A Slant on Warped Extra Dimensions
  We propose an orbifolded, warped, extra dimension scenario in which the visible brane is not parallel to the hidden brane. This leads automatically to Lorentz violation in the visible, four dimensional world. The background solution to the Einstein equations is a function of a parameter that can be identified with the amount of 'tilting' of the brane. The cosmological constant is found to coincide with the classic Randall-Sundrum value to first order in this tilt. Lorentz violating effects induced in the Standard Model are considered. We find that the strongest constraint on the tilt comes from determinations of the electron-proton mass ratio in six quasar spectra (four optical and two radio). Measurements of a third radio source could improve this by an order of magnitude. 
Daniel Hernandez and Marc Sher
hep-ph astro-ph.CO