Parton distributions from LHC, HERA, Tevatron and fixed target data: MSHT20 PDFs (2012.04684v4)

Abstract: We present the new MSHT20 set of parton distribution functions (PDFs) of the proton, determined from global analyses of the available hard scattering data. The PDFs are made available at NNLO, NLO, and LO, and supersede the MMHT14 sets. They are obtained using the same basic framework, but the parameterisation is now adapted and extended, and there are 32 pairs of eigenvector PDFs. We also include a large number of new data sets: from the final HERA combined data on total and heavy flavour structure functions, to final Tevatron data, and in particular a significant number of new LHC 7 and 8 TeV data sets on vector boson production, inclusive jets and top quark distributions. We include up to NNLO QCD corrections for all data sets that play a major role in the fit, and NLO EW corrections where relevant. We find that these updates have an important impact on the PDFs, and for the first time the NNLO fit is strongly favoured over the NLO, reflecting the wider range and in particular increased precision of data included in the fit. There are some changes to central values and a significant reduction in the uncertainties of the PDFs in many, though not all, cases. Nonetheless, the PDFs and the resulting predictions are generally within one standard deviation of the MMHT14 results. The major changes are the $u-d$ valence quark difference at small $x$, due to the improved parameterisation and new precise data, the $\bar d, \bar u$ difference at small $x$, due to a much improved parameterisation, and the strange quark PDF due to the effect of LHC $W,Z$ data and inclusion of new NNLO corrections for dimuon production in neutrino DIS. We discuss the phenomenological impact of our results, and in general find reduced uncertainties in predictions for processes such as Higgs, top quark pair and $W,Z$ production at post LHC Run-II energies.

• The paper presents a refined MSHT20 analysis that improves PDF parameterization with six Chebyshev polynomials for enhanced proton flavor characterization.
• It utilizes a global fit of diverse data from LHC, HERA, Tevatron, and fixed-target experiments, ensuring constraints across a broad kinematic range.
• The incorporation of NNLO QCD corrections reduces uncertainties in gluon and quark distributions, boosting precision predictions for high-energy processes.

Overview of MSHT20 Parton Distribution Functions

The paper presents the MSHT20 set of parton distribution functions (PDFs) for the proton, marking an update over the previous MMHT14 PDFs. These PDFs are developed through global analyses of a comprehensive set of hard scattering data sourced from a range of experiments, including the LHC, HERA, and the Tevatron. Notably, the MSHT20 PDFs are available across various orders of perturbation theory—Next-to-Leading Order (NLO), Next-to-Next-to-Leading Order (NNLO), and Leading Order (LO). The MSHT20 set signifies an advancement in the detail and precision of the PDF parameterization and incorporates numerous new data sets.

Key Updates and Methodological Advances

The MSHT20 analysis introduces significant improvements over MMHT14, both in terms of theoretical modeling and the data sets employed. Central to these improvements is the enhanced parameterization of the PDFs, which now includes six Chebyshev polynomials—a marked increase in complexity from the previous four, optimized to capture the doubly-differential structure of the flavor content in the proton. The antiquark asymmetries, a critical factor in discerning flavor dynamics, are captured via a refined parameter set describing the $\bar{d}/\bar{u}$ ratio rather than the difference, allowing for a more accurate representation, particularly at low Bjorken-$x$.

In addition, NNLO QCD corrections are introduced, notably impacting the precision fits where up to NNLO theoretical predictions are now available, including for the challenging neutrino-induced dimuon production processes. This detailed inclusion reflects a commitment to reducing theoretical uncertainties and aligning closely with high-precision experimental data, a notable feature in the era of precision QCD.

Data Sets and Global Fitting Approach

The MSHT20 set draws from a rich tapestry of data spanning inclusive DIS from HERA to precision electroweak data from the LHC, thereby providing constraints across a broad kinematic range. This exhaustive data inclusion ensures that the new PDFs reflect a contemporary understanding of partonic dynamics, particularly with the LHC providing inputs on processes like $W$+jets and $t\overline{t}$ production.

The implementation of theoretical uncertainties is handled using a Hessian framework with a dynamic tolerance approach. This method captures tension between different data sets and reflects uncertainties not purely captured by statistical analysis. However, results indicate that PDF4LHC15's value of $\alpha_S(M_Z^2)=0.118$ at NNLO is consistent with world averages, further corroborating the robustness of the presented fits.

Impact on PDF Compositions

The flavor decomposition highlights salient improvements—the MSHT20 PDFs exhibit refined uncertainty bands and demonstrate enhanced precision in capturing the valence quark distributions. Significant reductions in uncertainties are evident for the gluon PDF, especially at high $x$, driven by high precision jet data, as well as for quark flavor separations constrained by new results from E866 Drell-Yan and LHC vector boson data.

The strange quark sector, often subject to large uncertainties, benefits substantially from updated LHC $W/Z$ data. Notably, exploration of the strangeness suppression ratio $R_s$ indicates a trend towards reduced suppression, aligning with newer measurements.

Broader Implications and Future Directions

The MSHT20 PDFs provide an indispensable tool for high-energy physics, facilitating precise predictions for current and future collider experiments. Improved accuracy in the PDFs informs theoretical predictions relevant for high precision measurements, such as the Higgs boson production cross sections at varying energies, essential for probing new physics scenarios.

There is potential for further refinement using forthcoming data at 13 TeV from the LHC and advanced theoretical calculations, such as N$^3$LO corrections, which could alleviate existing tensions between data sets. Further development may also engage with the intricacies of electroweak corrections within the MSHT series of PDFs which are critical at high energies.

In conclusion, the MSHT20 analysis represents a pivotal step in PDF development, enhancing the accuracy and reliability of theoretical predictions across a wide array of QCD processes. Its comprehensive embedding of modern experimental insights alongside theoretical innovations marks MSHT20 as a cornerstone for next-generation analyses in particle physics.