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Improved Measurement of the $\bar d / \bar u$ Asymmetry in the Nucleon Sea

FNAL E866/NuSea Collaboration, :, R. S. Towell, P. L. McGaughey

TL;DR

The FNAL E866/NuSea experiment provides a high-precision, x-dependent measurement of the light antiquark asymmetry in the nucleon by analyzing the Drell-Yan cross-section ratio \\sigma^{pd}/2\\sigma^{pp} using 800 GeV/c protons on H2 and D2 targets. Through an iterative extraction anchored in LO cross sections folded with detector acceptance and modern PDFs (CTEQ5M, MRST), the study obtains \\bar{d}(x)/\\bar{u}(x) and \\bar{d}-\\bar{u} across 0.015 < x < 0.35, and an integral \\int_0^1 [\\bar{d}-\\bar{u}]dx = 0.118 \\pm 0.012 with extrapolation uncertainties. The results corroborate prior findings (NA51, HERMES, NMC) while providing tighter constraints on PDFs and challenging several nonperturbative models; meson-cloud and chiral-soliton scenarios are favored over instanton-dominated pictures. The analysis also assesses charge symmetry and nuclear shadowing, with small deuterium effects at higher x and implications for Gottfried-sum physics, motivating future measurements at higher x to fully map the asymmetry.

Abstract

Measurements of the ratio of Drell-Yan yields from an 800 \rm{GeV/c} proton beam incident on liquid hydrogen and deuterium targets are reported. Approximately 360,000 Drell-Yan muon pairs remained after all cuts on the data. From these data, the ratio of anti-down ($\bar{d}$) to anti-up ($\bar{u}$) quark distributions in the proton sea is determined over a wide range in Bjorken-$x$. These results confirm previous measurements by E866 and extend them to lower $x$. From these data, $(\bar{d}-\bar{u})$ and $\int(\bar{d}-\bar{u})dx$ are evaluated for $0.015<x<0.35$. These results are compared with parameterizations of various parton distribution functions, models and experimental results from NA51, NMC, and HERMES.

Improved Measurement of the $\bar d / \bar u$ Asymmetry in the Nucleon Sea

TL;DR

The FNAL E866/NuSea experiment provides a high-precision, x-dependent measurement of the light antiquark asymmetry in the nucleon by analyzing the Drell-Yan cross-section ratio \\sigma^{pd}/2\\sigma^{pp} using 800 GeV/c protons on H2 and D2 targets. Through an iterative extraction anchored in LO cross sections folded with detector acceptance and modern PDFs (CTEQ5M, MRST), the study obtains \\bar{d}(x)/\\bar{u}(x) and \\bar{d}-\\bar{u} across 0.015 < x < 0.35, and an integral \\int_0^1 [\\bar{d}-\\bar{u}]dx = 0.118 \\pm 0.012 with extrapolation uncertainties. The results corroborate prior findings (NA51, HERMES, NMC) while providing tighter constraints on PDFs and challenging several nonperturbative models; meson-cloud and chiral-soliton scenarios are favored over instanton-dominated pictures. The analysis also assesses charge symmetry and nuclear shadowing, with small deuterium effects at higher x and implications for Gottfried-sum physics, motivating future measurements at higher x to fully map the asymmetry.

Abstract

Measurements of the ratio of Drell-Yan yields from an 800 \rm{GeV/c} proton beam incident on liquid hydrogen and deuterium targets are reported. Approximately 360,000 Drell-Yan muon pairs remained after all cuts on the data. From these data, the ratio of anti-down () to anti-up () quark distributions in the proton sea is determined over a wide range in Bjorken-. These results confirm previous measurements by E866 and extend them to lower . From these data, and are evaluated for . These results are compared with parameterizations of various parton distribution functions, models and experimental results from NA51, NMC, and HERMES.

Paper Structure

This paper contains 12 sections, 23 equations, 15 figures, 13 tables.

Table of Contents

  1. Introduction
  2. Experimental Setup
  3. Trigger and Monitoring
  4. Analysis
  5. Calculation of $\sigma^{pd}/2\sigma^{pp}$
  6. Extraction of $\bar{d}(x)/\bar{u}(x)$
  7. Charge Symmetry and Shadowing
  8. Dependence on Other Kinematic Variables
  9. Comparison to Other Results
  10. Structure of the Nucleon Sea
  11. Origins of the Nucleon Sea
  12. Conclusions

Figures (15)

  • Figure 1: The FNAL E866/NuSea Spectrometer
  • Figure 2: The dimuon mass distributions for the three different mass settings. The inset figures are the same spectra shown on linear scales. The mass cuts used in the analysis to select Drell-Yan events are listed in Table \ref{['tab:massregions']}.
  • Figure 3: The dimuon distributions for $x_1$ versus $x_2$ for the high mass setting.
  • Figure 4: The dimuon distributions for $x_1$ versus $x_2$ for the intermediate mass setting.
  • Figure 5: The dimuon distributions for $x_1$ versus $x_2$ for the low mass setting.
  • ...and 10 more figures