Parton distributions in the LHC era: MMHT 2014 PDFs (1412.3989v2)

Abstract: We present LO, NLO and NNLO sets of parton distribution functions (PDFs) of the proton determined from global analyses of the available hard scattering data. These MMHT2014 PDFs supersede the MSTW2008' parton sets, but are obtained within the same basic framework. We include a variety of new data sets, from the LHC, updated Tevatron data and the HERA combined H1 and ZEUS data on the total and charm structure functions. We also improve the theoretical framework of the previous analysis. These new PDFs are compared to theMSTW2008' parton sets. In most cases the PDFs, and the predictions, are within one standard deviation of those of MSTW2008. The major changes are the $u-d$ valence quark difference at small $x$ due to an improved parameterisation and, to a lesser extent, the strange quark PDF due to the effect of certain LHC data and a better treatment of the $D \to \mu$ branching ratio. We compare our MMHT PDF sets with those of other collaborations; in particular with the NNPDF3.0 sets, which are contemporary with the present analysis.

• The paper introduces improved PDFs derived from extensive LHC, Tevatron, and HERA data with enhanced parameterization addressing valence distribution discrepancies.
• It implements refined deuteron corrections and optimal heavy quark treatment, leading to robust charge asymmetry predictions in W± production.
• The analysis offers comprehensive NNLO cross-section benchmarks that outperform MSTW2008, establishing MMHT2014 as a key tool for precise QCD studies.

Analysis of MMHT2014 Parton Distribution Functions

The paper "Parton distributions in the LHC era" presents the MMHT2014 parton distribution functions (PDFs) derived from an augmented set of experimental data and refined theoretical methodologies. This analysis supersedes the MSTW2008 PDFs, utilizing a combination of updated datasets, particularly from the Large Hadron Collider (LHC), alongside optimized theoretical frameworks to enhance the precision and reliability of proton structure modeling.

The MMHT2014 PDFs are derived at leading order (LO), next-to-leading order (NLO), and next-to-next-to-leading order (NNLO), improving aspects of both input parameterization and data inclusion. Notably, the analysis incorporates a vast array of new high-quality datasets from the LHC, complemented by updated datasets from earlier experiments such as the Tevatron and HERA. Moreover, methodological improvements are incorporated, particularly in the parameterization of input distributions, deuteron and nuclear corrections, and the treatment of heavy quark thresholds using an optimal general-mass variable flavor number scheme (GM-VFNS).

A significant focus of the MMHT2014 analysis lies in addressing discrepancies noted in previous PDFs, particularly the limitations in the $u_V-d_V$ valence quark distribution at low Bjorken $x$. This has been tackled via an enhanced parameterization framework based on Chebyshev polynomials, allowing for greater flexibility and stability. Consequently, these improvements address prior shortcomings in fitting LHC data, notably in charge asymmetry measurements for $W^{\pm}$ production, resulting in more robust predictions across the entire kinematic range.

The analysis explores the implications of refined deuteron correction factors, allowing them to be dynamically determined by the data with theoretical biases minimized. Estimated corrections exhibit convergence with theoretical expectations, providing improved stability and reduced bias in PDF determinations, particularly in distinguishing valence from sea quark effects.

The datasets from neutrino-induced muon pair production have been revisited, incorporating a more consistent treatment of charm quark decay branching ratios, leading to an elevated uncertainty estimate for the strange quark PDFs. This mirrors a general trend within this analysis: while central PDF estimates have been adjusted in light of new data, uncertainty bands often remain broad due to significant, systematic increases in modeled uncertainty. A notable example is the substantial increase in uncertainty for the $s+\bar{s}$ strange quark distribution.

Comparatively, MMHT2014 PDFs align well with other contemporary determinations from NNPDF, CT, and ABM, though differences persist due to varying methodologies and frameworks. For instance, the gluon distribution shows relatively strong agreement with NNPDF across much of its domain but diverges from ABM in regions dominated by differences in coupling constant treatment and heavy quark scheme variations.

Ultimately, the MMHT2014 analysis provides a comprehensive suite of PDFs accompanied by extensive cross-section predictions for collider processes, meticulously benchmarking against prior models and detailing potential shifts in phenomenological outcomes — most notably in processes sensitive to valence distributions. Though MSTW2008 PDFs remain performant in many cases, the improved precision and methodological transparency of MMHT2014 establish it as a preferred tool for ongoing and future explorations in proton structure and QCD processes at the LHC and beyond.

The release of MMHT2014 marks another evolutionary step in global PDF fitting, aiming to meet the stringent demands of current and future high-luminosity collider experiments where precise parton modeling is imperative. Subsequent developments will likely focus on refining NNLO corrections, particularly for jet and top-quark production, maximizing the constraining power of emerging LHC datasets.