[Paper Review] Status of QCD precision predictions for Drell-Yan processes

S. Alekhin, S. Amoroso+10 more|arXiv (Cornell University)|Jan 1, 2024

Quantum Chromodynamics and Particle Interactions1 citations

TL;DR

This paper presents a comprehensive comparison of next-to-next-to-leading order (NNLO) QCD predictions for Drell-Yan processes at the LHC and Tevatron, including fiducial cuts on final-state leptons. It demonstrates excellent agreement among four independent codes after including linear power corrections from fiducial cuts, and resolves instabilities in fixed-order predictions for symmetric transverse momentum cuts via all-order resummation of small lepton-pair transverse momenta.

ABSTRACT

We compute differential distributions for Drell-Yan processes at the LHC and the Tevatron colliders at next-to-next-to-leading order in perturbative QCD, including fiducial cuts on the decay leptons in the final state. The comparison of predictions obtained with four different codes shows excellent agreement, once linear power corrections from the fiducial cuts are included in those codes that rely on phase-space slicing subtraction schemes. For $Z$-boson production we perform a detailed study of the symmetric cuts on the transverse momenta of the decay leptons. Predictions at fixed order in perturbative QCD for those symmetric cuts, typically imposed in experiments, suffer from an instability. We show how this can be remedied by an all-order resummation of the fiducial transverse momentum spectrum, and we comment on the choice of cuts for future experimental analyses.

Motivation & Objective

• To validate fixed-order NNLO QCD predictions for Drell-Yan processes across multiple independent codes, including fiducial cuts on decay leptons.
• To identify and resolve discrepancies in predictions arising from neglected linear power corrections in phase-space slicing schemes.
• To investigate the unphysical instabilities in fixed-order predictions for symmetric transverse momentum cuts on final-state leptons.
• To demonstrate the effectiveness of all-order resummation in stabilizing the fiducial transverse momentum spectrum near small pT values.
• To provide guidance on fiducial cut choices for future high-precision experimental analyses at the LHC.

Proposed method

• Employed four independent NNLO QCD codes—based on different subtraction schemes and phase-space integration techniques—to compute differential distributions for Drell-Yan processes at the LHC and Tevatron.
• Applied fiducial cuts on the transverse momenta and pseudorapidities of final-state leptons, with a focus on symmetric cuts commonly used in experiments.
• Incorporated linear power corrections from fiducial cuts into codes using phase-space slicing, resolving prior discrepancies with other methods.
• Performed all-order resummation of large logarithms in the small transverse momentum region of the lepton pair using effective field theory techniques.
• Compared fixed-order NNLO predictions with resummed results to assess stability and sensitivity to fiducial cuts.
• Validated results against experimental data from ATLAS and DØ, using benchmark cross sections and kinematic configurations.

Experimental results

Research questions

• RQ1 Why do fixed-order NNLO QCD predictions for Drell-Yan processes exhibit instabilities under symmetric transverse momentum cuts on final-state leptons?
• RQ2 To what extent do neglected linear power corrections in phase-space slicing schemes affect the reliability of NNLO predictions for fiducial cross sections?
• RQ3 Can all-order resummation of transverse momentum logarithms stabilize the fiducial cross section in the small pT regime?
• RQ4 How do different NNLO codes compare after including fiducial power corrections, and what does this imply for theoretical uncertainty estimates?
• RQ5 What are the implications of these findings for the choice of fiducial cuts in future high-precision Drell-Yan measurements at the LHC?

Key findings

• After inclusion of linear power corrections from fiducial cuts, four independent NNLO QCD codes show excellent agreement across all benchmark observables.
• Fixed-order predictions for symmetric transverse momentum cuts exhibit unphysical instabilities due to the breakdown of perturbative convergence in the small pT region.
• All-order resummation of transverse momentum logarithms successfully stabilizes the fiducial cross section, resolving the instability observed in fixed-order calculations.
• The resummed predictions show only small differences from fixed-order codes, indicating that the instability is not due to large higher-order corrections but to missing power corrections.
• The study confirms that fiducial cuts significantly affect theoretical predictions, and their choice must be carefully considered to avoid unphysical sensitivities.
• The results support the use of resummed predictions for fiducial cross sections in precision measurements, especially when symmetric cuts are applied.