On models of new physics for the Tevatron top A_(FB)

Abstract

The Collider Detector at Fermilab (CDF) has observed a top forward-backward asymmetry discrepant with the standard model prediction at 3.4σ. We analyze models that could generate the asymmetry, including flavor-violating W′s, horizontal Z′_(H)s, triplet and sextet diquarks, and axigluons. We consider the detailed predictions of these models for the invariant mass and rapidity distributions of the asymmetry at the parton level, comparing against the unfolded parton-level CDF results. While all models can reproduce the asymmetry with the appropriate choice of mass and couplings, it appears at first examination that the extracted parton-level invariant mass distributions for all models are in conflict with Tevatron observations. We show, however, that, on closer examination, tt̄ events in Z′_H and W′ models have considerably lower selection efficiencies in high invariant mass bins as compared to the standard model, so that W′, Z′_H, and axigluon models can generate the observed asymmetry while being consistent with the total cross-section and invariant mass spectrum. Triplet and sextet models have greater difficulty producing the observed asymmetry while remaining consistent with the total cross-section and invariant mass distribution. To more directly match the models and the CDF results, we proceed to decay and reconstruct the tops, comparing our results against the raw CDF asymmetry and invariant mass distributions. We find that the models that successfully generate the corrected CDF asymmetry at the parton level reproduce the more finely binned uncorrected asymmetry very well. Finally, we discuss the early LHC reach for discovery of these models, based on our previous analysis [M. I. Gresham, I.-W. Kim, and K. M. Zurek, arXiv:1102.0018.].

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