2Q^2

We report on recently published experimental studies on spin sum rules, namely the generalized Gerasimov-Drell-Hearn, Bjorken, Burkhardt-Cottingham, Schwinger, and generalized spin polarizability sum rules. The data were taken at Jefferson Lab in Halls A and B by experiments E97-110 and EG4, respectively. They covered the very low $Q^2$ domain, down to $Q^2$$\simeq$0.02 GeV$^2$, where Chiral Effective Field Theory ($\chi$EFT) predictions should be valid. While some of the obervables agree with the state-of-the-art $\chi$EFT predictions, others are in tensions, including the Longitudinal-Transverse interference polarizability $\delta_{\rm LT}^n(Q^2)$ for which $\chi$EFT prediction was expected to be robust. This suggests that $\chi$EFT does not yet consistently describes nucleon spin observables, even in the very low $Q^2$ domain covered by the experiments.Comment: 10 pages, 2 figures. Contribution to the proceedings of the 25th International Symposium on Spin Physics (Spin 2023

The QCD Running Coupling

We review the present knowledge for $\alpha_s$, the fundamental coupling underlying the interactions of quarks and gluons in QCD. The dependence of $\alpha_s(Q^2)$ on momentum transfer $Q$ encodes the underlying dynamics of hadron physics -from color confinement in the infrared domain to asymptotic freedom at short distances. We review constraints on $\alpha_s(Q^2)$ at high $Q^2$, as predicted by perturbative QCD, and its analytic behavior at small $Q^2$, based on models of nonperturbative dynamics. In the introductory part of this review, we explain the phenomenological meaning of $\alpha_s$, the reason for its running, and the challenges facing a complete understanding of its analytic behavior in the infrared domain. In the second, more technical, part of the review, we discuss the behavior of $\alpha_s(Q^2)$ in the high $Q^2$ domain of QCD. We review how $\alpha_s$ is defined, including its renormalization scheme dependence, the definition of its renormalization scale, the utility of effective charges, as well as Commensurate Scale Relations which connect the various definitions of $\alpha_s$ without renormalization-scale ambiguity. We also report recent measurements and theoretical analyses which have led to precise QCD predictions at high energy. In the last part of the review, we discuss the challenge of understanding the analytic behavior $\alpha_s(Q^2)$ in the infrared domain. We also review important methods for computing $\alpha_s$, including lattice QCD, the Schwinger-Dyson equations, the Gribov-Zwanziger analysis and light-front holographic QCD. After describing these approaches and enumerating their conflicting predictions, we discuss the origin of these discrepancies and how to remedy them. Our aim is not only to review the advances in this difficult area, but also to suggest what could be an optimal definition of $\alpha_s$ in order to bring better unity to the subject.Comment: Invited review article for Progress in Particle and Nuclear Physics. 195 pages, 18 figures. V3: Minor corrections and addenda compared to V1 and V2. V4: typo fixed in Eq. (3.21