In this chapter we discuss the following topics:
In the previous chapter we have seen that the construction of interacting perturbative quantum field theories is given by perturbative S-matrix schemes (def. ), equivalently by time-ordered products (def. ) or equivalently by Feynman amplitudes (prop. ). These are uniquely fixed away from coinciding interaction points (prop. ) by the given local interaction (prop. ), but involve further choices of interactions whenever interaction vertices coincide (prop. ). This choice is called the choice of ("re"-)normalization (def. ) in perturbative QFT.
In this rigorous discussion no “infinite divergent quantities” (as in the original informal discussion due to Schwinger-Tomonaga-Feynman-Dyson) that need to be “re-normalized” to finite well-defined quantities are ever considered, instead finite well-defined quantities are considered right away, and the available space of choices is determined. Therefore making such choices is rather a normalization of the time-ordered products/Feynman amplitudes (as prominently highlighted in Scharf 95, see title, introduction, and section 4.3). Actual re-normalization is the the change of such normalizations.
The construction of perturbative QFTs may be explicitly described by an inductive extension of distributions of time-ordered products/Feynman amplitudes to coinciding interaction points. This is called
This inductive construction has the advantage that it gives accurate control over the space of available choices of (“re”-)normalizations (theorem below) but it leaves the nature of the “new interactions” that are to be chosen at coinciding interaction points somwewhat implicit.
Alternatively, one may re-define the interactions explicitly (by adding “counterterms”, remark below), depending on a chosen UV cutoff-scale (def. below), and construct the limit as the “cutoff is removed” (prop. below). This is called (“re”-)normalization by
This still leaves open the question how to choose the counterterms. For that it serves to understand the relative effective action induced by the choice of UV cutoff at any given cutoff scale (def. below). This is the perspective of effective quantum field theory (remark below).
The infinitesimal change of these relative effective actions follows a universal differential equation, known as Polchinski's flow equation (prop. below). This makes the problem of (“re”-)normalization be that of solving this differential equation subject to chosen initial data. This is the perspective on (“re”-)normalization called
The main theorem of perturbative renormalization (theorem below) states that different S-matrix schemes are precisely related by vertex redefinitions. This yields the
If a sub-collection of renormalization schemes is parameterized by some group , then the main theorem implies vertex redefinitions depending on pairs of elements of (prop. below). This is known as
Specifically scaling transformations on Minkowski spacetime yield such a collection of renormalization schemes (prop. below); the corresponding renormalization group flow is known as
The infinitesimal behaviour of this flow is known as the beta function, describing the running of the coupling constants with scale (def. below).
The construction of perturbative quantum field theories around a given gauge fixed relativistic free field vacuum is equivalently, by prop. , the construction of S-matrices in the sense of causal perturbation theory (def. ) for the given local interaction . By prop. the construction of these S-matrices is inductively in a choice of extension of distributions (remark and def. below) of the corresponding -ary time-ordered products of the interaction to the locus of coinciding interaction points. An inductive construction of the S-matrix this way is called Epstein-Glaser-("re"-)normalization (def. ).
By paying attention to the scaling degree (def. below) one may precisely characterize the space of choices in the extension of distributions (prop. below): For a given local interaction it is inductively in a finite-dimensional affine space. This conclusion is theorem below.
(("re"-)normalization is inductive extension of time-ordered products to diagonal)
Let be a gauge-fixed relativistic free vacuum according to def. ).
Assume that for , time-ordered products of arity have been constructed in the sense of def. . Then the time-ordered product of arity is uniquely fixed on the complement
of the image of the diagonal inclusion (where we regarded as a generalized function on according to remark ).
This statement appears in (Popineau-Stora 82), with (unpublished) details in (Stora 93), following personal communication by Henri Epstein (according to Dütsch 18, footnote 57). Following this, statement and detailed proof appeared in (Brunetti-Fredenhagen 99).
We will construct an open cover of by subsets which are disjoint unions of non-empty sets that are in causal order, so that by causal factorization the time-ordered products on these subsets are uniquely given by . Then we show that these unique products on these special subsets do coincide on intersections. This yields the claim by a partition of unity.
We now say this in detail:
For write . For , define the subset
Since the causal order-relation involves the closed future cones/closed past cones, respectively, it is clear that these are open subsets. Moreover it is immediate that they form an open cover of the complement of the diagonal:
(Because any two distinct points in the globally hyperbolic spacetime may be causally separated by a Cauchy surface, and any such may be deformed a little such as not to intersect any of a given finite set of points. )
Hence the condition of causal factorization on implies that restricted to any these have to be given (in the condensed generalized function-notation from remark ) on any unordered tuple with corresponding induced tuples and by
This shows that is unique on if it exists at all, hence if these local identifications glue to a global definition of . To see that this is the case, we have to consider any two such subsets
By definition this implies that for
a tuple of spacetime points which decomposes into causal order with respect to both these subsets, the corresponding mixed intersections of tuples are spacelike separated:
By the assumption that the satisfy causal factorization, this implies that the corresponding time-ordered products commute:
Using this we find that the identifications of on and on , accrding to (1), agree on the intersection: in that for we have
Here in the first step we expanded out the two factors using (1) for , then under the brace we used (2) and in the last step we used again (1), but now for .
