The Littlest Higgs

We present an economical theory of natural electroweak symmetry breaking,generalizing an approach based on deconstruction. This theory is the smallestextension of the Standard Model to date that stabilizes the electroweak scalewith 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, withthe cutoff $\Lambda \lsim 4\pi f \sim $ 10 TeV. A single electroweak doublet,the ``little Higgs'', is automatically much lighter than the other pseudoGoldstone bosons. The quartic self-coupling for the little Higgs is generatedby the gauge and Yukawa interactions with a natural size $O(g^2,\lambda_t^2)$,while the top Yukawa coupling generates a negative mass squared triggeringelectroweak symmetry breaking. Beneath the TeV scale the effective theory issimply the minimal Standard Model. The new particle content at TeV energiesconsists of one set of spin one bosons with the same quantum numbers as theelectroweak gauge bosons, an electroweak singlet quark with charge 2/3, and anelectroweak triplet scalar. One loop quadratically divergent corrections to theHiggs mass are cancelled by interactions with these additional particles.Comment: 15 pages. References added. Corrected typos in the discussion of the top Yukawa couplin