Commensurate Scale Relations in Quantum Chromodynamics

We use the BLM method to show that perturbatively-calculable observables in QCD can be related to each other without renormalization scale or scheme ambiguity. We define and study the commensurate scale relations. We show that the commensurate scales satisfy the renormalization group transitivity rule which ensures that predictions in PQCD are independent of the choice of an intermediate renormalization scheme. We generalize the BLM procedure to higher order. The application of this procedure to relate known physical observables in QCD gives surprisingly simple results. In particular, the annihilation ratio $R_{e^+e^-}$ and the Bjorken sum rule for polarized electroproduction are related through simple coefficients, which reinforces the idea of a hidden symmetry between these two observables.Comment: 35 pages (RevTeX), one PostScript figure included at the end. SLAC-PUB-6481, UMD Preprint #94-13

Quark Mass Corrections to the Z Boson Decay Rates

The results of perturbative QCD evaluation of the ~m_f^2/M_Z^2 contributions to the decay rates in Z--->bb and Z--->hadrons for the quark masses m_f << M_Z are presented. The recent results due to the combination of renormalization group constraints and the results of several other calculations are independently confirmed by the direct computation. Some existing confusion in the literature is clarified. In addition, the calculated O(alpha_s^2) correction to the correlation function in the axial channel is a necessary ingredient for the yet uncalculated axial part of the O(alpha_s^3) mass correction to the Z decay rates. The results can be applied to the \tau hadronic width.Comment: 10 pages, LATeX, +1 figure available upon request, preprint OITS-554 (University of Oregon, USA, September 1994

Mass Corrections to the Tau Decay Rate

In this note radiative corrections to the total hadronic decay rate of the $\tau$-lepton are studied employing perturbative QCD and the operator product expansion. We calculate quadratic quark mass corrections to the decay rate ration $R_{\tau}$ to the order ${\cal O}(\alpha_s^2 m^2)$ and find that they contribute appreciably to the Cabbibo supressed decay modes of the $\tau$-lepton. We also discuss corrections of mass dimension D=4, where we emphasize the need of a suitable choice of the renormalization scale of the quark and gluon condensates.Comment: 13 pages, LaTeX, no figures. This version fixes a typo in eq. (25) of the original paper (Z. Phys. C59 (1993) 525) and an errror in a numerical integration procedure which has resulted to a significant increase of the O(\alpha_s^2) coefficient in eq. (27). As a consequence also some tables in Section 4 have been modifie

QCD sum rules and radial excitaions of light pseudoscalar and scalar mesons

The calculations of masses and decay constants of the radial excitations of light pseudoscalar and scalar mesons within QCD sum rules method are briefly reviewed. The predictions are based on the $1/N_c$-supported model spectra, which consist of infinite number of infinitely narrow resonances, and on the assumption that the ground states of light scalar mesons may be considered as the $\bar{q}q$-bound states. The results of the studies are compared with the existing experimental data and with the predictions of other theoretical approaches.Comment: 10 pages, LaTeX, to be published in Phys.Atom.Nucl. Dedicated to 70th Birthday of Professor Yu.A. Simonov. Additional shortcomings eliminated. Results unchange

The Extraction of Light Quark Masses From Sum Rule Analyses of Axial and Vector Current Ward Identities

We re-examine the use of sum rules in the extraction of light quark masses and discuss a number of potential problems with existing analyses. The most important issue is that of the overall normalization of the hadronic spectral functions should not be fixed by assuming complete resonance dominance of the continuum threshold region as it can overestimate the resonance contributions to spectral integrals by factors as large as $\sim 5$. The second important uncertainty comes from the assumed location, $s_0$, of the onset of duality with perturbative QCD, as the extracted quark masses depend very sensitively on this parameter. The assumption of duality and the requirement of positivity of $\rho_5(s)$ imposes very severe constraints on the shape of the relevant spectral function in the dual region, and leads to rigorous lower bounds for $m_u+m_d$ as a function of $s_0$. In the extractions of $m_s$ we find that the conventional choice of the value of $s_0$ is not physical. For a more reasonable choice of $s_0$ we are not able to find a solution that is stable with respect to variations of the Borel transform parameter. This problem can be overcome only if the hadronic spectral function is determined up to significantly larger values of $s$ than is currently possible. Finally, we also estimate the error associated with the convergence of perturbative QCD expressions used in the sum rule analyses. Our conclusion is that, taking all of these issues into account, the resulting sum rule estimates for both $m_u+m_d$ and $m_s$ could easily have uncertainties as large as a factor of 2.Comment: 28 pages. Published version. Modified "axis" source for figures also include

