Breaking up the Proton: An Affair with Dark Forces

Abstract

Deep inelastic scattering of e± off protons is sensitive to contributions from "dark photon"exchange. Using HERA data fit to HERA's parton distribution functions (PDFs), we obtain the model-independent bound ϵ0.02 on the kinetic mixing between hypercharge and the dark photon for dark photon masses 10 GeV. This slightly improves on the bound obtained from electroweak precision observables. For higher masses, the limit weakens monotonically; ϵ1 for a dark photon mass of 5 TeV. Utilizing PDF sum rules, we demonstrate that the effects of the dark photon cannot be (trivially) absorbed into refit PDFs and, in fact, lead to non-Dokshitzer-Gribov-Lipatov-Altarelli-Parisi (Bjorken xB-independent) scaling violations that could provide a smoking gun in data. The proposed e±p collider operating at s=1.3 TeV (Large Hadron Electron Collider) is anticipated to accumulate 103 times the luminosity of HERA, providing substantial improvements in probing the effects of a dark photon: sensitivity to ϵ well below that probed by electroweak precision data is possible throughout virtually the entire dark photon mass range, as well as being able to probe to much higher dark photon masses, up to 100 TeV.