Constraints on large-$x$ parton distributions from new weak boson production and deep-inelastic scattering data (1602.03154v3)

Abstract: We present a new set of leading twist parton distribution functions, referred to as "CJ15", which take advantage of developments in the theoretical treatment of nuclear corrections as well as new data. The analysis includes for the first time data on the free neutron structure function from Jefferson Lab, and new high-precision charged lepton and W-boson asymmetry data from Fermilab. These significantly reduce the uncertainty on the d/u ratio at large values of x and provide new insights into the partonic structure of bound nucleons.

Analysis of Large-$x$ Parton Distributions

The paper under review introduces the CJ15 set of leading-twist parton distribution functions (PDFs), incorporating recent experimental data and theoretical advances. The work addresses the need to rigorously analyze the parton structures at large momentum fractions, $x$, an area that remains less explored compared to the small-$x$ domain extensively studied at high-energy colliders. This analysis stands out by integrating novel data from the Jefferson Lab on the free neutron structure function and precise $W$-boson production asymmetry data from Fermilab.

Key Contributions

The researchers enhance previous studies by applying more robust models for nuclear corrections in deuteron data, crucial for accurate PDF extraction at large $x$. They include data sensitive to the $d/u$ ratio, critically reducing uncertainties associated with this parameter. The CJ15 PDFs are determined using an improved description of the nuclear corrections, especially nucleon off-shell effects, fitted directly to data. This approach contrasts with prior analyses relying heavily on model-dependent corrections, demonstrating progress in minimizing model biases in such fits.

Methodological Improvements

The CJ collaboration advances the theoretical framework for PDF extraction notably by:

1. Nuclear Corrections: Implementing comprehensive deuteron corrections accounting for Fermi motion, binding, and off-shell effects. A phenomenological form for the off-shell correction is precisely fitted, providing a more adaptable and less biased correction mechanism.
2. Heavy Quark Treatment: Utilizing the S-ACOT scheme for a more accurate threshold behavior of PDFs involving charm and bottom quarks, improving on the ZMVFS scheme.
3. Flexible PDF Parametrization: Introducing a refined parametrization for the $\bar{d}/\bar{u}$ ratio, which directly impacts the understanding of sea quark distributions at large $x$.

Impact of New Data

A central strength of this analysis is the use of newly available data sets which contribute significantly to refining PDF uncertainties. The Jefferson Lab's novel approach in measuring neutron structure through spectator tagging in the BONuS experiment provides critical information previously unavailable due to the complexities of nuclear corrections. Additionally, the precision of $W$-boson asymmetry measurements at large rapidities presents robust constraints on the $d$-quark PDF, especially at larger $x$. These data sets enhance the reliability of the analysis, distinguishing CJ15 as a significant refinement over its predecessors (e.g., CJ12), particularly in the large-$x$ region where experimental data are sparse.

Implications and Future Directions

The improvement in the $d/u$ ratio determination has important implications for high-energy physics, especially in searches at the LHC for new physics signatures at the kinematic limits. The methodology opens pathways for future studies with upcoming data from experiments like SeaQuest and the Jefferson Lab’s further upgraded experiments, which are expected to probe deeper into the nucleon’s partonic landscape. These analyses will likely offer even more precise determinations of PDFs, influenced by the direct measurements of free neutron structure and further verification of nuclear correction models.

Conclusion

This work paves the way for a deeper understanding of nucleon structure at large momentum fractions. By incorporating new experimental insights and theoretical advancements, it sets a comprehensive standard for future analyses needed to address the complexities of nonperturbative QCD effects in nucleon structure. The CJ15 set of PDFs underscores the importance of considering both theoretical refinements and new data to inform our understanding of nucleon substructure, reinforcing the bridge between theoretical predictions and experimental observations in particle physics.