Extraction of the x-dependence of the non-perturbative QCD b-quark fragmentation distribution component

Using recent measurements of the b-quark fragmentation distribution obtained in $e^+e^- \to b \bar{b}$ events registered at the Z pole, the non-perturbative QCD component of the distribution has been extracted independently of any hadronic physics modelling. This distribution depends only on the way the perturbative QCD component has been defined. When the perturbative QCD component is taken from a parton shower Monte-Carlo, the non-perturbative QCD component is rather similar with those obtained from the Lund or Bowler models. When the perturbative QCD component is the result of an analytic NLL computation, the non-perturbative QCD component has to be extended in a non-physical region and thus cannot be described by any hadronic modelling. In the two examples used to characterize these two situations, which are studied at present, it happens that the extracted non-perturbative QCD distribution has the same shape, being simply translated to higher-x values in the second approach, illustrating the ability of the analytic perturbative QCD approach to account for softer gluon radiation than with a parton shower generator.Comment: 13 page

Progress in perturbative QCD

We briefly summarize some recent theoretical developments in perturbative QCD, emphasizing new ideas which have led to widening the domain of applicability of perturbation theory. In particular, it is now possible to calculate efficiently processes with many partons; the high order behavior of perturbation theory can be at least partly understood by going beyond leading twist accuracy; factorization with more than one hard scale (such as in DIS with heavy quarks) can be made consistent with the renormalization group; and large infrared logs can be resummed beyond the renormalization group. The use of the renormalization group to resum large longitudinal scales may allow the use of perturbation theory even in the absence of a large transverse scale.Comment: 10 pages Latex; summary of the theory WG at the DIS97 workshop; typo on pag. 4 correcte

Moriond 2009, QCD and High Energy Interactions: Theory Summary

These proceedings provide a brief summary of the theoretical topics that were covered at Moriond QCD 2009, including non-perturbative QCD, perturbative QCD at colliders, a small component of physics beyond the standard model and heavy-ion collisions.Comment: 13 pages, 7 figures; v2 adds a reference; v3 includes small changes to tex

High energy amplitude as an admixture of "soft" and "hard" Pomerons

In this paper an attempt is made to find an interface of the perturbative BFKL Pomeron with the non-perturbative Pomeron originating from non-perturbative QCD phenomena such as QCD instantons and/or scale anomaly. The main idea is that the non-perturbative Pomeron involves a large scale ($M_0 \approx 2 GeV$), which is larger than the scale from which perturbative QCD is applicable. One key result is that even for processes involving a large hard scale (such as DIS) the low $x$ behavior is determined by an effective Pomeron with an intercept having an essential non-perturbative QCD contribution.Comment: 29 pages, 13 fugures. Accepted for publication in Nucl. Phys.

Nonperturbative QCD, gauge-fixing, Gribov copies, and the lattice

Perturbative QCD uses the Faddeev-Popov gauge-fixing procedure, which leads to ghosts and the local BRST invariance of the gauge-fixed perturbative QCD action. In the asymptotic regime, where perturbative QCD is relevant, Gribov copies can be neglected. In the nonperturbative regime, one must adopt either a nonlocal Gribov-copy free gauge (e.g., Laplacian gauge) or attempt to maintain local BRST invariance at the expense of admitting Gribov copies. These issues are explored. In addition, we discuss the relationship between recent Dyson-Schwinger based model calculations of the infrared behavior of QCD Green's functions and the lattice calculation of these quantities.Comment: 9 pages, 2 figures, to appear in Prog. Theor. Phys. Suppl. in the proceedings of the Tokyo-Adelaide Joint Workshop on Quarks, Astrophysics and Space Physic

Understanding the penguin amplitude in B→ϕKB \to \phi K decays

We calculate branching ratios for pure penguin decay modes, $B\to \phi K$ decays using perturbative QCD approach. Our results of branching ratios are consistent with the experimental data and larger than those obtained from the naive factorization assumption and the QCD-improved factorization approach. This is due to a dynamical penguin enhancement in perturbative QCD approach.Comment: 7 pages, 2 figures, version to appear in PL