Flavor physics and fine-tuning in theory space

Abstract

Recently a new class of composite Higgs models have been developed which give rise to naturally light Higgs bosons without supersymmetry. Based on the chiral symmetries of "theory space," involving replicated gauge groups and appropriate gauge symmetry breaking patterns, these models allow the scale of the underlying strong dynamics giving rise to the composite particles to be as large as of order 10 TeV, without any fine tuning to prevent large corrections to Higgs boson mass(es) of order 100 GeV. In this note we show that the size of flavor violating interactions arising generically from underlying flavor dynamics constrain the scale of the Higgs boson compositeness to be greater than of order 75 TeV, implying that significant fine-tuning is required. Without fine-tuning, the low-energy structure of the composite Higgs model alone is not sufficient to eliminate potential problems with flavor-changing neutral currents or excessive CP violation; solving those problems requires additional information or assumptions about the symmetries of the underlying flavor or strong dynamics. We also consider the weaker, but more model-independent, bounds which arise from limits on weak isospin violation.

Big corrections from a little Higgs

“…These corrections in the SM can only come from custodial SU(2) violating interactions like hypercharge and Yukawa couplings. The importance of custodial SU(2) was noted in the early literature on composite Higgs models [8], and recently emphasized again in [6]. However, in the littlest Higgs model the heavy gauge bosons break custodial symmetry since the embeddings of SU(2) 1 and SU(2) 2 into SU(5) as 5 → (2, 1) + (1, 2) + (1, 1) break all non-Abelian global symmetries acting on the Higgs.…”

“…However in the little Higgs models the corrections are more dangerous since a "custodial" SU(2) symmetry is not automatically enforced as it is in the SM or in other words weak isospin is violated. (The importance of custodial SU (2) was recently emphasized in [6].) These weak isospin violating effects come both from heavy gauge boson exchange and to a lesser extent from the presence of a triplet Higgs VEV.…”

“…Recently there has been excitement generated by the revival [1,2,3,4,5,6] of the idea that the Higgs is a pseudo-Goldstone boson [7,8,9]. New models have been generated that provide plausible realizations of this scenario with the feature that they cancel all quadratically divergent contributions to the Higgs mass at one-loop.…”

IMPLICATIONS OF $D^0 - \overline D ^0 $ MIXING FOR NEW PHYSICS

“…We thus refer to Section VIII C for the details of the analysis for these contributions. The resulting constraints on scalar leptoquark are governed by the translation The key idea in the composite Higgs picture is that the flavor physics responsible for generation of the Yukawa couplings can induce flavor-changing neutral currents [80]. Applied to charm physics they generically lead to the following effective hamiltonian, (16),…”

Dynamical Electroweak Superconductivity From a Composite Little Higgs

“…In some little Higgs theories, corrections to precision electroweak observables are comparable in size to one-loop standard model effects over much of the natural parameter space [14,[16][17][18], providing important constraints. It is, however, straightforward to find natural, simple, and experimentally viable little Higgs theories where the corrections are much smaller [10,19].…”