arXiv subjects
Publications and source records attributed to Levan R. Surguladze.
The results of evaluation of mixed QED X QCD corrections to R(s) in e^+e^- annihilation and R_τ in hadronic decay of the τlepton to O(α^nα_s^m), m+n \leq 3, are presented. The net effect on α_s(M_Z) from the Z decay is only about 0.1% and in the τdecay case the net effect is negligible, as expected.
Depending on their masses, Supersymmetric particles can affect various measurements in Z decay. Among these are the total width (or consequent extracted value of $α_s$), enhancement or suppression of various flavors, and left-right and forward-backward asymmetries. The latter depend on squark mass splittings and are, therefore, a possible test of the Supergravity related predictions. We calculate leading order corrections for these quantities considering in particular the case of light photino and gluino where the SUSY effects are enhanced. In this limit the effect on $α_s$ is appreciable, the effect on $R_b$ is small, and the effect on the asymmetries is extremely small.
The results of calculation of light gluino contributions to Gamma(Z--->hadrons) and Gamma(tau--->hadrons) to O(alpha_s^3) are presented. The net effect in the case of Z decay is noticeable. For the tau width the effect is very large and, if a light gluino exists, suggests that alpha_s increases by more than 15% relative to the Standard Model analysis.
A summary of the current understanding of methods of analytical higher order perturbative computations of total cross sections and decay widths in Quantum Chromodynamics is presented. As examples, the total cross section in electron positron annihilation, the hadronic decay rates of the tau lepton and Higgs boson up to O(α_s^2) and O(α_s^3) are considered. The evaluation of the four-loop QED β- function at an intermediate step of the calculation is briefly described. The problem of renormalization group ambiguity of perturbative results is considered and some of the existing prescriptions are discussed. The problem of estimation of theoretical uncertainty in perturbative calculations is briefly discussed.
We review the current status of the high order perturbative QCD evaluation of the hadronic decay rates of the Z and Higgs bosons. A systematic classification of the various types of QCD corrections to $O(α_s^2)$ and $O(α_s^3)$ is made and their numerical status is clarified.
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 τhadronic width.
The results of analytical evaluation of $O(α_s^2)$ QCD contributions due to the nonvanishing quark masses to $Γ_{H\rightarrow q_f\overline{q}_f}$ are presented. The ``triangle anomaly'' type contributions are included. As a byproduct the $O(α_s^3)$ logarithmic contributions are evaluated. The results are presented both in terms of running and pole quark masses. The partial decay modes $H \rightarrow b\overline{b}$ and $H \rightarrow c\overline{c}$ are considered. The calculated corrections decrease the absolute value of large and negative $O(α_s^2)$ massless limit coefficient by $\leq 1\%$ in the intermediate mass region and by 1\%$ - $20\% in the low mass region which, however, is experimentally ruled out. The results are relevant for $H \rightarrow t\overline{t}$ decay mode for the higher Higgs mass region where the mass effects are large and important. The high order corrections remove a very large discrepancy between the results for $Γ_{H\rightarrow q_f\overline{q}_f}$ in terms of running and pole quark masses almost completely and reduce the scale dependence from about 40\% to nearly 5\%.
We evaluate the effect of a virtual top quark on the coefficient of $α_{s}^{2}$ in the decay rates $Γ(Z \rightarrow {\rm hadrons})$ and $Γ(Z \rightarrow b\overline{b} )$. We treat the dependence on the top quark mass exactly instead of using a large mass expansion. The present work completes the evaluation of the $α_{s}^{2}$ contributions to these quantities. The calculation uses both the $\overline{\rm MS}$ and Collins-Wilczek-Zee renormalization prescriptions. The results can be applied to the hadronic decays of the $τ$-lepton.
A short presentation of the results of the analytical evaluation of the O(α_s^2) QCD contributions in the fermionic decay rates of the neutral CP-odd minimal supersymmetric Higgs particle is made. The corrections due to the nonvanishing quark masses are included. The results are presented both in terms of running and pole quark masses.