To conclude, let
be a partition of unity subordinate to the open cover formed by the :
Then the above implies that setting for any
is well defined and satisfies causal factorization.
(time-ordered products of fixed interaction as distributions)
Let be a gauge-fixed relativistic free vacuum according to def. , and assume that the field bundle is a trivial vector bundle (example )
and let
be a polynomial local observable as in def. , to be regarded as a adiabatically switched interaction action functional. This means that there is a finite set
of Lagrangian densities which are monomials in the field and jet coordinates, and a corresponding finite set
of adiabatic switchings, such that
is the transgression of variational differential forms (def. ) of the sum of the products of these adiabatic switching with these Lagrangian densities.
In order to discuss the S-matrix and hence the time-ordered products of the special form it is sufficient to restrict attention to the restriction of each to the subspace of local observables induced by the finite set of Lagrangian densities .
This restriction is a continuous linear functional on the corresponding space of bump functions , hence a dstributional section of a corresponding trivial vector bundle.
In terms of this, prop. says that the choice of time-ordered products is inductively in a choice of extension of distributions to the diagonal.
If is Minkowski spacetime and we impose the renormalization condition “translation invariance” (def. ) then each is a distribution on and the extension of distributions is from the complement of the origina .
Therefore we now discuss extension of distributions (def. below) on Cartesian spaces from the complement of the origin to the origin. Since the space of choices of such extensions turns out to depend on the scaling degree of distributions, we first discuss that (def. below).
Let . For a positive real number write
for the diffeomorphism given by multiplication with , using the canonical real vector space-structure of .
Then for a distribution on the Cartesian space the rescaled distribution is the pullback of along
Explicitly, this is given by
Similarly for an open subset which is invariant under , the rescaling of a distribution is is .
(scaling degree of a distribution)
Let and let be an open subset of Cartesian space which is invariant under rescaling (def. ) for all , and let be a distribution on this subset. Then
The scaling degree of is the infimum
of the set of real numbers such that the limit of the rescaled distribution (def. ) vanishes. If there is no such one sets .
The degree of divergence of is the difference of the scaling degree by the dimension of the underlying space:
(scaling degree of non-singular distributions)
If is a non-singular distribution given by bump function , then its scaling degree (def. ) is non-positive
Specifically if the first non-vanishing partial derivative of at 0 occurs at order , then the scaling degree of is .
By definition we have for any bump function that
where in last line we applied change of integration variables.
The limit of this expression is clearly zero for all , which shows the first claim.
If moreover the first non-vanishing partial derivative of occurs at order , then Hadamard's lemma says that is of the form
where the are smooth functions. Hence in this case
This makes manifest that the expression goes to zero with precisely for , which means that
in this case.
(scaling degree of derivatives of delta-distributions)
Let be a multi-index and the corresponding partial derivatives of the delta distribution supported at . Then the degree of divergence (def. ) of is the total order the derivatives
where .
By definition we have for any bump function that
where in the last step we used the chain rule of differentiation. It is clear that this goes to zero with as long as . Hence .
(scaling degree of Feynman propagator on Minkowski spacetime)
Let
be the Feynman propagator for the massive free real scalar field on -dimensional Minkowski spacetime (prop. ). Its scaling degree is
(Brunetti-Fredenhagen 00, example 3 on p. 22)
Regarding as a generalized function via the given Fourier-transform expression, we find by change of integration variables in the Fourier integral that in the scaling limit the Feynman propagator becomes that for vannishing mass, which scales homogeneously:
(basic properties of scaling degree of distributions)
Let and be a distribution as in def. , such that its scaling degree is finite: (def. ). Then
For , the partial derivative of distributions increases scaling degree at most by :
For , the product of distributions with the smooth coordinate functions decreases scaling degree at least by :
Under tensor product of distributions their scaling degrees add:
for another distribution on ;
for and for ;
(Brunetti-Fredenhagen 00, lemma 5.1, Dütsch 18, exercise 3.34)
The first three statements follow with manipulations as in example and example .
For the fourth…
(scaling degree of product distribution)
Let be two distributions such that
both have finite degree of divergence (def. )
their product of distributions is well-defined
(in that their wave front sets satisfy Hörmander's criterion)
then the product distribution has degree of divergence bounded by the sum of the separate degrees:
With the concept of scaling degree of distributions in hand, we may now discuss extension of distributions:
Let be an inclusion of open subsets of some Cartesian space. This induces the operation of restriction of distributions
Given a distribution , then an extension of to is a distribution such that
(unique extension of distributions with negative degree of divergence)
For , let be a distribution on the complement of the origin, with negative degree of divergence at the origin
Then has a unique extension of distributions to the origin with the same degree of divergence
(Brunetti-Fredenhagen 00, theorem 5.2, Dütsch 18, theorem 3.35 a))
Regarding uniqueness:
Suppose and are two extensions of with . Both being extensions of a distribution defined on , this difference has support at the origin . By prop. this implies that it is a linear combination of derivatives of the delta distribution supported at the origin:
for constants . But by example the degree of divergence of these point-supported distributions is non-negative
This implies that for all , hence that the two extensions coincide.
