HEP In the literature: open

Is the Higgs boson elementary or a composite bound state?

In plain words

The Higgs could be a point-like particle or a bound state of new strongly interacting constituents, much as a pion is made of quarks. A composite Higgs would show small deviations in how it couples to other particles.

Precise statement

In composite pseudo-Goldstone Higgs models, Higgs couplings to W, Z and fermions deviate from SM values by $\mid \kappa - 1 \mid \sim \xi = v^{2}/f^{2}$ up to $O(1)$ factors, with f the compositeness scale. Determine whether $\xi > 0$ by measuring $\kappa_V$ and $\kappa_f$ to 0.1 to 1 percent, or find the associated resonances of mass $m_\rho \sim g_\rho f$ with $1 < g_\rho < 4\pi$.

What would settle it

Higgs coupling measurements at a Higgs factory (FCC-ee, CEPC or a linear collider) at the per-mille level for $\kappa_Z$, combined with high-energy searches for resonances.

Status in the literature

Unverified note

LHC coupling fits (2024 to 2025) agree with the SM at about the 5 to 10 percent level, implying $\xi < \sim 0.1$ and $f > \sim 0.8\,\mathrm{TeV}$.