$W^\pm Z$ production at the LHC: fiducial cross sections and distributions in NNLO QCD (1703.09065v1)

Abstract: We report on the first fully differential calculation for $W^\pm Z$ production in hadron collisions up to next-to-next-to-leading order (NNLO) in QCD perturbation theory. Leptonic decays of the $W$ and $Z$ bosons are consistently taken into account, i.e. we include all resonant and non-resonant diagrams that contribute to the process $pp\to \ell^{'\pm} \nu_{\ell^{'}} \ell^+\ell^-+X$ both in the same-flavour ($\ell'=\ell$) and the different-flavour ($\ell'\neq \ell$) channel. Fiducial cross sections and distributions are presented in the presence of standard selection cuts applied in the experimental $W^\pm Z$ analyses by ATLAS and CMS at centre-of-mass energies of 8 and 13\,TeV. As previously shown for the inclusive cross section, NNLO corrections increase the NLO result by about $10\%$, thereby leading to an improved agreement with experimental data. The importance of NNLO accurate predictions is also shown in the case of new-physics scenarios, where, especially in high-$p_T$ categories, their impact can reach ${\cal O}(20\%)$. The availability of differential NNLO predictions will play a crucial role in the rich physics programme that is based on precision studies of $W^\pm Z$ signatures at the LHC.

• The paper introduces an NNLO QCD framework using qT subtraction to improve predictions and reduce theoretical uncertainties in W±Z production at 8 and 13 TeV.
• The study shows that NNLO corrections can increase cross section predictions by up to 20% in high-pT regions, closely matching ATLAS and CMS measurements.
• The analysis provides a robust foundation for precise Standard Model modeling and background assessments in searches for new physics at the LHC.

Overview of NNLO QCD Calculations for W Boson Production at the LHC

The paper under discussion offers a comprehensive examination of W boson production in hadron collisions, with a focus on next-to-next-to-leading order (NNLO) Quantum Chromodynamics (QCD) perturbation theory. This research fills a significant gap in precise calculations, crucial for high-precision tests at the Large Hadron Collider (LHC). The authors, Massimiliano Grazzini and colleagues, employ a thorough analytical and numerical framework to achieve fully differential predictions, incorporating the leptonic decays of both resonant and non-resonant processes.

Important Findings and Methodology

Their work extends the precision of fiducial cross sections and kinematic distributions for W production at the LHC, operating at center-of-mass energies of 8 and 13 TeV. Key to this effort is their utilization of the q_T subtraction formalism, integrated within a Monte Carlo simulation framework to handle the intrinsic complexity of higher-order QCD corrections. This computational approach enables more accurate modeling by accounting for off-shell effects, spin correlations, and interference effects throughout the calculation.

The paper highlights the importance of NNLO corrections, which provide a roughly 10% increase in predicted cross sections, aligning theoretical outcomes with experimental data from LHC collaborations ATLAS and CMS. In some high transverse momentum (p_T) categories, the authors note that these higher-order QCD corrections can impact predictions by up to 20%, underscoring the necessity of NNLO precision for accurate modeling.

Key Numerical Insights

Grazzini et al. present strong numerical evidence supporting the inclusion of NNLO terms. In various scenarios, the paper quantifies the incremental improvements in predictive accuracy compared to previous leading-order (LO) and next-to-leading-order (NLO) models. The NNLO predictions demonstrate significant reductions in theoretical uncertainty, a critical factor in distinguishing Standard Model (SM) processes from potential new physics signals.

Implications for New Physics and SM Background Studies

The improved precision of GM predictions for W production is particularly vital in contexts where W processes are considered irreducible backgrounds, such as in trilepton, missing energy, and Higgs studies. This work not only enhances our understanding of SM processes but also provides a solid foundation for examining scenarios involving hypothetical particles or interactions beyond the Standard Model (BSM).

The detailed data and methodical progression presented can be leveraged for future studies aiming to constrain or discover BSM phenomena, particularly where precision in the tail of p_T distributions is critical. The authors suggest that NNLO precision will be a cornerstone in upcoming high-luminosity LHC runs, where the data volume will accentuate the need for precise theoretical models.

Future Directions

Building on their findings, Grazzini and his team speculate on the future developments of automated computational tools for higher-order corrections, aiming for even broader applicability in various processes relevant to hadron colliders. As computational techniques evolve, the integration of electroweak corrections alongside QCD will likely become a focal point for further refinements.

In summary, this paper stands as a pivotal contribution to the field of high-energy particle physics. The thorough inclusion of NNLO QCD corrections in W production calculations represents a significant step toward a more complete and accurate understanding of both the SM and potential new physics at LHC energy scales.