Global QCD Analysis and Dark Photons

We perform a global QCD analysis of high energy scattering data within the JAM Monte Carlo framework, including a coupling to a dark photon that augments the standard model electroweak coupling via kinetic mixing with the hypercharge $B$ boson. Including the most recent measurement of the anomalous magnetic moment of the muon as a constraint, we find a significant reduction in the combined $\chi^2$, favoring the inclusion of a dark photon, with a statistical significance in excess of 8$\sigma$. With respect to the experimental data, the improvements in the theoretical predictions are spread across a wide range of $x$ and $Q^2$, with the largest improvement corresponding to neutral current data from HERA, while the best fit yields a value of $g-2$ which significantly reduces the disagreement with the latest experimental determination. The best fit yields a dark photon mass in the range 4.2--6.2 GeV and a mixing parameter of order 0.1.