Implications of CTEQ global analysis for collider observables (0802.0007v3)

Abstract: The latest CTEQ6.6 parton distributions, obtained by global analysis of hard scattering data in the framework of general-mass perturbative QCD, are employed to study theoretical predictions and their uncertainties for significant processes at the Fermilab Tevatron and CERN Large Hadron Collider. The previously observed increase in predicted cross sections for the standard-candle W and Z boson production processes in the general-mass scheme (compared to those in the zero-mass scheme) is further investigated and quantified. A novel method to constrain PDF uncertainties in LHC observables, by effectively exploiting PDF-induced correlations with benchmark standard model cross sections, is presented. Using this method, we show that the top-antitop pair cross section can potentially serve as a standard candle observable for the LHC processes dominated by initial-state gluon scattering. Among other benefits, precise measurements of $t\bar{t}$ cross sections would reduce PDF uncertainties in predictions for single-top quark and Higgs boson production in the standard model and minimal supersymmetric standard model.

• The paper introduces the CTEQ6.6 PDFs using a general-mass QCD scheme to improve key collider predictions.
• It employs a Hessian correlation analysis to link PDF uncertainties with observables, notably impacting W, Z, and top-quark cross sections.
• The study highlights the potential of precise t-tbar measurements to constrain PDFs and serve as standard candles for luminosity monitoring at the LHC.

Analysis and Implications of the CTEQ6.6 Parton Distributions for Collider Observables

The paper "Implications of CTEQ global analysis for collider observables" by Nadolsky et al. introduces the CTEQ6.6 parton distribution functions (PDFs) within the general-mass (GM) scheme of perturbative QCD and explores their influence on important collider processes, especially at facilities like Fermilab's Tevatron and CERN's Large Hadron Collider (LHC). The paper aims to refine theoretical predictions of processes such as $W$, $Z$, and top-quark production and to minimize uncertainties by utilizing recent experimental data and advancements in QCD analysis.

Main Contributions

1. CTEQ6.6 PDF and Scheme Transition: The CTEQ6.6 PDFs involve a comprehensive treatment of heavy-quark mass effects in perturbative QCD, employing the ACOT general-mass approach. The transition from the zero-mass (ZM) to the GM scheme yields enhanced predictions for cross sections, notably increasing $W$ and $Z$ production cross sections at the LHC by approximately 6-7% compared to those obtained using the CTEQ6.1 PDFs. This is credited to the improved modeling of light-quark distributions, which are found to have increased at small $x$ due to relaxation in strange quark PDF constraints and incorporation of new experimental datasets.
2. Methodological Enhancements: The authors integrate a novel correlation analysis based on the Hessian method that assesses the dependency of collider observables on PDFs, directly linking uncertainties in partonic cross sections with specific PDF flavors at given momentum fractions $x$ and scales $\mu$. This enables a detailed understanding of how theoretical uncertainties propagate through complex QCD calculations to impact predicted observables.
3. PDF-Induced Correlation and Uncertainty Reduction: The paper identifies strong correlations between different collider cross sections driven by shared PDF uncertainties, making a compelling case for measuring $t\bar{t}$ cross sections with precision, potentially serving as a standard candle for processes dominated by initial-state gluon scattering at the LHC. The analysis indicates that precise measurements of $t\bar{t}$ rates, especially if theoretical uncertainties can be lowered to reflect those of PDFs, can help constrain the PDFs and serve for luminosity monitoring.
4. Strangeness and Intrinsic Charm Impact: The freedom introduced in the parametrization of the strangeness PDF influences predictions of the $Z/W$ cross section ratio, particularly important for LHC luminosity measurements. The potential inclusion of intrinsic charm in the PDFs could significantly affect predictions for specific Higgs boson production mechanisms in the MSSM.

Practical and Theoretical Implications

The results presented have substantial implications for QCD precision tests and collider physics. The improved predictions with CTEQ6.6 PDFs, underscored by variance from ZM approaches, imply that the incorporation of heavy-quark effects is vital for accurate theoretical modeling at the TeV scale. Moreover, identifying new standard candle observables like $t\bar{t}$ production and reducing related theoretical uncertainties can finely tune predictions for new physics phenomena at the LHC, such as Higgs production in various models.

Future Developments in AI

Given the intricate dependence of collider observables on QCD parameters, there exists an exciting opportunity for AI and ML techniques to further enhance PDF analysis. AI models could potentially refine correlations, predict experimentally challenging cross sections, and simulate more complex interactions with high accuracy, thereby supporting experimental analyses in refining their measurements further.

This paper sets a benchmark for future QCD analyses, highlighting the potential for further precision improvements that are essential as experimental data proliferates and more sophisticated AI-driven methodologies emerge to deal with complex domain uncertainties.