Modified Higgs Physics from Composite Light Flavors

TL;DR

This work analyzes how a composite Higgs that is a pseudo-Nambu–Goldstone boson can exhibit sizable modifications to Higgs production and decay when first- and second-generation quarks are partially composite. Using an EFT framework below resonances and a two-site toy model, it shows that top-partner contributions to radiative Higgs couplings can cancel in pNGB scenarios, Leaving radiative couplings controlled by light-flavor compositeness. In the concrete MCHM setup, Higgs production via gluon fusion and decays to $\gamma\gamma$ and $WW^*/ZZ^*$ depend on the degree of light-quark compositeness ($N_i$, $\theta_i$, $x_i$, $r_i$) and the Higgs nonlinearity parameter $\xi=v^2/f^2$, enabling potential order-one shifts that could be probed by flavor-blind Higgs measurements at the LHC. The paper also outlines flavor-structure classifications (Anarchy, MFV, Exhilaration) and their implications for collider phenomenology, suggesting that Higgs data can indirectly illuminate the underlying composite flavor dynamics.

Abstract

We point out that Higgs rates into gauge bosons can be significantly modified in composite pseudo Nambu--Goldstone boson (pNGB) Higgs models if quarks belonging to the first two generation are relatively composite objects as well. Although the lightness of the latter a priori screen them from the electroweak symmetry breaking sector, we show, in an effective two-site description, that their partners can lead to order one shifts in radiative Higgs couplings to gluons and photons. Moreover, due to the pseudo-Goldstone nature of the Higgs boson, the size of these corrections is completely controlled by the degree of compositeness of the individual light quarks. The current measurements of flavor-blind Higgs decay rates at the LHC thus provide an indirect probe of the flavor structure of the framework of pNGB Higgs compositeness.

Figures (9)

• Figure 1: Generic one-loop diagrams contributing to the gluon fusion Higgs production NP amplitude in the presence of composite fermionic resonances. Mass eigenstates are understood in the loops. Diagram (a) is the SM quark contribution where the black filled circle denotes the modified Yukawa coupling, whose deviation from the SM value is caused both by mixing with the composite states and possibly by Higgs non-linearities. Diagram (b) is the contribution from heavy resonances.
• Figure 2: Two-site model: the elementary quarks, $q_L$ and $u_R$, mix with vector-like massive quarks, $Q$ and $U$, that belong to the composite sector and have Yukawa interaction with the Higgs field.
• Figure 3: Leading contributions to the generic diagrams of Fig. \ref{['diag_hgg']}, in an expansion in small elementary/composite mixings, $\lambda_x$ with $x=q^u,q^d,u,d$. We use in the text expressions valid to all orders. Non-linear Higgs interactions arising in pNGB models are not represented. Mass eigenstates are understood on the left-hand side of the equalities, whereas, on the right-hand side, single and double lines stand for elementary quarks and composite resonances, respectively, and the crossed-circle denotes $\lambda_x$ insertions. Black squares are effective Yukawa interactions for the elementary fields generated through mixings (see Fig. \ref{['diag_tree']}). Dots denote diagrams of higher order in $\lambda_x$. Diagram (a1) is the SM contribution, while diagrams (a2) and (a3) are corrections due to mixing with the composite resonances. Diagrams (b1) and (b2) are contributions from the composite resonances. For the top sector, diagrams (a2) and (a3) cancels, to leading order in $m_h^2/(4m_t^2)$, against diagrams (b1) and (b2), respectively, provided the determinant of the associated mass matrix can be factorized as in Eq. \ref{['detMfacto']}. For light quark generations, diagram (a) is $4m_q^2/m_h^2$ suppressed and the composite partners yield large contributions through diagrams (b1) and (b2). In pNGB Higgs model, diagrams (a2) and (b1) vanish individually for all flavors due to the global symmetry of the Goldstone bosons.
• Figure 4: Tree-level diagrams generating the effective vertices used in Fig. \ref{['diagrams']}. Single and double lines stand for elementary quarks and composite resonances, respectively, and the crossed-circle denotes elementary/composite mixing insertion. Non-linear Higgs interactions arising in pNGB models are not represented.
• Figure 5: $X_{gg}$ ratio of gluon fusion Higgs production cross-section in MCHM relative to SM as a function of $\xi$ (setting $N_ix_i \sin^2\theta_i=0$) [left] and $N_ix_i \sin^2\theta_i$ (setting $\xi=0$) [right], for $i=u$ or $d$. We defined $\xi=v^2/f^2$, $x_i=(Y_iv/M_i)^2$ and $r_i=M/(Y_if)$.