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$varphi^4$ via renormalization group with hard cut-off

Physics Asked by Slz2718 on April 14, 2021

I am studying the application of the renormalization group to the $varphi^4$ theory:

$$mathcal{L} = -frac{1}{2} varphi (Box + m^2)varphi -frac{lambda}{4!}varphi^4.$$

In particular I wanted to follow two different regularization methods, and verify that the resulting critical exponents $nu$ did not differ, as I expect.

If you want to calculate the contributions up to one loop, you encounter two diagrams which diverge: the tadpole for the 2-points correlation function, and the loop for the 4-points correlation function. The first goes like $int frac{d^4k}{k^2-m^2}$, the second like $int frac{d^4k}{(k^2-m^2)^2}$.

In every textbook I know (e.g. Schwartz, Quantum Field Theory and the Standard Model, 23.5.1), you see the treamtment of the $varphi^4$ and of the renormalization group with the dimensional regularization, which leads to the following equations:

$$beta = -varepsilonlambda + frac{3lambda^2}{16pi^2}$$

$$mufrac{d}{dmu}m^2 = frac{lambda}{16pi^2}m^2$$

where $mu$ is the renormalization scale, $lambda$ and $m^2$ are the renormalized constant of interaction and mass, and $d=4-varepsilon$ is the dimension.

Solving for the Wilson-Fisher fixed point, we find $lambda^* = frac{16pi^2varepsilon}{3}$ and $m^{2*} =0$.

At the fixed point, the anomalous dimension of the mass is then $gamma_m=frac{lambda^{*}}{16pi^2}=frac{varepsilon}{3}$ giving $nu=frac{1}{2-gamma_m}=frac{3}{6-varepsilon}$.

Fine. Now, if I try to introduce an explicit cut-off $Lambda$ in the divergent integrals, in four dimensions I get something like:
$int frac{d^4k}{k^2-m^2} propto frac{Lambda^2}{m^2} + log(1+frac{Lambda^2}{m^2})$

$$int frac{d^4k}{(k^2-m^2)^2} propto log(1+frac{Lambda^2}{m^2}).$$

This is something worrisome, since we have two very different behaviours between the two integrals, and in the critical exponent they enter as a ratio.

But let’s get into the calculation. I switch to the Euclidean integrals, and I define:

$f(Lambda,m^2,d) = int_Lambda frac{d^dk}{k^2+m^2}$

and

$g(Lambda,m^2,d) = int_Lambda frac{d^dk}{(k^2+m^2)^2}$

Taking into account that

$Lambda frac{d}{dLambda} f(Lambda,m^2,d) = S_d frac{Lambda^d}{Lambda^2+m^2} + frac{partial f(Lambda,m^2,d)}{partial m^2}Lambda frac{d}{dLambda}m^2$

and

$Lambda frac{d}{dLambda} g(Lambda,m^2,d) = S_d frac{Lambda^d}{(Lambda^2+m^2)^2} + frac{partial g(Lambda,m^2,d)}{partial m^2}Lambda frac{d}{dLambda}m^2$,

where $S_d$ is the area of the d-dimensional unit sphere,

I obtain the following equations for d-dimensions:

$beta = -(4-d)lambda – frac{3}{2^{d+1}pi^d}lambda^2(-(4-d)f + frac{Lambda^d}{Lambda^2+m^2}S_d)$

$Lambdafrac{d}{dLambda}m^2 = -frac{lambda}{2^{d+1}pi^d}m^2 (-(4-d)g + frac{Lambda^d}{(Lambda^2+m^2)^2}S_d)$

If I now try to evaluate $gamma_m$ at the fixed point, I find something like:

$gamma_m = frac{4-d}{3}frac{-(4-d)g + frac{Lambda^d}{(Lambda^2+m^2)^2}S_d}{-(4-d)f + frac{Lambda^d}{Lambda^2+m^2}S_d}$

If I did not make any error, which is an assumption, that $gamma_m$ is not equivalent to the one obtained via dimensional regularization. I am probably missing something.

Any suggestions?

EDIT: I realized (thanks @TehMeh) that I defined the functions $f$ and $g$ differently from my pen and paper calculation, and came up with a mixed notation and a lot of mess, which ended up in a lot of errors. Sorry to everyone. Let me now correct.