Regarding existence:
Let
be a bump function which is and constant on 1 over a neighbourhood of the origin. Write
graphics grabbed from Dütsch 18, p. 108
and for a positive real number, write
Since the product has support of a distribution on a complement of a neighbourhood of the origin, we may extend it by zero to a distribution on all of , which we will denote by the same symbols:
By construction coincides with away from a neighbourhood of the origin, which moreover becomes arbitrarily small as increases. This means that if the following limit exists
then it is an extension of .
To see that the limit exists, it is sufficient to observe that we have a Cauchy sequence, hence that for all the difference
becomes arbitrarily small.
It remains to see that the unique extension thus established has the same scaling degree as . This is shown in (Brunetti-Fredenhagen 00, p. 24).
(space of point-extensions of distributions)
For , let be a distribution of degree of divergence .
Then does admit at least one extension (def. ) to a distribution , and every choice of extension has the same degree of divergence as
Moreover, any two such extensions and differ by a linear combination of partial derivatives of distributions of order of the delta distribution supported at the origin:
for a finite number of constants .
This is essentially (Hörmander 90, thm. 3.2.4). We follow (Brunetti-Fredenhagen 00, theorem 5.3), which was inspired by (Epstein-Glaser 73, section 5). Review of this approach is in (Dütsch 18, theorem 3.35 (b)), see also remark below.
For a smooth function, and , we say that vanishes to order at the origin if all partial derivatives with multi-index of total order vanish at the origin:
By Hadamard's lemma, such a function may be written in the form
for smooth functions .
Write
for the subspace of that of all bump functions on those that vanish to order at the origin.
By definition this is equivalently the joint kernel of the partial derivatives of distributions of order of the delta distribution supported at the origin:
Therefore every continuous linear projection
may be obtained from a choice of dual basis to the , hence a choice of smooth functions
such that
by setting
hence
Together with Hadamard's lemma in the form (5) this means that every is decomposed as
Now let
Observe that (by prop. ) the degree of divergence of the product of distributions with is negative
Therefore prop. says that each for has a unique extension to the origin. Accordingly the composition has a unique extension, by (8):
That says that is of the form
for a finite number of constants .
Notice that for any extension the exact value of the here depends on the arbitrary choice of dual basis used for this construction. But the uniqueness of the first summand means that for any two choices of extensions and , their difference is of the form
where the constants are independent of any choices.
It remains to see that all these in fact have the same degree of divergence as .
By example the degree of divergence of the point-supported distributions on the right is .
Therefore to conclude it is now sufficient to show that
This is shown in (Brunetti-Fredenhagen 00, p. 25).
(“W-extensions”)
Since in Brunetti-Fredenhagen 00, (38) the projectors (7) are denoted “”, the construction of extensions of distributions via the proof of prop. has come to be called “W-extensions” (e.g Dütsch 18).
In conclusion we obtain the central theorem of causal perturbation theory:
(existence and choices of ("re"-)normalization of S-matrices/perturbative QFTs)
Let be a gauge-fixed relativistic free vacuum, according to def. , such that the underlying spacetime is Minkowski spacetime and the Wightman propagator is translation-invariant.
Then:
an S-matrix scheme (def. ) around this vacuum exists;
for a local observable as in def. , regarded as an adiabatically switched interaction action functional, the space of possible choices of S-matrices
hence of the corresponding perturbative QFTs, by prop. , is, inductively in , a finite dimensional affine space, parameterizing the extension of the time-ordered product to the locus of coinciding interaction points.
By prop. the Feynman propagator is finite scaling degree of a distribution, so that by prop. the binary time-ordered product away from the diagonal has finite scaling degree.
By prop. this implies that in the inductive description of the time-ordered products by prop. , each induction step is the extension of distributions of finite scaling degree of a distribution to the point. By prop. this always exists.
This proves the first statement.
Now if a polynomial local interaction is fixed, then via remark each induction step involved extending a finite number of distributions, each of finite scaling degree. By prop. the corresponding space of choices is in each step a finite-dimensional affine space.
Stückelberg-Petermann renormalization group
A genuine re-normalization is the passage from one S-matrix ("re"-)normalization scheme to another such scheme . The inductive Epstein-Glaser ("re"-normalization) construction (prop. ) shows that the difference between any and is inductively in a choice of extra term in the time-ordered product of factors, equivalently in the Feynman amplitudes for Feynman diagrams with vertices, that contributes when all of these vertices coincide in spacetime (prop. ).
A natural question is whether these additional interactions that appear when several interaction vertices coincide may be absorbed into a re-definition of the original interaction . Such an interaction vertex redefinition (def. below)
should perturbatively send local interactions to local interactions with higher order corrections.
The main theorem of perturbative renormalization (theorem below) says that indeed under mild conditions every re-normalization is induced by such an interaction vertex redefinition in that there exists a unique such redefinition so that for every local interaction we have that scattering amplitudes for the interaction computed with the ("re"-)normalization scheme equal those computed with but applied to the re-defined interaction :
This means that the interaction vertex redefinitions form a group under composition which acts transitively and freely, hence regularly, on the set of S-matrix ("re"-)normalization schemes; this is called the Stückelberg-Petermann renormalization group (theorem below).
(perturbative interaction vertex redefinition)
Let be a gauge fixed free field vacuum (def. ).
