Direct Observation Limits on Antimatter Gravitation
  The proposed Antihydrogen Gravity experiment at Fermilab (P981) will directly measure the gravitational attraction ("gbar") between antihydrogen and the Earth, with an accuracy of 1% or better. The following key question has been asked by the PAC:   Is a possible 1% difference between gbar and g already ruled out by other evidence?   This memo presents the key points of existing evidence, to answer whether such a difference is ruled out (a) on the basis of direct observational evidence; and/or (b) on the basis of indirect evidence, combined with reasoning based on strongly held theoretical assumptions. The bottom line is that there are no direct observations or measurements of gravitational asymmetry which address the antimatter sector. There is evidence which by indirect reasoning can be taken to rule out such a difference, but the analysis needed to draw that conclusion rests on models and assumptions which are in question for other reasons and are thus worth testing. There is no compelling evidence or theoretical reason to rule out such a difference at the 1% level. 
Mark Fischler, Joe Lykken, and Tom Roberts