Constraining the pion distribution amplitude using Drell-Yan reactions on a proton

TL;DR

This paper presents a reaction model to predict pion-induced Drell-Yan muon angular distributions, highlighting their sensitivity to the pion distribution amplitude and emphasizing the need for higher-precision data for effective mapping.

Contribution

The study introduces a parameter-free reaction model incorporating pion bound state effects to predict muon angular distributions sensitive to the pion distribution amplitude.

Findings

Angular distributions are sensitive to the pion distribution amplitude.

Current data precision is insufficient for detailed mapping of $\

The model predicts a nonzero single-spin azimuthal asymmetry without $T$-odd DFs.

Abstract

Using a reaction model that incorporates pion bound state effects and continuum results for proton parton distributions and the pion distribution amplitude, $\varphi_\pi$, we deliver parameter-free predictions for the $\mu^+$ angular distributions in $\pi N \to \mu^+ \mu^- X$ reactions on both unpolarised and polarised targets. The analysis indicates that such angular distributions are sensitive to the pointwise form of $\varphi_\pi$ and suggests that unpolarised targets are practically more favourable. The precision of extant data is insufficient for use in charting $\varphi_\pi$; hence, practical tests of this approach to charting $\varphi_\pi$ must await data with improved precision from new-generation experiments. The reaction model yields a nonzero single-spin azimuthal asymmetry, without reference to $T$-odd parton distribution functions (DFs). This may necessitate additional care when attempting to extract such $T$-odd DFs from data.