The analytical singlet αs4\alpha_s^4 QCD contributions into the e+e−e^+e^--annihilation Adler function and the generalized Crewther relations

The generalized Crewther relations in the channels of the non-singlet and vector quark currents are considered. They follow from the double application of the operator product expansion approach to the same axial vector-vector-vector triangle amplitude in two regions, adjoining to the angle sides $(x,y)$ (or $p^2,q^2$). We assume that the generalized Crewther relations in these two kinematic regimes result in the existence of the same perturbation expression for two products of the coefficient functions of annihilation and deep-inelastic scattering processes in the non-singlet and vector channels. Taking into account the 4-th order result for $S_{GLS}$ and the perturbative effects of the violation of the conformal symmetry in the generalized Crewther relation, we obtain the analytical contribution to the singlet $\alpha_s^4$ correction to the $D_A^{V}$-function. Its a-posteriori comparison with the recent result of direct diagram-by-diagram evaluation of the singlet 4-th order corrections to $D_A^{V}$- function demonstrates the coincidence of the predicted and obtained $\zeta_3^2$-contributions to the singlet term. They can be obtained in the conformal invariant limit from the original Crewther relation. On the contrary to previous belief, the appearance of $zeta_3$-terms in perturbative series in gauge models does not contradict to the property of conformal symmetry and can be considered as ragular feature. The Banks-Zaks motivated relation between our predicted and obtained 4-th order corrections is mentioned. This confirms Baikov-Chetyrkin-Kuhn expectation that the generalized Crewther relation in the channel of vector currents receives additional singlet contribution, which in this order of perturbation theory is proportional to the first coefficient of the QCD $\beta$-function.Comment: Concrete new foundations explained, abstract updated, presentation improved, 2 references added, extra acknowledgements added. This work is dedicated to K. G. Chetyrkin on the occasion of his 60th anniversary, to be published in Jetp. Lett supposedly in vol.94, issue 1

New high order relations between physical observables in perturbative QCD

We exploit the fact that within massless perturbative QCD the same Green's function determines the hadronic contribution to the $\tau$ decay width and the moments of the $e^+e^-$ cross section. This allows one to obtain relations between physical observables in the two processes up to an unprecedented high order of perturbative QCD. A precision measurement of the $\tau$ decay width allows one then to predict the first few moments of the spectral density in $e^+e^-$ annihilations integrated up to $s\sim m_\tau^2$ with high accuracy. The proposed tests are in reach of present experimental capabilities.Comment: 7 pages, Latex, no figure

The order O(αˉ αˉs)O(\bar{\alpha}~\bar{\alpha}_s) and O(αˉ2)O(\bar{\alpha}^2) corrections to the decay width of the neutral Higgs boson to the bˉb\bar{b}b pair

We present the analytical expressions for the contributions of the order $O(\bar{\alpha}~\bar{\alpha}_s)$ and $O(\bar{\alpha}^2)$ corrections to the decay width of the Standard Model Higgs boson into the $\bar{b}b$-pair. The numerical value of the mixed QED and QCD correction of order $O(\bar{\alpha}~\bar{\alpha}_s)$ is comparable with the previously calculated terms in the perturbative series for $\Gamma(H^0\to\bar{b}b)$.Comment: LaTeX 5 pages, accepted for publication in Pisma Zh. Eksp. Teor. Fiz. v 66, N5 (1997

Production of Heavy Quarks Close to Threshold

We calculate production by vector and axial currents of heavy quark pairs ($c\bar{c}$, $b\bar{b}$, $t\bar{t}$) close to threshold. We take into account strong interaction contributions (including radiative corrections and leading nonperturbative effects) by using the Fermi-Watson final state interaction theorem. We use the results obtained to compare with experiment for open production of $c\bar{c}$, $b\bar{b}$ near threshold, and to give a reliable estimate of the so-called ``threshold effects'' contribution to vector and axial correlators, for $t \bar{t}$, $i.e.$, the contribution of regions close to $4 m_t^2$ to $\Pi(t)$, for small values of $t$ ( 0 < t \lower2pt\hbox{\lesssim} M_Z^2 ).Comment: 36 pages, uses RevTeX, 7 postscript figures available upon reques

The three-loop QED photon vacuum polarization function in the -scheme and the four-loop QED [beta]-function in the on-shell scheme

We present the three-loop analytical QED results for the asymptotic expression of the photon vacuum polarization function [Pi] in the -scheme. Using the recently obtained four-loop results for the QED [beta]-function in the MS-like and MOM schemes ([beta]MS- and [Psi]-functions) and for (g-2)[mu] we get the four-loop QED [beta]-function in the on-shell scheme.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/28983/1/0000010.pd