Is there a theoretical upper limit for the Yukawa-like coupling constant in a theory with a cut-off scale?

Question

In the fermion mass terms in the Standard Model,

$y_x\bar{L}H d_x$ or $y_x\bar{L}\tilde{H} u_x$ where $y_x$ are the Yukawa couplings,

we have $y_x<<1$ except for the case of the top quark. For the top quark, $y_t \sim 1$. As I understand, this is around the maximum value the coupling constant can have because of the conditions imposed by perturbativity and unitarity. On the other hand, if I write a non-renormalizable mass term,

$y'_x\frac{\Phi}{\Lambda}\bar{L}H d_x$ where $y'_x$ is the Yukawa-like coupling, $\Phi$ is a hypothetical gauge singlet scalar field, $\Lambda$ is the cut-off scale (for example the GUT scale),

does such a maximum limit exist for $y'_x$? Here, the Standard Model Yukawa coupling is obtained as $y_x = y'_x \frac{\langle\Phi\rangle}{\Lambda}$ where $\langle\Phi\rangle$ is the Vacuum expectation value of $\Phi$.

Answer 1

The Yukawa couplings are not constrained theoretically.

That said, from 'tHooft's technical naturalness point of view, all the dimensionless Yukawa couplings $$ y_x $$ should be of order 1.

Hence, only top quark's $y_t$ is natural, while the rest $y_x$ are not natural. There should be an insofar unknown symmetry (and spontaneous symmetry breaking thereof) protecting the smallness of non-top $y_x$.

So if your gauge singlet scalar $\Phi$ is endowed with some global symmetry, then non-top $y_x$ are more exciting, since you hit the jackpot of explaining the Fermion mass hierarchies.

Mind you that the Higgs mass naturalness/hierarchy is a different beast.