The Littlest Higgs
N. Arkani-Hamed, A. G. Cohen, E. Katz, A. E. Nelson
TL;DR
This paper introduces the littlest Higgs model, a minimal, weakly coupled extension of the Standard Model in which the Higgs emerges as a pseudo-Goldstone boson from SU(5)/SO(5). Collective symmetry breaking, realized through a two-site gauge structure, cancels one-loop quadratic divergences with a small set of TeV-scale new states, yielding a naturally light Higgs and a cutoff around 10 TeV. Radiative corrections control electroweak symmetry breaking via a negative top-loop contribution, while a TeV-scale scalar triplet and heavy vectors ensure the Higgs quartic arises without a mass term. Precision electroweak constraints are discussed, with UV completions ranging from strong dynamics to supersymmetric UV completions, and distinctive LHC signatures identified, including the φ triplet, W′/B′ bosons, and a vector-like T′ quark. The model aims to provide an economical, testable alternative to supersymmetry or technicolor for natural EWSB.
Abstract
We present an economical theory of natural electroweak symmetry breaking, generalizing an approach based on deconstruction. This theory is the smallest extension of the Standard Model to date that stabilizes the electroweak scale with a naturally light Higgs and weakly coupled new physics at TeV energies. The Higgs is one of a set of pseudo Goldstone bosons in an $SU(5)/SO(5)$ nonlinear sigma model. The symmetry breaking scale $f$ is around a TeV, with the cutoff $Λ\lsim 4πf \sim $ 10 TeV. A single electroweak doublet, the ``little Higgs'', is automatically much lighter than the other pseudo Goldstone bosons. The quartic self-coupling for the little Higgs is generated by the gauge and Yukawa interactions with a natural size $O(g^2,λ_t^2)$, while the top Yukawa coupling generates a negative mass squared triggering electroweak symmetry breaking. Beneath the TeV scale the effective theory is simply the minimal Standard Model. The new particle content at TeV energies consists of one set of spin one bosons with the same quantum numbers as the electroweak gauge bosons, an electroweak singlet quark with charge 2/3, and an electroweak triplet scalar. One loop quadratically divergent corrections to the Higgs mass are cancelled by interactions with these additional particles.