# 11th international workshop on High-pT Physics in the RHIC & LHC Era

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

## Elimination of pQCD renormalization scale ambiguities for collider reactions

12 Apr 2016, 09:05
30m
Large Seminar Room (BNL Physics Building)

### Speaker

Prof. Stanley Brodsky (SLAC National Accelerator Laboratory)

### Description

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)

 Slides