Calculations for deep inelastic scattering using fast interpolation grid techniques at NNLO in QCD and the extraction of alpha _{mathrm {s}} from HERA data

D Britzger,T Gehrmann,E W N Glover,K Rabbertz,C Gwenlan,J Pires,J Niehues,A Huss,A Gehrmann-De Ridder,J Currie,M R Sutton,T Morgan

doi:10.1140/epjc/s10052-019-7351-x

Abstract

The extension of interpolation-grid frameworks for perturbative QCD calculations at next-to-next-to-leading order (NNLO) is presented for deep inelastic scattering (DIS) processes. A fast and flexible evaluation of higher-order predictions for any a posteriori choice of parton distribution functions (PDFs) or value of the strong coupling constant is essential in iterative fitting procedures to extract PDFs and Standard Model parameters as well as for a detailed study of the scale dependence. The APPLfast project, described here, provides a generic interface between the parton-level Monte Carlo program NNLOjet and both the APPLgrid and fastNLO libraries for the production of interpolation grids at NNLO accuracy. Details of the interface for DIS processes are presented together with the required interpolation grids at NNLO, which are made available. They cover numerous inclusive jet measurements by the H1 and ZEUS experiments at HERA. An extraction of the strong coupling constant is performed as an application of the use of such grids and a best-fit value of alpha _{mathrm {s}} (M_{{mathrm {Z}}}) = 0.1170,(15)_text {exp},(25)_text {th} is obtained using the HERA inclusive jet cross section data.

Highlights

The NNLOJET program [1] is a recent and continuously developing framework for the calculation of fully differential cross sections for collider experiments
Comparisons must be performed between the data and the next-to-next-to-leading order (NNLO) predictions for the many thousands of points that are drawn from the multidimensional parameter space used in the minimisation
This paper describes developments in the APPLfast project which provides a common interface for the APPLgrid and fastNLO grid libraries to link to the NNLOJET program for the calculation of the perturbative coefficients

Summary

Introduction

The NNLOJET program [1] is a recent and continuously developing framework for the calculation of fully differential cross sections for collider experiments. For a detailed study of NNLO predictions and the estimation of theoretical uncertainties, these calculations must be repeated with different input conditions This includes, for example, using different values for the strong coupling αs(MZ), different parametrisations for the PDFs, or different choices for the factorisation or renormalisation scales. C (2019) 79:845 tion of the strong coupling constant and the parton densities in the proton In such fits, comparisons must be performed between the data and the NNLO predictions for the many thousands of points that are drawn from the multidimensional parameter space used in the minimisation. It is computationally prohibitive to run the full calculation at NNLO for each required input condition encountered in such a fit Applications of this nature critically require an efficient approach to perform the convolution of the partonic hard scattering with PDFs, change the value of the strong coupling constant, and vary the scales. Additional details of the grid methodology for DIS are discussed, together with the NNLO extraction of αs(MZ) using data on inclusive jet production from both H1 and ZEUS

DIS at NNLO and the NNLOJET framework

The APPLgrid and fastNLO packages

Renormalisation and factorisation scale dependence

The APPLfast project

Application: determination of the strong coupling constant

Conclusions and outlook