$f(Lambda,m^2,d) = int_Lambda frac{d^dk}{(k^2+m^2)^2}$

and

$g(Lambda,m^2,d) = frac{1}{m^2}int_Lambda frac{d^dk}{k^2+m^2}$

Taking into account that

$Lambda frac{d}{dLambda} f(Lambda,m^2,d) = S_d frac{Lambda^d}{(Lambda^2+m^2)^2} + frac{partial f(Lambda,m^2,d)}{partial m^2}Lambda frac{d}{dLambda}m^2$,

and

$Lambda frac{d}{dLambda} g(Lambda,m^2,d) = frac{S_d}{m^2} frac{Lambda^d}{Lambda^2+m^2} + frac{partial g(Lambda,m^2,d)}{partial m^2}Lambda frac{d}{dLambda}m^2$

where $S_d$ is the area of the d-dimensional unit sphere,

I obtain the following equations for d-dimensions:

$beta = -(4-d)lambda – frac{3}{2^{d+1}pi^d}lambda^2(-(4-d)f + frac{Lambda^d}{(Lambda^2+m^2)^2}S_d)$

$Lambdafrac{d}{dLambda}m^2 = -frac{lambda}{2^{d+1}pi^d}m^2 (-(4-d)g + frac{Lambda^d}{Lambda^2+m^2}frac{S_d}{m^2})$

If I now try to evaluate $gamma_m$ at the fixed point, I find something like:

$gamma_m = frac{4-d}{3}frac{-(4-d)g + frac{Lambda^d}{Lambda^2+m^2}frac{S_d}{m^2}}{-(4-d)f + frac{Lambda^d}{(Lambda^2+m^2)^2}S_d}$

$f$ and $g$ are representable with the hypergeometric function, but if we take the limit for small $4-d$ it should not anyway matter their expression.

2 Answers

Taking a derivative (with regard to a parameter) to a quadratically divergent integral $$ Lambda frac{d}{dLambda} f(Lambda,m^2,d) = S_d frac{Lambda^d}{Lambda^2+m^2} + frac{partial f(Lambda,m^2,d)}{partial m^2}Lambda frac{d}{dLambda}m^2 $$ will open a can of worms (e.g. the order of $partial m^2$ and $d^4k$ is not interchangeable) when hard cutoff is involved, albeit $$ Lambda frac{d}{dLambda} g(Lambda,m^2,d) $$ is OK, since $g(Lambda,m^2,d)$ is only logarithmically divergent.

The cutoff and boundary conditions are very tricky for divergent Feynman integrals beyond logarithmic divergence. A typical example is the triangular diagram (linearly divergent) of the ABJ anomaly, where seemingly innocuous shift of integrals are prohibited.

Answered by MadMax on April 14, 2021

I've been doing the very same problem (23.6 - right?), hopefully, this is still helpful.

First of all, I notice that your $beta$ function is probably incorrect. In $d=4$ it should be dimensionless and yet one of the terms is of mass dimension 2, which is also different from the two other terms. If you fix that, maybe in $d=4-epsilon$ dimensions your $gamma_m$ is correct once expanded in $epsilon$ as you have the seemingly correct prefactor of $frac{4-d}{3}$, which would immediately give the correct answer.

I myself did the problem in a slightly different manner. We have to work in $d=4-epsilon$ dimensions and I introduced the usual subtraction point $mu$, also there were no $epsilon$ poles as these are regulated by the cut off $Lambda$.
To get $beta$ functions I differentiated bare parameters with respect to $mu$, for example,

$mu frac{dlambda_0}{dmu}=mu frac{d(lambda_R mu^{epsilon}(mu)Z_lambda)}{dmu}$

Where $lambda_R(mu)$ is the renormalized coupling and $Z_lambda=1+delta_lambda$ is the renormalization constant - all as in Schwartz. I got the counterterms by expanding the integrands in powers of $m^2$ and keeping only divergent (after integration) terms. The counterterms are quite nasty due to $epsilon$ and the regulator $lambda$, hence, I used Mathematica to do expansions and solve for $beta$ functions. In the end, the result matches the one from dimensional regularization.

Answered by TehMeh on April 14, 2021

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