  The CDF and D0 collaborations have observed a forward-backward asymmetry in
t-tbar production at large invariant mass in excess of the standard model
prediction. One explanation involves a heavy color octet particle with axial
vector couplings to quarks (an axigluon). We describe and contrast various
aspects of axigluons obtained from the breaking of a chiral SU(3)_L x SU(3)_R
gauge theory both from the standpoint of a string-inspired field theory and
from a quiver analysis of a local type IIa intersecting brane construction.
Special attention is paid to the additional constraints and issues that arise
from these classes of top-down constructions compared with the more common
effective field theory approach. These include the implications of a
perturbative connection to a large scale; Yukawa couplings, which must be
generated from higher-dimensional operators in many constructions; anomaly
cancellation, in particular the implications of the required exotics for the
axigluon width, perturbativity, and the signatures from exotic decays; the
possibility of family nonuniversality via mirror representations, mixing with
exotics, or additional SU(3) factors; the additional constraints from anomalous
U(1) factors in the string constructions; tadpole cancellation, which implies
new uncolored matter; the prevention of string-scale masses for vector pairs;
and various phenomenological issues involving FCNC, CKM constraints, and the
axigluon coupling strength. It is concluded that the construction of viable
axigluon models from type IIa or similar constructions is problematic and would
require considerable fine tuning, but is not entirely excluded. These
considerations illustrate the importance of top-down constraints on possible
TeV-scale physics, independent of the ultimate explanation of the t-tbar
asymmetry.
