12-15 April 2016
BNL Physics Building
US/Eastern timezone

Elimination of pQCD renormalization scale ambiguities for collider reactions

12 Apr 2016, 09:05
Large Seminar Room (BNL Physics Building)

Large Seminar Room

BNL Physics Building


Prof. Stanley Brodsky (SLAC National Accelerator Laboratory)


A primary problem for perturbative QCD analyses is how to set the renormalization scale of the QCD running coupling in order to achieve maximally precise fixed-order predictions for physical observables. The Principle of Maximum Conformality (PMC) eliminates the ambiguities associated with the conventional renormalization scale-setting procedure, giving predictions which are independent of the choice of the renormalization scheme. The scales of the QCD couplings and the effective number of quark flavors are set order-by-order in the pQCD series. The PMC has a rigorous theoretical foundation, satisfying renormalization group invariance and all of the self-consistency conditions derived from the renormalization group. The PMC scales at each order are obtained by shifting the arguments of $\alpha_s$ to eliminate all non-conformal $\{\beta_i\}$-terms in the pQCD series. The $\{\beta_i\}$ terms are determined from renormalization group equations without ambiguity. One then obtains the correct scale of the running coupling at each order and at each phase-space point. The PMC reduces in the $N_C \to 0$ Abelian limit to the Gell-Mann-Low method. In this talk, I will summarize recent PMC applications for a number of collider processes. If one applies the PMC to compute the top-quark pair forward-backward asymmetry at the next-to-next-to-leading order level, one obtains a comprehensive, self-consistent pQCD explanation for the Tevatron measurements of the asymmetry, accounting for the ``increasing-decreasing" behavior observed by D0 collaboration as the $t \bar t$ invariant mass is increased. In the case of hadronic $Z$ decay, one can achieve precise scheme-independent predictions for the decay widths at each order without scale ambiguities by applying the PMC. The application of the PMC systematically eliminates a major theoretical uncertainty for pQCD predictions, thus increasing the sensitivity of the colliders to possible new physics beyond the Standard Model.

Primary author

Prof. Stanley Brodsky (SLAC National Accelerator Laboratory)

Presentation Materials