A perturbative interaction vertex redefinition (or just vertex redefinition, for short) is an endofunction
on local observables with formal parameters adjoined (def. ) such that there exists a sequence of continuous linear functionals, symmetric in their arguments, of the form
such that for all the following conditions hold:
(perturbation)
and
(field independence) The local observable depends on the field histories only through its argument , hence by the chain rule:
The following proposition should be compared to the axiom of causal additivity of the S-matrix scheme (?):
(local additivity of vertex redefinitions)
Let be a gauge fixed free field vacuum (def. ) and let be a vertex redefinition (def. ).
Then for all local observables with spacetime support denoted (def. ) we have
(local additivity)
(preservation of spacetime support)
hence in particular
Under the inclusion
of local observables into polynomial observables we may think of each as a generalized function, as for time-ordered products in remark .
Hence if
is the transgression of a Lagrangian density we get
Now by definition is in the subspace of local observables, i.e. those polynomial observables whose coefficient distributions are supported on the diagonal, which means that
Together with the axiom “field independence” (10) this means that the support of these generalized functions in the integrand here must be on the diagonal, where .
By the assumption that the spacetime supports of and are disjoint, this means that only the summands with and those with contribute to the above sum. Removing the overcounting of those summands where all we get
This directly implies the claim.
As a corollary we obtain:
(composition of S-matrix scheme with vertex redefinition is again S-matrix scheme)
Let be a gauge fixed free field vacuum (def. ) and let be a vertex redefinition (def. ).
Then for
and S-matrix scheme (def. ), the composite
is again an S-matrix scheme.
Moreover, if satisfies the renormalization condition “field independence” (prop. ), then so does .
(e.g Dütsch 18, theorem 3.99 (b))
It is clear that causal order of the spacetime supports implies that they are in particular disjoint
Therefore the local additivity of (prop. ) and the causal factorization of the S-matrix (remark ) imply the causal factorization of the composite:
But by prop. this implies in turn causal additivity and hence that is itself an S-matrix scheme.
Finally that satisfies “field indepndence” if does is immediate by the chain rule, given that satisfies this condition by definition.
(any two S-matrix renormalization schemes differ by unique vertex redefinition)
Let be a gauge fixed free field vacuum (def. ).
Then for any two S-matrix schemes (def. ) which both satisfy the renormalization condition “field independence”, the there exists a unique vertex redefinition (def. ) relating them by composition, i. e. such that
By applying both sides of the equation to linear combinations of local observables of the form and then taking derivatives with respect to at (as in example ) we get that the equation in question implies
which in components means that
where are the time-ordered products corresponding to (by example ) and those correspondong to .
Here the sum on the right runs over all ways that in the composite a -ary operation arises as the composite of an -ary time-ordered product applied to the -ary components of , for running from 1 to ; except for the case , which is displayed separately in the second line
This shows that if exists, then it is unique, because its coefficients are inductively in given by the expressions
(The symbol under the brace is introduced as a convenient shorthand for the term above the brace.)
Hence it remains to see that the defined this way satisfy the conditions in def. .
The condition “perturbation” is immediate from the corresponding condition on and .
Similarly the condition “field independence” follows immediately from the assumoption that and satisfy this condition.
It only remains to see that indeed takes values in local observables. Given that the time-ordered products a priori take values in the larrger space of microcausal polynomial observables this means to show that the spacetime support of is on the diagonal.
But observe that, as indicated in the above formula, the term over the brace may be understood as the coefficient at order of the exponential series-expansion of the composite , where
is the truncation of the vertex redefinition to degree . This truncation is clearly itself still a vertex redefinition (according to def. ) so that the composite is still an S-matrix scheme (by prop. ) so that the are time-ordered products (by example ).
So as we solve inductively in degree , then for the induction step in degree the expressions and agree and are both time-ordered products. By prop. this implies that and agree away from the diagonal. This means that their difference is supported on the diagonal, and hence is indeed local.
In conclusion this establishes the following pivotal statement of perturbative quantum field theory:
(main theorem of perturbative renormalization – Stückelberg-Petermann renormalization group of vertex redefinitions)
Let be a gauge fixed free field vacuum (def. ).
the vertex redefinitions (def. ) form a group under composition;
the set of S-matrix ("re"-)normalization schemes (def. ), remark ) satisfying the renormalization condition “field independence” (prop. ) is a torsor over this group, hence equipped with a regular action in that
the set of S-matrix schemes is non-empty;
any two S-matrix ("re"-)normalization schemes , are related by a unique vertex redefinition via composition:
This group is called the Stückelberg-Petermann renormalization group.
Typically one imposes a set of renormalization conditions (def. ) and considers the corresponding subgroup of vertex redefinitions preserving these conditions.
The group-structure and regular action is given by prop. and prop. . The existence of S-matrices follows is the statement of Epstein-Glaser ("re"-)normalization in theorem .
UV-Regularization via counterterms
While Epstein-Glaser renormalization (prop. ) gives a transparent picture on the space of choices in ("re"-)normalization (theorem ) the physical nature of the higher interactions that it introduces at coincident interaction points (via the extensions of distributions in prop. ) remains more implicit. But the main theorem of perturbative renormalization (theorem ), which re-expresses the difference between any two such choices as an interaction vertex redefinition, suggests that already the choice of ("re"-)normalization itself should have an incarnation in terms of interaction vertex redefinitions.
