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We use a recent upper limit from CDF on like-sign top pair production toplace constraints on general new vector bosons and scalars mediating u u -> tt. The possible vector bosons comprise neutral colour singlets or octets, andcharge 4/3 colour triplets or sextets, whereas the new scalars can be neutralcolour singlets or octets and charge 4/3 colour sextets. We also estimate theexpected bounds from like-sign top pair production at LHC in the near future.Then, we address the implications of these limits for the forward-backwardasymmetry in t tbar production measured at Tevatron. In particular, we findthat models explaining the observed asymmetry by the exchange of a singlet-channel heavy Z' boson are already excluded. On the other hand, light Z'bosons with a mass M_Z' ~ 150 GeV, which could also account for a recent CDFdijet excess in W+jet production, are barely allowed.

No like-sign tops at Tevatron: Constraints on extended models and implications for the t tbar asymmetry