Wednesday, March 29, 2017

Setting of renormalization scale in field theory calculations


In dimensional regularization an arbitrary mass parameter μ must be introduced in going to 4ϵ 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)212m2Bϕ2Bλ4!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 ϵ0:


m2P=m2R{1+λR2(4π)2[log(m2R4πμ2)]+γ1}


λP=λR+3λ2R2(4π)2[log(m2R4πμ2)+γ2+13A(m2RsE,m2RtE,m2RuE)]



where A(m2RsE,m2RtE,m2RuE)=zE=sE,tE,uEA(m2RzE) and A(x)1+4xlog(1+4x+11+4x1).


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 pEpE0. Since I have two equations in three unknowns I need to guess a suitable value for μ (say μ=sE) which allows me to fix λR and mR and then calculate λP(pE). 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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