Parton distributions from high-precision collider data (1706.00428v2)

Abstract: We present a new set of parton distributions, NNPDF3.1, which updates NNPDF3.0, the first global set of PDFs determined using a methodology validated by a closure test. The update is motivated by recent progress in methodology and available data, and involves both. On the methodological side, we now parametrize and determine the charm PDF alongside the light quarks and gluon ones, thereby increasing from seven to eight the number of independent PDFs. On the data side, we now include the D0 electron and muon W asymmetries from the final Tevatron dataset, the complete LHCb measurements of W and Z production in the forward region at 7 and 8 TeV, and new ATLAS and CMS measurements of inclusive jet and electroweak boson production. We also include for the first time top-quark pair differential distributions and the transverse momentum of the Z bosons from ATLAS and CMS. We investigate the impact of parametrizing charm and provide evidence that the accuracy and stability of the PDFs are thereby improved. We study the impact of the new data by producing a variety of determinations based on reduced datasets. We find that both improvements have a significant impact on the PDFs, with some substantial reductions in uncertainties, but with the new PDFs generally in agreement with the previous set at the one sigma level. The most significant changes are seen in the light-quark flavor separation, and in increased precision in the determination of the gluon. We explore the implications of NNPDF3.1 for LHC phenomenology at Run II, compare with recent LHC measurements at 13 TeV, provide updated predictions for Higgs production cross-sections and discuss the strangeness and charm content of the proton in light of our improved dataset and methodology. The NNPDF3.1 PDFs are delivered for the first time both as Hessian sets, and as optimized Monte Carlo sets with a compressed number of replicas.

• The paper introduces an updated NNPDF3.1 methodology that independently parametrizes the charm PDF among eight flavors.
• It integrates extensive high-precision data from Tevatron and LHC experiments to reduce uncertainties in parton distribution functions.
• The study enhances accuracy in gluon and light-quark distributions, supporting advanced analyses in particle physics.

An Overview of Parton Distributions from High-Precision Collider Data

The paper explores the NNPDF3.1 set of parton distribution functions (PDFs), offering an update on the previously established NNPDF3.0, with both methodological enhancements and the integration of novel experimental data. This update is vital for accuracy in contemporary and future particle physics research, founded on parton distributions for theoretical understanding of hadronic structure.

Methodological Advancements

NNPDF3.1 incorporates significant methodological progress by introducing an independently parametrized charm PDF, in addition to the traditional quark and gluon distributions. This adjustment results in the number of independently parameterized PDF flavors increasing from seven to eight—improving the assessment of uncertainties and the overall stability of the PDFs.

Integration of Novel High-Precision Data

The dataset for NNPDF3.1 has been extensively expanded, incorporating recent precise measurements from the Tevatron and LHC, including but not limited to D0 $W$ electron and muon asymmetries, complete LHCb measurements of $W$ and $Z$ production, as well as inclusive jet production data from ATLAS and CMS. For the first time, differential distributions of top-quark pairs and $Z$ bosons’ transverse momentum from ATLAS and CMS are also included in the analysis. This wealth of data allows for a more comprehensive understanding and a reduction in the uncertainties of the PDFs.

Impact and Analysis

Parametrizing charm standalone has proven crucial for the robustness of the data fitting process. The paper indicates a significant impact on the accuracy and reliability of the PDFs, chiefly by removing biases related to the assumption of perturbative generation of charm. Compared to NNPDF3.0, there are notable improvements in constraining the gluon distribution and refining light-quark flavor separation.

The paper also explores the compatibility and impact of including or excluding different experimental data subsets, clarifying their significance in PDF determinations. In conclusion, NNPDF3.1 finds a remarkable improvement over its predecessor especially in accuracy and the reduction of theoretical uncertainties, thus aligning better with high-precision LHC data, informing a variety of analyses, including strangeness and charm content of the proton.

Prospects and Future Directions

NNPDF3.1 sets a new precedent in PDF determinations, striving towards reduced uncertainties and higher precision, critical for analyses relevant to Higgs cross-section measurements and beyond-standard-model searches at the LHC. With continued advancements in both theoretical methodologies and experimental dataset expansion, future research may aim to address the remaining discrepancies in some flavor decompositions and tackle the intricacies surrounding intrinsic charm components, potentially leveraging upcoming LHC runs.

In this landscape, as precision measurements advance, the intricate balance between theoretical paradigms and experimental realities will continue to necessitate rigorous and versatile PDF update methodologies, similar to those demonstrated in this paper.