Hello Gaurav,

the maximal value of Yukawa coupling is usually chosen to be consistent with the perturbation theory -- i.e. to satisfy the perturbativity limit. Since Yukawa coupling \lambda enters into higher order corrections as \lambda/(4 \pi) the very generic limit from perturbativity constraint is \lambda/(4 \pi) < 1

There could be more stringent constraints, connected to unitarity, which depend on the specific theory, so called perturbative unitarity limits, which allows

maximal fermion masses of fermions to be up to ∼ 500 − 600GeV.

See, for example these papers

A. Denner et al., Eur.Phys.J. C72, 1992 (2012), 1111.6395.

M. S. Chanowitz, M. Furman, and I. Hinchliffe, Phys.Lett. B78, 285 (1978).

M. S. Chanowitz, M. Furman, and I. Hinchliffe, Nucl.Phys. B153, 402 (1979).

Hope this helps

Regards,

Alex

Look at the literature on Higgs-Yukawa models. They've been extensively studied, e.g.

http://arxiv.org/abs/1111.4544 where they do use overlap fermions-which you'll need, since, in the Standard Model the fermions are chiral.

Heavy fermion doesn't mean by itself anything-with respect to what scale? You should be at a second order transition point and from the Higgs phase, since you do want a massive fermion.

Thank you very much for your kind response. These answers help a lot.

The top quark mass is written as,

h_t /Sqrt[2] where =256 GeV.

Therefore h_t is approximately 1

The unitarity bound is,

h^2_t / 4/pi < 1

or

h_t < Sqrt[4 pi]=3.54

Same upper bound will apply for general heavy

fermions. So, upper limit is

3.54 x 256/Sqrt[2]= 640 GeV approx.

I must add that if the heavy fermion is strongly

interacting then the pole mass is

m_pole= m ( 1 + (4/(3 pi)) alpha_s)

here, m=640 and alpha_s=0.11 (approx)

then,

m_pole= 670 GeV approx.