This is an old question, and there will probably be better answers, but in the meantime, check out [Browning and Dietmann's paper][1], and (our own) [Paul Garrett's notes.][2]


  [1]: http://www.maths.bris.ac.uk/~matdb/preprints/qleast.pdf
  [2]: http://www.math.umn.edu/~garrett/students/reu/hasse_minkowski.pdf