4 research outputs found
Nonperturbative Contributions in an Analytic Running Coupling of QCD
In the framework of analytic approach to QCD the nonperturbative
contributions in running coupling of strong interaction up to 4-loop order are
obtained in an explicit form. For all they are shown to be
represented in the form of an expansion in inverse powers of Euclidean momentum
squared. The expansion coefficients are calculated for different numbers of
active quark flavors and for different number of loops taken into
account. On basis of the stated expansion the effective method for precise
calculation of the analytic running coupling can be developed.Comment: 9 pages, LaTeX, 1 table, 1 eps figur
Strong Coupling Constant with Flavour Thresholds at Four Loops in the MS-bar Scheme
We present in analytic form the matching conditions for the strong coupling
constant alpha_s^(n_f)(mu) at the flavour thresholds to three loops in the
modified minimal-subtraction scheme. Taking into account the recently
calculated coefficient beta_3 of the Callan-Symanzik beta function of quantum
chromodynamics, we thus derive a four-loop formula for alpha_s^(n_f)(mu)
together with appropriate relationships between the asymptotic scale parameters
Lambda^(n_f) for different numbers of flavours n_f.Comment: 10 pages (Latex), 3 figures (Postscript
Form Factors and QCD in Spacelike and Timelike Region
We analyze the basic hard exclusive processes: \pi\gamma*\gamma - transition,
pion and nucleon electromagnetic form factors, and discuss the analytic
continuation of QCD formulas from the spacelike q^2<0 to the timelike region
q^2 >0 of the relevant momentum transfers. We describe the construction of the
timelike version of the coupling constant \alpha_s. We show that due to the
analytic continuation of the collinear logarithms each eigenfunction of the
evolution equation acqiures a phase factor and investigate the resulting
interference effects which are shown to be very small. We found no sources for
the K-factor-type enhancements in the perturbative QCD contribution to the
hadronic form factors. To study the soft part of the pion electromagnetic form
factor, we use a QCD sum rule inspired model and show that there are
non-canceling Sudakov double logarithms which result in a K-factor-type
enhancement in the timelike region.Comment: 12 pages, LaTeX; a few typos corrected, references adde