Is Renormalisation Unique for Physically Measurable Quantities?

[Sunset]

Hi!

you seemed to have simply used H^2 (so you are doing it as if a_1 was one!).[\QUOTE]

Yes I made a mistake here:

$$-ia_{1} H - ia_{2} H^2 + iC a_{1}^2 H^2 [point0] + O(H^3)$$
$$=-iH -5H -i a_{2} H^2 + iC H^2 [point0] + 5iC H^2 [point0]$$

I didn't use a_1=1 but I forgot the "²" : right side is -i(1-5i)H - ia_2H² + iC(1-5i)²H²[point0]
=> a_2=-C(1-5i)²[point0]

all your coefficients a_i will contain expansions in poers of 1/H[\QUOTE]
So that is a contradiction to our assumption that you can expand g in powers of H when the coefficients depend on H!

Best regards

 

[Sunset]

If you haven't said the coefficients can depend on H, I would say the assumption g=f_1G + f_2G² + ... makes the choice M_{0}=-iG unique, because only with this definition you can make M' independant of Lambda.

But you see that this type of definition is a pain because all your coefficients a_i will contain expansions in poers of 1/H and that makes it a pain to work with. This is why defining the amplitude to simply be your coupling constant (as opposed to teh coupling constant plus some number) makes things *much* easier!

After what you say, I have to assume there exist other possible definitions for H than M_{0}=-iH . Here's the interesting point: does a different definition change my predictions?
Remember what I have in mind with "predictions":

By experiment, you can measure the probability for (2Meson/2Meson-scattering to happen for
certain scattering-angles) for different s',t' and u' . You can plot M' in dependence of s (or t or u) and the prediction of our renormalised theory (i.e. there might be seen something else in experiment!) is: there exists a constant G, for which you can fit your curve described by formula (*) to your datapoints, whereas the value of G depends on s0,t0,u0 ! This is not trivial, you may find in experiment that there is no such constant.

I would say NO, because although you have a different expression for M', you will get the same curve. Let's assume - as you said - you can cancel out Lambda when you're defining M_{0}:=-iH-5H . M' is a expression different to (*) depending on H. This time your prediction is, that there exists a constant H for which you can fit your datapoints. This is exactly the same KIND of information, the same essence as before.
The two expressions describe the same curve, only fitting-process might be in some cases more comfortable.

Is it the same principle here? I mean, using a different renormalisation scheme doesn't change the KIND of information, although the way how it is packaged looks a bit different...?

Otoh the choice of renormalization scheme is more a technical issue in the sense that while it does change the results, it just means one of them is approaching some attractor point better than the other as was pointed out earlier. Sometimes you can actually improve the results to even obtain some nonperturbative sectors of the theory.

This would be very satisfying to me.


Although it would be interesting, how g=f_1G + f_2G² + ... can be motivated further. I mean, we're dealing with infinities, so it is not trivial that there exists a power-expansion, or?
We haven't used a certain Renormalisation scheme in that case, right? Or does it correspond to the assumption g=f_1G + f_2G² + ... ?

 

[Sunset]

using a different renormalisation scheme doesn't change the KIND of information, although the way how it is packaged looks a bit different...?

I came to the opinion that this should be true:

To be more precise, let's take the MS scheme applied to Phi^4 theory (I read it Ryder):

He uses dimensional regularisation (it's clear to me why regularisation doesn't change the predictions, I'm ok with that). He regularises and renormalises only the 2-point-vertex-function and the 4-pt-vertex-function. I guess because every possible M is constructable form these two things. Renormalisation of 4-pt-function is in principle the same as we discussed so far (renormalisation of coupling constant).

The regularised self energy is
$$\Sigma= - (g m^2)/(16 \pi^2 \epsilon) - (g m^2)/(32 \pi^2) (1- 0.577 + ln((4 \pi \mu^2)/m^2) + O(\epsilon) + O(g^2)$$
$$\Gamma^{(2)}=p^2 - m^2 -\Sigma$$
He drops all the finite terms (MS-scheme) so he receives
$$\Gamma^{(2)}=p^2 -m^2[1- g/(16 \pi^2 \epsilon)$$

He defines the renormalised mass M as
$$\Gamma^{(2)}=p^2 - M^2$$

So he gets
$$m^2=M^2[1+ g/(16 \pi^2 \epsilon)]$$

Dropping the O(\epsilon) terms is ok, because you let epsilon go to zero. µ is arbitrary, because in the regularised Lagrangian it appears as $$\mu^\epsilon$$ so it can be choosen as m^2/4pi so the only finite term (remember: at order g) which contributes is
$$(g m^2)/(32 \pi^2) [1- 0.577]$$
So if you compute M for a certain process you end up with a different expression (as in the case discused before). So If you fit your curve for measured values for M and measured mass M (this is NOT a parameter of your theory, it is measured with well-known apparatures, which have never heard of Quantum-Field-theory - you don't have to fit it like g!) you receive again different values for g but it's exactly the same kind of prediction as discussed above.

Best regards Martin