This may be realized via a construction of ("re"-)normalization in terms of UV-regularization (prop. below): For any choice of “UV-cutoff”, given by an approximation of the Feynman propagator by non-singular distributions (def. below) there is a unique “effective S-matrix” induced at each cutoff scale (def. below). While the “UV-limit” does not in general exist, it may be “regularized” by applying suitable interaction vertex redefinitions ; if the higher-order corrections that these introduce serve to “counter” (remark below) the coresponding UV-divergences.
This perspective of ("re"-)normalization via via counterterms is often regarded as the primary one. Its elegant proof in prop. below, however relies on the Epstein-Glaser renormalization via inductive extensions of distributions and uses the same kind of argument as in the proof of the main theorem of perturbative renormalization (theorem via prop. ) that establishes the Stückelberg-Petermann renormalization group.
Let be a gauge fixed relativistic free vacuum over Minkowski spacetime (according to def. ), where is the corresponding Wightman propagator inducing the Feynman propagator
by .
Then a choice of UV cutoffs for perturbative QFT around this vacuum is a collection of non-singular distributions parameterized by positive real numbers
such that:
each satisfies the following basic properties
(translation invariance)
(symmetry)
i.e.
the interpolate between zero and the Feynman propagator, in that, in the Hörmander topology:
the limit as exists and is zero
the limit as exists and is the Feynman propagator:
(relativistic momentum cutoff)
Recall from this prop. that the Fourier transform of distributions of the Feynman propagator for the real scalar field on Minkowski spacetime is,
To produce a UV cutoff in the sense of def. we would like to set this function to zero for wave numbers (hence momenta ) larger than a given .
This needs to be done with due care: First, the Paley-Wiener-Schwartz theorem (prop. ) says that to be a test function and hence compactly supported, its Fourier transform needs to be smooth and of bounded growth. So instead of multiplying by a step function in , we may multiply it with an exponential damping.
(Keller-Kopper-Schophaus 97, section 6.1, Dütsch 18, example 3.126)
Let be a gauge fixed relativistic free vacuum (according to def. ) and let be a choice of UV cutoffs for perturbative QFT around this vacuum (def. ).
We say that the effective S-matrix scheme at cutoff scale
is the exponential series
with respect to the star product induced by the (def. ).
This is evidently defined on all polynomial observables as shown, and restricts to an endomorphism on microcausal polynomial observables as shown, since the contraction coefficients are non-singular distributions, by definition of UV cutoff.
(("re"-)normalization via UV regularization)
Let be a gauge fixed relativistic free vacuum (according to def. ) and let a polynomial local observable as in def. , regarded as an adiabatically switched interaction action functional.
Let moreover be a UV cutoff (def. ); with the induced effective S-matrix schemes (12).
Then
there exists a -parameterized interaction vertex redefinition (def. ) such that the limit of effective S-matrix schemes (12) applied to the -redefined interactions
exists and is a genuine S-matrix scheme around the given vacuum (def. );
every S-matrix scheme around the given vacuum arises this way.
These are called counterterms (remark below) and the composite is called a UV regularization of the effective S-matrices .
Hence UV-regularization via counterterms is a method of ("re"-)normalization of perturbative QFT (def. ).
This was claimed in (Brunetti-Dütsch-Fredenhagen 09, (75)), a proof was indicated in (Dütsch-Fredenhagen-Keller-Rejzner 14, theorem A.1).
Let be a sequence of projection maps as in (6) defining an Epstein-Glaser ("re"-)normalization (prop. ) of time-ordered products as extensions of distributions of the , regarded as distributions via remark , by the choice in (9).
We will construct that in terms of these projections .
First consider some convenient shorthand:
For , write . Moreover, for write for the -ary coefficient in the expansion of the composite , as in equation (11) in the proof of the main theorem of perturbative renormalization (theorem , via prop. ).
In this notation we need to find such that for each we have
We proceed by induction over .
Since by definition , and , the statement is trivially true for and .
So assume now and has been found such that (13) holds.
Observe that with the chosen renormalizing projection the time-ordered product may be expressed as follows:
Here in the first step we inserted the causal decomposition (4) of in terms of the away from the diagonal, as in the proof of prop. , which is admissible because the image of vanishes on the diagonal. In the second step we replaced the star-product of the Feynman propagator with the limit over the star-products of the regularized propagators , which converges by the nature of the Hörmander topology (which is assumed by def. ).
Hence it is sufficient to find and such that
subject to these two conditions:
is local;
.
Now by expanding out the left hand side of (15) as
(which uses the condition ) we find the unique solution of (15) for , in terms of the and (the latter still to be chosen) to be:
We claim that the following choice works:
To prove this, we need to show that 1) the resulting is local and 2) the limit of vanishes as .
First regarding the locality of : By inserting (17) into (16) we obtain
By definition is the identity on test functions (adiabatic switchings) that vanish at the diagonal. This means that is supported on the diagonal, and is hence local.
Second we need to show that :
By applying the analogous causal decomposition (4) to the regularized products, we find
Using this we compute as follows:
Here in the first step we inserted (18); in the second step we used that in the Hörmander topology the product of distributions preserves limits in each variable and in the third step we used the induction assumption (13) and the definition of UV cutoff (def. ).
