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The CDF and D0 collaborations have observed a forward-backward asymmetry int-tbar production at large invariant mass in excess of the standard modelprediction. One explanation involves a heavy color octet particle with axialvector couplings to quarks (an axigluon). We describe and contrast variousaspects of axigluons obtained from the breaking of a chiral SU(3)_L x SU(3)_Rgauge theory both from the standpoint of a string-inspired field theory andfrom a quiver analysis of a local type IIa intersecting brane construction.Special attention is paid to the additional constraints and issues that arisefrom these classes of top-down constructions compared with the more commoneffective field theory approach. These include the implications of aperturbative connection to a large scale; Yukawa couplings, which must begenerated from higher-dimensional operators in many constructions; anomalycancellation, in particular the implications of the required exotics for theaxigluon width, perturbativity, and the signatures from exotic decays; thepossibility of family nonuniversality via mirror representations, mixing withexotics, or additional SU(3) factors; the additional constraints from anomalousU(1) factors in the string constructions; tadpole cancellation, which impliesnew uncolored matter; the prevention of string-scale masses for vector pairs;and various phenomenological issues involving FCNC, CKM constraints, and theaxigluon coupling strength. It is concluded that the construction of viableaxigluon models from type IIa or similar constructions is problematic and wouldrequire considerable fine tuning, but is not entirely excluded. Theseconsiderations illustrate the importance of top-down constraints on possibleTeV-scale physics, independent of the ultimate explanation of the t-tbarasymmetry.

Ultraviolet Completions of Axigluon Models and Their Phenomenological Consequences