  Lorentz violation in the quark sector induces a sidereal time dependence in
electron-proton, proton-antiproton and proton-proton cross sections. At high
energies nonperturbative effects are buried in universal nucleon parton
distribution functions and Lorentz violating effects are calculable in
perturbation theory. We focus on deep inelastic electron-proton scattering data
collected from ZEUS and H1 at HERA and show that a sideral time analysis of
these events is able to set strong constraints on most of the coefficients we
consider.