Inserting this for the first summand in (17) shows that .
In conclusion this shows that a consistent choice of counterterms exists to produce some S-matrix . It just remains to see that for every other S-matrix there exist counterterms such that .
But by the main theorem of perturbative renormalization (theorem ) we know that there exists a vertex redefinition such that
and hence with counterterms for given, then counterterms for any are given by the composite .
Let be a gauge fixed relativistic free vacuum (according to def. ) and let be a choice of UV cutoffs for perturbative QFT around this vacuum (def. ).
Consider
a local observable, regarded as an adiabatically switched interaction action functional.
Then prop. says that there exist vertex redefinitions of this interaction
parameterized by , such that the limit
exists and is an S-matrix for perturbative QFT with the given interaction .
In this case the difference
(which by the axiom “perturbation” in def. is at least of second order in the coupling constant/source field, as shown) is called a choice of counterterms at cutoff scale . These are new interactions which are added to the given interaction at cutoff scale
In this language prop. says that for every free field vacuum and every choice of local interaction, there is a choice of counterterms to the interaction that defines a corresponding ("re"-)normalized perturbative QFT, and every (re"-)normalized perturbative QFT arises from some choice of counterterms.
Wilson-Polchinski effective QFT flow
We have seen above that a choice of UV cutoff induces effective S-matrix schemes at cutoff scale (def. ). To these one may associated non-local relative effective actions (def. below) which are such that their effective scattering amplitudes at scale coincide with the true scattering amplitudes of a genuine local interaction as the cutoff is removed. This is the Wilsonian picture of effective quantum field theory at a given cutoff scale (remark below). Crucially the “flow” of the relative effective actions with the cutoff scale satisfies a differential equation that in itself is independent of the full UV-theory; this is Polchinski's flow equation (prop. below). Solving this equation for given choice of initial value data is hence another way of choosing ("re"-)normalization constants.
(effective S-matrix schemes are invertible functions)
Let be a gauge fixed relativistic free vacuum (according to def. ) and let be a choice of UV cutoffs for perturbative QFT around this vacuum (def. ).
Write
for the subspace of the space of formal power series in with coefficients polynomial observables on those which are at least of first order in , i.e. those that vanish for (as in def. ).
Write moreover
for the subspace of polynomial observables which are the sum of 1 (the multiplicative unit) with an observable at least linear n .
Then the effective S-matrix schemes (def. ) restrict to linear isomorphisms of the form
Since each is symmetric (def. ) if follows by general properties of star products (prop. ) just as for the genuine time-ordered product on regular polynomial observables (prop. ) that eeach the “effective time-ordered product” is isomorphic to the pointwise product (def. )
for
as in (?).
In particular this means that the effective S-matrix arises from the exponential series for the pointwise product by conjugation with :
(just as for the genuine S-matrix on regular polynomial observables in def. ).
Now the exponential of the pointwise product on has as inverse function the natural logarithm power series, and since evidently preserves powers of this conjugates to an inverse at each UV cutoff scale :
Let be a gauge fixed relativistic free vacuum (according to def. ) and let be a choice of UV cutoffs for perturbative QFT around this vacuum (def. ).
Consider
a local observable regarded as an adiabatically switched interaction action functional.
Then for
two UV cutoff-scale parameters, we say the relative effective action is the image of this interaction under the composite of the effective S-matrix scheme at scale (12) and the inverse function of the effective S-matrix scheme at scale (via prop. ):
For chosen counterterms (remark ) hence for chosen UV regularization (prop. ) this makes sense also for and we write:
(effective quantum field theory)
Let be a gauge fixed relativistic free vacuum (according to def. ), let be a choice of UV cutoffs for perturbative QFT around this vacuum (def. ), and let be a corresponding UV regularization (prop. ).
Consider a local observable
regarded as an adiabatically switched interaction action functional.
Then def. and def. say that for any the effective S-matrix (12) of the relative effective action (21) equals the genuine S-matrix of the genuine interaction :
In other words the relative effective action encodes what the actual perturbative QFT defined by effectively looks like at UV cutoff .
Therefore one says that defines effective quantum field theory at UV cutoff .
Notice that in general is not a local interaction anymore: By prop. the image of the inverse of the effective S-matrix is microcausal polynomial observables in and there is no guarantee that this lands in the subspace of local observables.
Therefore effective quantum field theories at finite UV cutoff-scale are in general not local field theories, even if their limit as is, via prop. .
(effective action is relative effective action at )
Let be a gauge fixed relativistic free vacuum (according to def. ) and let be a choice of UV cutoffs for perturbative QFT around this vacuum (def. ).
Then the relative effective action (def. ) at is the actual effective action (def. ) in the sense of the the Feynman perturbation series of Feynman amplitudes (def. ) for connected Feynman diagrams :
More generally this holds true for any
where denotes the evident version of the Feynman amplitude (def. ) with time-ordered products replaced by effective time ordered product at scale as in (def. ).
Observe that the effective S-matrix scheme at scale (12) is the exponential series with respect to the pointwise product (def. )
Therefore the statement to be proven says equivalently that the exponential series of the effective action with respect to the pointwise product is the S-matrix:
The definition of the relative effective action in def. invokes a choice of UV regularization (prop. ). While (by that proposition and the main theorem of perturbative renormalization, theorem )this is guaranteed to exist, in practice one is after methods for constructing this without specifying it a priori.
