Tomography of pions and protons via transverse momentum dependent distributions

TL;DR

This paper compares the transverse momentum dependent parton distribution functions of pions and protons using low-energy Drell-Yan data, revealing differences in their internal quark structure and nuclear modifications.

Contribution

First simultaneous extraction of collinear and transverse parton distributions from fixed-target data, highlighting differences in pion and proton internal structure.

Findings

Pion's quark transverse separation is over 4 sigma smaller than proton's.

Transverse separation decreases with lower longitudinal momentum fraction.

Evidence of a transverse EMC effect in nuclear modifications.

Abstract

We perform the first simultaneous extraction of parton collinear and transverse degrees of freedom from low-energy fixed-target Drell-Yan data in order to compare the transverse momentum dependent (TMD) parton distribution functions (PDFs) of the pion and proton. We demonstrate that the transverse separation of the quark field encoded in TMDs of the pion is more than $4 \sigma$ smaller than that of the proton. Additionally, we find the transverse separation of the quark field decreases as its longitudinal momentum fraction decreases. In studying the nuclear modification of TMDs, we find clear evidence for a transverse EMC effect. We comment on possible explanations for these intriguing behaviors, which call for a deeper examination of tomography in a variety of strongly interacting quark-gluon systems.