Abstract
A holographic model of chiral symmetry breaking is used to study the dynamics plus the meson and baryon spectrum of the underlying strong dynamics in composite Higgs models. The model is inspired by top-down D-brane constructions. We introduce this model by applying it to Nf = 2 QCD. We compute meson masses, decay constants and the nucleon mass. The spectrum is improved by including higher dimensional operators to reflect the UV physics of QCD. Moving to composite Higgs models, we impose perturbative running for the anomalous dimension of the quark condensate in a variety of theories with varying number of colors and flavours. We compare our results in detail to lattice simulations for the following theories: SU(2) gauge theory with two Dirac fundamentals; Sp(4) gauge theory with fundamental and sextet matter; and SU(4) gauge theory with fundamental and sextet quarks. In each case, the holographic results are encouraging since they are close to lattice results for masses and decay constants. Moreover, our models allow us to compute additional observables not yet computed on the lattice, to relax the quenched approximation and move to the precise fermion content of more realistic composite Higgs models not possible on the lattice. We also provide a new holographic description of the top partners including their masses and structure functions. With the addition of higher dimension operators, we show the top Yukawa coupling can be made of order one, to generate the observed top mass. Finally, we predict the spectrum for the full set of models with top partners proposed by Ferretti and Karateev.
Highlights
The AdS/CFT correspondence was first proposed for conformal field theories such as the N = 4 Super Yang Mills (SYM) theory [1, 2]
Holographic work on technicolour includes [15,16,17,18,19,20,21,22]. Another class of Beyond the Standard Model (BSM) models that have generated considerable study are composite Higgs models [23, 24] (the idea that the Standard Model (SM) fields might be composite has a long history — see for example [25]) — in these models we study the Higgs emerges as a bound state of a strongly coupled gauge theory at the 1–5 TeV scale
The ability to see these effects is straightforward holographically but on the lattice needs both unquenched simulations and a wide separation of scales. This is a long paper, it is designed so that the reader can drop in to the self-contained sections of interest to them: Those interested in the specfics of the holographic model should read section 2 — we review the model, including determining the vacuum, the meson spectrum computation and the baryon spectrum computation and describe the addition of higher dimensional operators (HDOs)
Summary
The key component of composite Higgs models is that a strongly coupled gauge theory causes chiral symmetry breaking in the quark sector, generating four or more Nambu-Goldstone. We refer to this model as Dynamic AdS/YM (Anti-de Sitter/YangMills) to emphasise that it can be used to holographically describe the chiral symmetry breaking dynamics of any gauge theory (not just QCD), including with quarks in several, potentially inequivalent, representations. We refer to the corresponding configuration that describes the vacuum (for a given Nc, Nf , and quark mass) as L0(ρ) with IR value LIR (this is effectively the constituent quark mass) at the IR cut off ρIR Note at this point we observe a crucial difference between our approach and previous papers on holgraphic composite Higgs models [31, 32], which use the boundary conditions to impose chiral symmetry breaking. We will discuss such cases and their subtleties in more specific models below
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