Little Higgs models and electroweak measurements

Using effective field theory approach, we study the constraints from electroweak data on the $SU(5)/SO(5)$, $SU(6)/SP(6)$, SU(3)×U(1) little Higgs models and their variations. We construct an effective theory valid above the electroweak symmetry breaking scale in these models that includes dimension-six operators induced by integrating out heavy fields. We calculate the constraints on the models using the results of [Z. Han and W. Skiba, Phys. Rev. D 71, 075009 (2005)], where bounds on arbitrary linear combinations of flavor and $CP$ conserving dimension-six operators are given. We present the constraints in terms of the bounds on the masses of heavy fermions and heavy gauge bosons. The constraints are often stringent, but in some regions of the parameter space the constraints are mild enough and do not imply significant fine-tuning.