Confronting Top AFB with Parity Violation Constraints

We consider the implications of low-energy precision tests of parity violation on t-channel mediator models explaining the top AFB excess measured by CDF and D0. Flavor-violating u-t or d-t couplings of new scalar or vector mediators generate at one-loop an anomalous contribution to the nuclear weak charge. As a result, atomic parity violation constraints disfavor at >3 sigma t-channel models that give rise to a greater than 20% AFB at the parton level for M_tt > 450 GeV while not producing too large a top cross-section. Even stronger constraints are expected through future measurements of the proton weak charge by the Q-Weak experiment.