New CTEQ global analysis of quantum chromodynamics with high-precision data from the LHC

Abstract: We present the new parton distribution functions (PDFs) from the CTEQ-TEA collaboration, obtained using a wide variety of high-precision Large Hadron Collider (LHC) data, in addition to the combined HERA I+II deep-inelastic scattering data set, along with the data sets present in the CT14 global QCD analysis. New LHC measurements in single-inclusive jet production with the full rapidity coverage, as well as production of Drell-Yan pairs, top-quark pairs, and high-$p_T$ $Z$ bosons, are included to achieve the greatest sensitivity to the PDFs. The parton distributions are determined at NLO and NNLO, with each of these PDFs accompanied by error sets determined using the Hessian method. Fast PDF survey techniques, based on the Hessian representation and the Lagrange Multiplier method, are used to quantify the preference of each data set to quantities such as $\alpha_s(m_Z)$, and the gluon and strange quark distributions. We designate the main resulting PDF set as CT18. The ATLAS 7 TeV precision $W/Z$ data are not included in CT18, due to their tension with other data sets in the global fit. Alternate PDF sets are generated including the ATLAS precision 7 TeV $W/Z$ data (CT18A), a new scale choice for low-$x$ DIS data (CT18X), or all of the above with a slightly higher choice for the charm mass (CT18Z). Theoretical calculations of standard candle cross sections at the LHC (such as the $gg$ fusion Higgs boson cross section) are presented.

• The paper presents a refined global QCD analysis integrating high-precision LHC data to advance the accuracy of CT18 PDFs.
• It employs state-of-the-art NNLO calculations and advanced statistical techniques for precise cross-section predictions.
• The improved PDFs reduce uncertainties in gluon and strangeness distributions, aligning theoretical predictions with collider measurements.

Overview of CTEQ Global Analysis on Quantum Chromodynamics with LHC Data

The paper presents the latest advancements in the global analysis of Quantum Chromodynamics (QCD) undertaken by the CTEQ-TEA collaboration. With a focus on incorporating high-precision data from the Large Hadron Collider (LHC), the analysis utilizes updated parton distribution functions (PDFs) termed CT18. This work revisits and expands upon previous analyses, notably CT14, by integrating more recent datasets, thereby offering refined PDF sets that enhance our understanding and predictions of collider processes at NNLO and NLO precision.

Key Developments and Methodology

The CTEQ-TEA collaboration has made significant strides by incorporating comprehensive data from the LHC, alongside legacy data from HERA and fixed-target experiments, to refine the partonic content of the proton. The use of diverse LHC measurements, including $W/Z$ boson and top-quark pair production, as well as inclusive jet cross sections, marks a substantial enlargement of the global dataset pivotal for PDF constraints.

1. Data Inclusion and Selection: The inclusion of new LHC Run-I data has been carefully curated, ensuring consistency and impact. The integration accounts for tensions between various datasets, applying rigorous selection criteria based on sensitivity and compatibility analyses. Notably, the analysis critically evaluates the inclusion of the ATLAS 7 TeV $W/Z$ data due to its conflicting pulls on the PDFs relative to other key datasets like those from NuTeV and CCFR dimuon production.
2. Theoretical Precision: The analysis employs sophisticated theoretical calculations, incorporating NNLO predictions for cross sections using tools like fastNLO and APPLGrid. These methods allow predictions to keep pace with the precision of the collated experimental data. The study also examines the effect of theoretical uncertainties, such as scale choices, on PDFs, and proposes alternative treatments to gauge their impacts.
3. Advanced Statistical Techniques: To effectively handle the vast and complex dataset, the CTEQ-TEA group employs advanced statistical methodologies. Lagrange Multiplier scans, $L_2$ sensitivity metrics, and Hessian profiling are used to map the data's constraints upon the PDFs. These techniques are vital for identifying tensions between datasets and understanding their collective impact on the derived PDFs.
4. Parametrization and Uncertainties: The paper discusses the implementation of flexible parametrization forms to capture the nuanced x-dependencies of PDFs. By exploring diverse functional forms and estimating uncertainties, the analysis provides robust PDF sets that reflect the theoretical and experimental landscape's precision.

Numerical Results and Implications

• PDF Highlights: The CT18 PDFs exhibit improvements over previous sets, especially in reducing uncertainties in the gluon distribution at relevant x values for LHC processes like Higgs production. Adjustments in the strangeness and antiquark distributions reflect enhanced constraints from new LHC data.
• Strong Coupling and Heavy Quark Masses: Scans for $\alpha_s(M_Z)$ suggest a central value slightly lower than past averages but consistent with global fits. The analysis also considers variations in the charm mass, revealing sensitivities that influence PDF behavior and fit quality.
• Parton Luminosities and Cross Sections: Updated parton luminosities are provided for LHC energies, essential for predicting key processes. The predictive power of CT18 extends to reliable calculations of Higgs, top-quark pair, and vector boson production, aligning closely with observed values.

Future Directions and Conclusions

The CT18 PDF release represents a significant advancement in QCD analysis, leveraging high-precision LHC data to refine the partonic picture of the proton. The paper acknowledges ongoing challenges, such as dataset tensions and theoretical uncertainties, suggesting continuous methodological advancements and new measurements are vital for future progress. The findings of this analysis are crucial for precision predictions at current and future colliders, facilitating more stringent tests of the Standard Model and searches for new physics phenomena. The flexibility of the CTEQ-TEA frameworks, coupled with their comprehensive treatment of uncertainties, ensures that CT18 and its follow-up analyses will be instrumental in meeting the demands of an evolving experimental landscape.