# What is the physical meaning of this simplification to calculate the effective coupling constants for a Gaussian model with quartic interactions?

To calculate the effective coupling constants $u'_2(q)$ and $u'_4(q)$ of the effective Hamiltinian eq (4.9) of this paper

$$H' = -\frac{1}{2}\int\limits_q u'_2(q)\sigma'_q\sigma'_{-q} - \int\limits_{q_1}\int\limits_{q_2}\int\limits_{q_3}u'_4(q_1,q_2,q_3,-q_1-q_2-q_3) \sigma'_{q_1}\sigma'_{q_2}\sigma'_{q_3}\sigma'_{q_-q_1-q_2-q_3}$$

The following simplifications are introduced into eq (4.20) and (4.21) to calculate $u'_2(q)$ and $u'_4(q)$ respectively

1. $u'_2(q)$ is only evaluated to order u, which means only tree level diagrams are considered

2. Higher order than quartic interactions are neglected

3. $\int\limits_{\frac{1}{2} < ¦p¦ < 1} \frac{1}{p^2+r} \rightarrow \frac{1}{1+r}\int\limits_{\frac{1}{2} < ¦p¦ < 1} 1$

4. $\int\limits_{\frac{1}{2} < ¦p¦ < 1} \frac{1}{p^2+r}\frac{1}{[(\frac{1}{2}q_1+\frac{1}{2}q_2-p)^2 +r]} \rightarrow \frac{1}{(1+r)^2}\int\limits_{\frac{1}{2} < ¦p¦ < 1} 1$

What is the physical meaning of 3. and 4. ? Is there an "intuitive explanation for the physical meaning of these two simplifications?

PS: here is an alternative link to the paper that maybe works better.

• As they are written down, these equations are nonsensical, as they don't even have the right units. $r$ cannot have dimension mass^2 and 0 at the same time. – Vibert Feb 10 '13 at 13:59

1. $\int\limits_{\frac{1}{2} < ¦p¦ < 1} \frac{1}{p^2+r} \rightarrow \frac{1}{1+r}\int\limits_{\frac{1}{2} < ¦p¦ < 1} 1$
2. $\int\limits_{\frac{1}{2} < ¦p¦ < 1} \frac{1}{p^2+r}\frac{1}{[(\frac{1}{2}q_1+\frac{1}{2}q_2-p)^2 +r]} \rightarrow \frac{1}{(1+r)^2}\int\limits_{\frac{1}{2} < ¦p¦ < 1} 1$
Since the referenced document is behind a paywall, it is a little difficult to know for sure of the context, but when I see these two equations, it would appear they are making a simplification about the amplitudes associated with propagators (internal lines of Feynman diagrams). It appears to be saying the upper half of the range of values associate with momentum follow the rules of geometric progression or more appropriately a geometric series and the region of integration has a constant slope (e.g. there is a uniform accumulation in the identified region of integration) Represented as. $$\int\limits_{\frac{1}{2} < ¦p¦ < 1} 1$$