# Beta function QED

The beta function for QED is given by:

$$\beta=\frac{e^3}{16 \pi^2}*\frac{4}{3}*(Q_i)^2$$

where $$(Q_i)^2$$ represents the sum of the squares of the charges of all Dirac fields.

For one generation, for the charge squared you have (2/3)^2 for the up quark, (-1/3)^2 for the down quark, but this is all multiplied by 3 for the 3 colors of quarks, and then you have (-1)^2 for the electron and (0)^2 for its neutrino.

So all in all, 3[(2/3)^2+(-1/3)^2]+(-1)^2=8/3

However this gives a beta function that is not equal to the book value of:

$$\beta=\frac{e^3}{16 \pi^2}*\frac{20}{9}$$

So is the book wrong?

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Avodyne
20/9 is the coefficient for the contribution of one quark-lepton generation to the beta function for hypercharge, not QED.

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Ah that's it. But now I have to re-calculate the beta function for the hypercharge. Darn it.

Do you remember Srednicki ever providing justification that the coupling constants of SU(3), SU(2), and U(1) are unified to the coupling constant of SU(5) (well within a factor of 3/5 for the U(1) coupling constant) at the mass of the X-particle? I mean you can show that the 3 couplings of the product group meet at a point, but after that point what's the justification for using SU(5) instead of just continuing to follow the three beta functions past that point, i.e., continue with SU(3)xSU(2)xU(1)? I don't recall ever seeing Srednicki talk about the phase transition.

Avodyne