FQFT and cohomology
Types of quantum field thories
One way to make sense of the path integral used for quantization of classical field theory to quantum field theory is to define it as a resummation of the the Feynman perturbation series of Feynman diagrams.
But for a generic action functional this prescription produces in each term of this series an ill-defined diverging term. Renormalization is a process of adjusting the action functional such that the classical field theory remains the same, but while at the same time adding interactions whose effect is to counteract these divergences to yield a series that is termwise finite. The terms for these extra interactions in the action functional are therefore called counterterms.
A well-developed method for renormalization, described in more detail below, expresses each of the terms in the perturbation series as a Laurent series in a certain parameter which is divergent at the parameter value of interest. In this description the process of renormalization corresponds to discarding the poles of this series.
The precise way in which this Laurent series is constructed has recently been understood to have an elegant desciption in terms of Hopf algebra and is now known as Hopf algebraic renormalization .
There are different procedures of renormalization
In the study of perturbative quantum field theory one is concerned with functions – called amplitudes – that take a collection of graphs – called Feynman graphs – to Laurent polynomials in a complex variable – called the (dimensional) regularization parameter
and wishes to extract a “meaningful” finite component when evaluated at vanishing regularization parameter .
A prescription – called renormalization scheme – for adding to a given amplitude in a certain recursive fashion further terms – called counterterms – such that the resulting modified amplitude – called the renormalized amplitude – is finite at was once given by physicists and is called the BPHZ-procedure .
This procedure justifies itself mainly through the remarkable fact that the numbers obtained from it match certain numbers measured in particle accelerators to fantastic accuracy.
The combinatorial Hopf algebraic approach to perturbative quantum field theory, see for instance
starts with the observation that the BPHZ-procedure can be understood
by noticing that there is secretly a natural group structure on the collection of amplitudes;
which is induced from the fact that there is secretly a natural Hopf algebra structure on the vector space whose basis consists of graphs;
and with respect to which the BPHZ-procedure is simply the Birkhoff decomposition of group valued functions on the circle into a divergent and a finite part.
The Hopf algebra structure on the vector space whose basis consists of graphs can be understood most conceptually in terms of pre-Lie algebras.
A Birkhoff decomposition of a loop in a complex group is a continuation of the loop to
a holomorphic function on the standard disk inside the circle;
a holomorphic function on the complement of this disk in the projective complex plane
such that on the unit circle the original loop is reproduced as
with the product and the inverse on the right taken in the group .
Notice that by the assumption of holomorphicity is a well defined element of .
then the Birkhoff decomposition always exists and is given by the formula
There is naturally the structure of a Hopf algebra, , on the graphs considered in quantum field theory. As an algebra this is the free commutative algebra on the “1-particle irreducible graphs”. Hence QFT amplitudes can be regarded as characters on this Hopf algebra.
The BPHZ renormalization-procedure for amplitudes is nothing but the first item applied to the special case of the second item.
The proof is given in
Computing the renormalization of an amplitude amounts to using the above formula to compute the counterterm and then evaluating the right hand side of
where the product is the group product on characters, hence the convolution product of characters.
Every elegant reformulation has in it the potential of going beyond mere reformulation by allowing to see structures invisible in a less natural formulation. For instance Dirk Kreimer claims that the Hopf algebra language allows him to see patterns in perturbative quantum gravity previously missed.
In his nicely readable
he reviews the central idea: the BRST formulation of Yang-Mills theory manifests itself at the level of the resulting bare i.e. unnormalized amplitudes in certain relations satisfied by these, the Slavnov-Taylor identities .
Renormalization of gauge theories is consistent only if these relations are still respected by renormalized amplitudes, too. We can reformulate this in terms of Hopf algebra now:
the relations between amplitudes to be preserved under renormalization must define a Hopf ideal in the Hopf algebra of graphs.
Walter von Suijlekom proves this to be the case for Slavnov-Taylor in his theorem 9 on p. 12
As a payoff, he obtains a very transparent way to prove the generalization of Dyson’s formula to nonabelian gauge theory, which expresses renormalized Green’s functions in terms of unrenormalized Green’s functions “at bare coupling”. This is his corollary 12 on p. 13.
is an interaction that is at least cubic.
Most of the fundamental theories in physics are of this form, notably Yang-Mills theory. In particular also all theories of infinity-Chern-Simons theory-type coming from binary invariant polynomials are perturbatively of this form, notably ordinary 3d Chern-Simons theory.
For a discussion of just the simple special case of 3d CS see (Costello 11, chapter 5.4 and 5.14).
For comparison of the following with other renormalization schemes, see at (Costello 07, section 1.7).
of degree -1
for the induced pairing on sections
The paring being non-degenerate means that we have an isomorphism and we write
of degree 1 such that
From this data we obtain:
An interaction term
For the corresponding convolution operator is
This is (Costello 07, p. 32).
where is given by def. 6.
This is (Costello 07, lemma 6.6.2).
does not in general exist. Renormalization is the process of adding -corrections to the action – the counterterms – such as to make it exist after all. In this case we may regard the limit, by prop. 3, as the definition of the path integral.
such that the limit
by def. 8, exists.
The are called the counterterms.
such that the functions are non-singular in that exists.
Hence this is a choice of picking the singularities in functions that are not necessarily defined at .
This is (Costello 07, theorem B, p. 38).
Given a renormalization , write for all
Review of renormalization and specifically of the Hopf algebra description of the BPHZ method is in
G. Peter Lepage, What is Renormalization?, talk 1989 (arXiv:hep-ph/0506330)
(in the context of effective field theory)
BPHZ renormalization was introduced in particular in
Discussion in the context of BV-BRST formalism is in
See also at factorization algebra of observables.
A vaguely related approach earlier appeared in
A discussion of renormalization in the context of AQFT, as well as a detailed comparison of major renormalization schemes is in
An introduction and review to the Hopf-algebraic description of renormalization is in
A textbook treatment is
Some heavywheight automated computations using this formalism are discussed in
Alexander B. Goncharov, Marcus Spradlin, C. Vergu, Anastasia Volovich, Classical polylogarithms for amplitudes and Wilson loops, Phys.Rev.Lett.105:151605,2010 arxiv/1006.5703
Nima Arkani-Hamed, Jacob L. Bourjaily, Freddy Cachazo, Alexander B. Goncharov, Alexander Postnikov, Jaroslav Trnka, Scattering amplitudes and the positive Grassmannian, arxiv/1212.5605