- The paper presents the first computation of NNLO QCD corrections in ZZ production, increasing NLO cross sections by 11–17% across varying energies.
- The authors use advanced tools like OpenLoops and Collier to evaluate tree-level and loop amplitudes while managing IR singularities via the q subtraction method.
- Numerical results reveal that loop-induced gluon fusion accounts for over 50% of the NNLO effect, with scale uncertainties near ±3% validated against LHC measurements.
An Analysis of NNLO QCD Corrections in ZZ Production at Hadron Colliders
The paper under discussion presents the inaugural computation of next-to-next-to-leading order (NNLO) QCD corrections pertinent to the inclusive production of Z-boson pairs at hadron colliders. Specifically, the paper addresses proton-proton (pp) collisions with center-of-mass energies ranging from 7 to 14 TeV. The authors report that NNLO corrections augment the next-to-leading order (NLO) results by approximately 11% to 17% as energy scales increase across the studied range. Notably, the loop-induced gluon fusion contribution remains significant, constituting more than half of the complete NNLO effect, despite scale uncertainties persisting at approximately ±3%.
Computation Framework
The authors employ a meticulous computation approach to evaluate NNLO corrections, necessitating tree-level scattering amplitudes involving two additional unresolved partons, one-loop amplitudes with an additional parton, and a combination of one-loop-squared and two-loop corrections to the primary subprocess qq̅ → ZZ. Integral to this work is the use of OpenLoops for automated generation of relevant tree and one-loop matrix elements and the Collier library for stable tensor integral evaluations, leveraging Denner–Dittmaier reduction techniques.
This work represents a complex endeavor in confronting infrared (IR) singularities inherent in NNLO calculations. The q subtraction method [35] plays a critical role in canceling these singularities, enabling the precise formulation of the cross-section for ZZ production at NNLO accuracy.
Numerical Results and Implications
Numerical analyses reveal that gluon fusion processes, enhanced by gluon luminosity, are considerable contributors to the NNLO corrections, even as their relative importance dwindles. The paper's figures illustrate that NLO corrections alone augment the leading-order (LO) cross section by ~45%, while NNLO corrections further enhance NLO results by 11% to 17%, contingent on energy levels.
The comparison with recent experimental data from ATLAS and CMS indicates a reasonable alignment, albeit with significant experimental uncertainties. Furthermore, the authors acknowledge that Z-boson pair production cross-sections derived by LHC experiments may exclude contributions from off-shell Z bosons and have not incorporated electroweak (EW) corrections, predicted to impart a negative influence on total cross sections.
Theoretical and Practical Implications
The advancement of NNLO QCD corrections for ZZ production is pivotal for ensuring precise SM predictions at these energy scales, holding substantial ramifications for both Higgs boson studies and new-physics searches. The detailed computation of the two-loop helicity amplitudes proposed in this paper sets the stage for an expansive array of phenomenological explorations at NNLO, encompassing off-shell phenomena and differential analyses of Z boson decay products.
In conclusion, the authors have provided a comprehensive computation of NNLO QCD corrections to ZZ production, paving the path for more in-depth theoretical analyses and comparisons with experimental findings, fortifying the precision backbone required in advancing SM physics studies. Future developments are expected to build on this foundation, potentially including further refined NNLO implementations and the integration of EW corrections for more comprehensive theoretical predictions.