Exclusive heavy-vector meson photoproduction is a prominent signal in collider experiments with hadron beams. At the highest photon-hadron collision energies, this process is considered as a candidate to constrain the gluon parton distribution function (PDF) at small longitudinal momentum fractions. However, in the framework of collinear factorization, exclusive particle production is described in terms of generalized parton distributions (GPDs). In this contribution, we investigate the connection between GPDs and PDFs at the leading order in <math><msub><mi>α</mi><mi>s</mi></msub></math>. Our main result is a proposal to quantify the systematic uncertainty inherent to this connection. We put our approach into context with respect to the Shuvaev transform. Our uncertainty estimate can be straightforwardly adapted to higher fixed orders and small-<math><mi>x</mi></math> resummation. The question of extrapolating GPDs to vanishing skewness is paramount for the program of the Electron Ion Collider, notably for the extraction of the radial distributions of partons.

France

Hervé Dutrieux

Michael Winn

Valerio Bertone

Exclusive meets inclusive particle production at small Bjorken <math><msub><mi>x</mi><mi>B</mi></msub></math>: How to relate exclusive measurements to PDFs based on evolution equations

The production of dileptons with an invariant mass in the range <math><mn>1</mn><mspace width="0.25em"></mspace><mtext>GeV</mtext><mo>&lt;</mo><mi>M</mi><mo>&lt;</mo><mn>5</mn></math> GeV provides unique insight into the approach to thermal equilibrium in ultrarelativistic nucleus-nucleus collisions. In this mass range, they are produced through the annihilation of quark-antiquark pairs in the early stages of the collision. They are sensitive to the anisotropy of the quark momentum distribution, and also to the quark abundance, which is expected to be underpopulated relative to thermal equilibrium. We take into account both effects based on recent theoretical developments in QCD kinetic theory. We argue that the dilepton mass spectrum provides a measure of the shear viscosity to entropy ratio that controls the equilibration time. We evaluate the background from the Drell-Yan process and argue that future detector developments can suppress the additional background from semileptonic decays of heavy flavors.