But the collection relative effective actions for “flows” with the cutoff-parameters and in particular also with (remark below) which suggests that examination of this flow yields information about full theory at .
This is made precise by Polchinski's flow equation (prop. below), which is the infinitesimal version of the “Wilsonian RG flow” (remark ). As a differential equation it is independent of the choice of and hence may be used to solve for the Wilsonian RG flow without knowing in advance.
The freedom in choosing the initial values of this differential equation corresponds to the ("re"-)normalization freedom in choosing the UV regularization . In this sense “Wilsonian RG flow” is a method of ("re"-)normalization of perturbative QFT (def. ).
(Wilsonian groupoid of effective quantum field theories)
Let be a gauge fixed relativistic free vacuum (according to def. ) and let be a choice of UV cutoffs for perturbative QFT around this vacuum (def. ).
Then the relative effective actions (def. ) satisfy
This is similar to a group of UV-cutoff scale-transformations. But since the composition operations are only sensible when the UV-cutoff labels match, as shown, it is really a groupoid action.
This is often called the Wilsonian RG.
We now consider the infinitesimal version of this “flow”:
Let be a gauge fixed relativistic free vacuum (according to def. ), let be a choice of UV cutoffs for perturbative QFT around this vacuum (def. ), such that is differentiable.
Then for every choice of UV regularization (prop. ) the corresponding relative effective actions (def. ) satisfy the following differential equation:
where on the right we have the star product induced by (def. ).
This goes back to (Polchinski 84, (27)). The rigorous formulation and proof is due to (Brunetti-Dütsch-Fredenhagen 09, prop. 5.2, Dütsch 10, theorem 2).
First observe that for any polynomial observable we have
Here denotes the functional derivative of the th tensor factor of , and the binomial coefficient counts the number of ways that an unordered pair of distinct labels of tensor factors may be chosen from a total of tensor factors, where we use that the star product is commutative (by symmetry of ) and associative (by prop. ).
With this and the defining equality (22) we compute as follows:
Acting on this equation with the multiplicative inverse (using that is a commutative product, so that exponentials behave as usual) this yields the claimed equation.
In perturbative quantum field theory the construction of the scattering matrix , hence of the interacting field algebra of observables for a given interaction perturbing around a given free field vacuum, involves choices of normalization of time-ordered products/Feynman diagrams (traditionally called "re"-normalizations) encoding new interactions that appear where several of the original interaction vertices defined by coincide.
Whenever a group acts on the space of observables of the theory such that conjugation by this action takes ("re"-)normalization schemes into each other, then these choices of ("re"-)normalization are parameterized by – or “flow with” – the elements of . This is called renormalization group flow (prop. below); often called RG flow, for short.
The archetypical example here is the group of scaling transformations on Minkowski spacetime (def. below), which induces a renormalization group flow (prop. below) due to the particular nature of the Wightman propagator resp. Feynman propagator on Minkowski spacetime (example below). In this case the choice of ("re"-)normalization hence “flows with scale”.
Now the main theorem of perturbative renormalization (theorem ) states that (if only the basic renormalization condition called “field independence” is satisfied) any two choices of ("re"-)normalization schemes and are related by a unique interaction vertex redefinition , as
Applied to a parameterization/flow of renormalization choices by a group this hence induces an interaction vertex redefinition as a function of . One may think of the shape of the interaction vertices as fixed and only their (adiabatically switched) coupling constants as changing under such an interaction vertex redefinition, and hence then one has coupling constants that are parameterized by elements of :
This dependendence is called running of the coupling constants under the renormalization group flow (def. below).
One example of renormalization group flow is that induced by scaling transformations (prop. below). This is the original and main example of the concept (Gell-Mann & Low 54)
In this case the running of the coupling constants may be understood as expressing how “more” interactions (at higher energy/shorter wavelength) become visible (say to experiment) as the scale resolution is increased. In this case the dependence of the coupling on the parameter happens to be differentiable; its logarithmic derivative (denoted “” in Gell-Mann & Low 54) is known as the beta function (Callan 70, Symanzik 70):
The running of the coupling constants is not quite a representation of the renormalization group flow, but it is a “twisted” representation, namely a group 1-cocycle (prop. below). For the case of scaling transformations this may be called the Gell-Mann-Low renormalization cocycle (Brunetti-Dütsch-Fredenhagen 09).
Let
be a relativistic free vacuum (according to def. ) around which we consider interacting perturbative QFT.
Consider a group equipped with an action on the Wick algebra of off-shell microcausal polynomial observables with formal parameters adjoined (as in def. )
hence for each a continuous linear map which has an inverse and is a homomorphism of the Wick algebra-product (the star product induced by the Wightman propagator of the given vauum )
such that the following conditions hold:
the action preserves the subspace of off-shell polynomial local observables, hence it restricts as
the action respects the causal order of the spacetime support (def. ) of local observables, in that for we have
for all .
Then:
The operation of conjugation by this action on observables induces an action on the set of S-matrix renormalization schemes (def. , remark ), in that for
a perturbative S-matrix scheme around the given free field vacuum , also the composite
is an S-matrix scheme, for all .
