In dimensional regularization an arbitrary mass parameter μ must be introduced in going to 4−ϵ dimensions. I am trying to understand to what extent this parameter can be eliminated from physical observables.
Since μ is arbitrary, physical quantities such as pole masses and scattering amplitudes must be independent of it. Nevertheless at any fixed order in perturbation theory these quantities contain residual μ-dependence. One expects this dependence to decrease at higher orders in perturbation theory.
For concreteness, consider dimensional regularization with minimal subtraction of ϕ4 theory, which has bare Lagrangian
LB=12(∂ϕB)2−12m2Bϕ2B−λ4!ϕ4B
Here are the 1-loop expressions for the physical mass mP and 4-point coupling λP≡(√Z)4Γ(4) after minimal subtraction of poles and taking ϵ→0:
m2P=m2R{1+λR2(4π)2[log(m2R4πμ2)]+γ−1}
λP=λR+3λ2R2(4π)2[log(m2R4πμ2)+γ−2+13A(m2RsE,m2RtE,m2RuE)]
where A(m2RsE,m2RtE,m2RuE)=∑zE=sE,tE,uEA(m2RzE) and A(x)≡√1+4xlog(√1+4x+1√1+4x−1).
Both of these quantities (mP and λP) are physically observable.
Suppose we conduct an experiment at a reference momentum pE0≡(sE0,tE0,uE0) and make measurements of the pole mass and 4-point coupling with the result λP0,mP0. We now have a system of two equations in the three unknowns (λR,mR,μ). This means that in principle I can solve for λR=λR(μ) and mR=mR(μ).
I would now like to make a prediction for the 4-point amplitude at a different momentum p′E≠pE0. Since I have two equations in three unknowns I need to guess a suitable value for μ (say μ′=√s′E) which allows me to fix λR and mR and then calculate λP(p′E). This procedure seems quite ad hoc to me because a different (arbitrary) choice of μ (e.g. μ′/2 or 2μ′) will lead a different physical answer (albeit only logarithmically different).
From what I can gather from the literature, the problem of determining the renormalization scale I described above is a genuine problem in actual calculations of QCD (e.g. http://arxiv.org/abs/1302.0599) which leads theorists to introduce so-called "systematic uncertainties".
What concerns me is that I haven't been able to find any mention of this problem in any textbook on quantum field theory that deals with QED or QCD (anyone know of a reference?). Since this problem appears already in arguably the simplest QFT of λϕ4, I would expect it also to occur in e.g. 1-loop calculations of Bhabha scattering but I haven't been able to find any mention of it in this context.
Does anyone know how this problem is dealt with in real loop calculations (e.g. at LEP or LHC?)
Also, I would be interested to know if there is any analogue of this problem in condensed matter theory.
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