Renormalisation of the Fermionic triangle loop

In summary, the conversation discusses the renormalization of a loop diagram in the Standard Model using the Feynman rules. The amplitude is written in terms of external Higgs and loop momenta, and the fermion mass. After evaluating the loop integral functions using dimensional regularization, the expression is found to be divergent. The conversation then mentions the need for counter-terms to make the amplitude finite, and references Srednicki's book for further information.
  • #1
Ramtin123
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I am trying to renormalise the following loop diagram in the Standard Model:
Fermionic_triangle.png

Using the Feynman rules, we can write the amplitude as follows:
$$ \Gamma_f \sim - tr \int \frac{i}{\displaystyle{\not}\ell -m_f}
\frac{i^2}{(\displaystyle{\not}\ell+ \displaystyle{\not}k -m_f)^2}
\frac{d^4 \ell}{(2 \pi)^4} $$
where ##k## and ##\ell## are external Higgs and loop momenta, and ##m_f## being the fermion mass. I have ignored the coefficients coming from vertex factors.
After evaluating the loop integral functions using dimensional regularisation, I ended up with the following expression:
$$ \Gamma_f \sim \frac{m_f}{4 \pi^2}
\left( 3\Delta -6\ln \frac{m_f}{\mu} +4
-6 \sqrt{1-4x} \ \ln \frac{1 +\sqrt{1-4x}}{2\sqrt{x}} \right)$$
The loop integral is clearly divergent due to ##\Delta \equiv \ln \pi - \gamma_E + 2/\epsilon## term, when ##\epsilon = 4 -d \to 0##.
Here, ##x \equiv (m_f/k)^2##, ##d## is dimension, ##\mu## regularisation scale, and ##\gamma_E## being Euler-Mascheroni constant.
How should I introduce the counter-terms to make the expression for the amplitude finite?
 
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  • #2
As always in the books.
Check Srednicki.
 
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