Direct search limits on the littlest Higgs model

Recent direct searches for new massive particles place constraints on the free parameters of the littlest Higgs model. Depending on the choice of model free parameters, the direct-search limit on the global symmetry-breaking scale f can range from as low as a few hundred GeV to in excess of 4.5 TeV. The most stringent constraints are from exclusion of the A(H) using high-mass dilepton resonance searches. The Z(H) provides the best constraint in parameter regions where the A(H) decouples from leptons. Current top pair resonance data approach but do not yet reach a useful limit in the anomaly-cancelling case, but do provide a constraint for a limited range of parameters in other cases. A neutral gauge boson is shown to be undetectable in dilepton resonances for a significant range of parameter space due to decoupling from standard model leptons, providing a counterexample to broad claims that a new neutral gauge boson (sometimes generically referred to as a Z(')) is ruled out to a high-mass scale.