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I read that the Higgs field is a pair of complex numbers at each point of spacetime, and since we know that the Higgs boson has a mass, I'm imagining that these complex numbers oscillate over time at a frequency proportional to the mass, just like any quantum field -- even though the magnitude of the complex doublet is pretty much uniform (the Higgs VEV), and even though this complex doublet is a Lorentz scalar.

If I've got that right, then what role does this oscillation play in the giving of mass to fermions? A qualitative description would be great, but I'll take a technical answer if that's what it comes down to.

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The mass of the Higgs particle is conceptually independent from the mass Yukawa couplings impart to fermions. If you opened a SM QFT book or review, you'd find an illustration of spontaneous symmetry breaking summary, illustrated by the abelian version of the linear σ model of Gell-Mann and Levy (1964), whose relevant terms in the Lagrangian are $$ ...+ y\bar \psi (\sigma +i\pi \gamma_5)\psi+ \frac{\lambda}{4} (\sigma^2 + \pi^2 -v^2)^2 +... $$ The σ is the analog of the Higgs particle and the π the analog of the Nambu-Goldstone boson.

By contrivance, at the bottom of the ("sombrero") potential, it happens that $\langle \sigma \rangle =v$ and $\langle \pi\rangle=0$. Define $\sigma = h+v$, so the above chunk of the Lagrangian reads $$ y\bar \psi (v+ h+i\pi \gamma_5)\psi+ \frac{\lambda}{4} ( h^2+ 2v h \pi^2)^2\\ = yv\bar \psi \psi+ {\lambda} v^2 h^2+ ... ~, $$ where only terms bilinear in the fields are kept.

Note there is no mass term for the goldston, 𝜋, and masses for the fermion and the Higgs, $$ m_\psi= - yv, \qquad m_h^2=2\lambda v^2. $$

They have nothing to do with each other. They are related to v, the location of the bottom of the potential, but the relevant dimensionless couplings multiplying them are y and 𝜆, respectively, completely, and mysteriously, unrelated. The Higgs mass determines its fluctuations around the bottom of the potential, while the goldston is massless and "fluctuates" with enormous/infinite wavelength into and out of the vacuum.

But it's all math: you can't spin qualitative tales on math, unless you are blasé about the math underlying it...

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