Basic question on the pertubative Standard Model

[Breo]

the couplings in this case can play some role (eg. know that the higgs coupling to fermions depends on their masses),because you are taking their squares

Why it depends on their masses? and why the square of the couplings makes them to play some role?

Note: When I face for the first time with a new theory I like to hit myself with mistakes to learn from them and ask a lot of things even very basic ones to finally build the correct idea. Sorry if I am a bit of tiresome.

 

[mfb]

Oh, gluons. Sorry, I thought of photons in my previous posts.

But again don't think of this so lightly...for example, even though the Higgs couples to heavy fermions, there will be searches for the Higgs channel of H \rightarrow \tau \tau in the next ATLAS run

The decay has been found:
Evidence for the Higgs-boson Yukawa coupling to tau leptons with the ATLAS detector: "observed (expected) significance of 4.5 (3.4) standard deviations"
Evidence for the 125 GeV Higgs boson decaying to a pair of tau leptons: "with a local significance larger than 3 standard deviations"
Edit: observed 3.2, expected 3.7 for CMS

Why it depends on their masses?

The Higgs is a field where the mass is part of the coupling strength. Why? Well, this is an observation.

 

[ChrisVer]

The decay has been found:
Evidence for the Higgs-boson Yukawa coupling to tau leptons with the ATLAS detector: "observed (expected) significance of 4.5 (3.4) standard deviations"
Evidence for the 125 GeV Higgs boson decaying to a pair of tau leptons: "with a local significance larger than 3 standard deviations"

Yup, it's still an evidence... I guess I typed it in a wrong way. First the sigma should be better and the new study will be about Higgs properties (such as its CP that we can figure out by the taus -and possibly other particles?)...

Why it depends on their masses? and why the square of the couplings makes them to play some role?

Because when you write the Yukawa term:
y_q \bar{q} H q
you have that the higgs gets H= v +h
and then this leads to a known fermion mass term of the form : y_q v \bar{q}q \equiv m_q \bar{q} q
So you have: y_q = \frac{m_q}{v}
I have dropped the factors of \sqrt{2} but you can see from this that the yukawa coupling y_q \propto m_q...
Now if y_{q1}>y_{q2} then y_{q1}^2>y_{q2}^2 is even larger...so that's why the squares matter.

 

[mfb]

Oh well, you expect the decay, and then you find it with 4.5 and 3.2 sigma and consistent with the expected strength, that does not leave much room for doubts. It is not some random excess seen where no one would expect one. Sure, the next years will improve the precision a lot.

 

[Breo]

Thank you very much to you both for your help :)

This was very useful to me to understand and correct some basic ideas. I will continue with my study tomorrow so if I have more questions I will post them here :P
For now I have another doubt about technicolours and bound states. But I will throw it tomorrow after a bit of study.

 

[mfb]

For now I have another doubt about technicolours and bound states.

I suggest you start with easier things first. Learn to walk before you try to run.