More generally, let
be a collection of gauge fixed free field vacua parameterized by elements , all with the same underlying field bundle; and consider as above, except that it is not an automorphism of any Wick algebra, but an isomorphism between the Wick algebra-structures on various vacua, in that
for all
Then if
is a collection of S-matrix schemes, one around each of the gauge fixed free field vacua , it follows that for all pairs of group elements the composite
is an S-matrix scheme around the vacuum labeled by .
Since therefore each element in the group picks a different choice of normalization of the S-matrix scheme around a given vacuum at , we call the assignment a re-normalization group flow.
(Brunetti-Dütsch-Fredenhagen 09, sections 4.2, 5.1, Dütsch 18, section 3.5.3)
It is clear from the definition that each satisfies the axiom “perturbation” (in def. ).
In order to verify the axiom “causal additivity”, observe, for convenience, that by prop. it is sufficient to check causal factorization.
So consider two local observables whose spacetime support is in causal order.
We need to show that the
for all .
Using the defining properties of and the causal factorization of we directly compute as follows:
Let
be a relativistic free vacuum (according to def. ) around which we consider interacting perturbative QFT, let be an S-matrix scheme around this vacuum and let be a renormalization group flow according to prop. , such that each re-normalized S-matrix scheme satisfies the renormalization condition “field independence”.
Then by the main theorem of perturbative renormalization (theorem , via prop. ) there is for every pair a unique interaction vertex redefinition
which relates the corresponding two S-matrix schemes via
If one thinks of an interaction vertex, hence a local observable , as specified by the (adiabatically switched) coupling constants multiplying the corresponding interaction Lagrangian densities as
(where denotes transgression of variational differential forms) then exhibits a dependency of the (adiabatically switched) coupling constants of the renormalization group flow parameterized by . The corresponding functions
are then called running coupling constants.
(Brunetti-Dütsch-Fredenhagen 09, sections 4.2, 5.1, Dütsch 18, section 3.5.3)
(running coupling constants are group cocycle over renormalization group flow)
Consider running coupling constants
as in def. . Then for all the following equality is satisfied by the “running functions” (25):
(Brunetti-Dütsch-Fredenhagen 09 (69), Dütsch 18, (3.325))
Directly using the definitions, we compute as follows:
This demonstrates the equation between vertex redefinitions to be shown after composition with an S-matrix scheme. But by the uniqueness-clause in the main theorem of perturbative renormalization (theorem ) the composition operation as a function from vertex redefinitions to S-matrix schemes is injective. This implies the equation itself.
We discuss (prop. below) that, if the field species involved have well-defined mass dimension (example below) then scaling transformations on Minkowski spacetime (example below) induce a renormalization group flow (def. ). This is the original and main example of renormalization group flows (Gell-Mann& Low 54).
(scaling transformations and mass dimension)
Let
be a field bundle which is a trivial vector bundle over Minkowski spacetime .
For a positive real number, write
for the operation of multiplication by using the real vector space-structure of the Cartesian space underlying Minkowski spacetime.
By pullback this acts on field histories (sections of the field bundle) via
Let then
be a 1-parameter collection of relativistic free vacua on that field bundle, according to def. , and consider a decomposition into a set of field species (def. ) such that for each the collection of Feynman propagators for that species scales homogeneously in that there exists
such that for all we have (using generalized functions-notation)
Typically rescales a mass parameter, in which case is also called the mass dimension of the field species .
Let finally
be the function on off-shell polynomial observables given on field observables by pullback along followed by multiplication by taken to the negative power of the mass dimension, and extended from there to all polynomial observables as an algebra homomorphism.
This constitutes an action of the group
of positive real numbers (under multiplication) on polynomial observables, called the group of scaling transformations for the given choice of field species and mass parameters.
(mass dimension of scalar field)
Consider the Feynman propagator of the free real scalar field on Minkowski spacetime for mass parameter ; a Green function for the Klein-Gordon equation.
Let the group of scaling transformations on Minkowski spacetime (def. ) act on the mass parameter by inverse multiplication
Then we have
and hence the corresponding mass dimension (def. ) of the real scalar field on is
By prop. the Feynman propagator in question is given by the Cauchy principal value-formula (in generalized function-notation)
By applying change of integration variables in the Fourier transform this becomes
(scaling transformations are renormalization group flow)
Let
be a relativistic free vacua on that field bundle, according to def. equipped with a decomposition into a set of field species (def. ) such that for each the collection of Feynman propagators the corresponding field species has a well-defined mass dimension (def. )
Then the action of the group of scaling transformations (def. ) is a renormalization group flow in the sense of prop. .
It is clear that rescaling preserves causal order and the renormalization condition of “field indepencen”.
The condition we need to check is that for two microcausal polynomial observables we have for any that
By the assumption of decomposition into free field species , it is sufficient to check this for each species . Moreover, by the nature of the star product on polynomial observables, which is given by iterated contractions with the Wightman propagator, it is sufficient to check this for one such contraction.
Observe that the scaling behaviour of the Wightman propagator is the same as the behaviour (26) of the correspponding Feynman propagator. With this we directly compute as follows:
This concludes our discussion of renormalization.
Last revised on August 29, 2018 at 09:27:15. See the history of this page for a list of all contributions to it.