Feynman Graph Amplitudes Research Articles

Differential equations and dispersion relations for Feynman amplitudes. The two-loop massive sunrise and the kite integral

It is shown that the study of the imaginary part and of the corresponding dispersion relations of Feynman graph amplitudes within the differential equations method can provide a powerful tool for the solution of the equations, especially in the massive case.The main features of the approach are illustrated by discussing the simple cases of the 1-loop self-mass and of a particular vertex amplitude, and then used for the evaluation of the two-loop massive sunrise and the QED kite graph (the problem studied by Sabry in 1962), up to first order in the (d−4) expansion.

Schouten identities for Feynman graph amplitudes; The Master Integrals for the two-loop massive sunrise graph

A new class of identities for Feynman graph amplitudes, dubbed Schouten identities, valid at fixed integer value of the dimension d is proposed. The identities are then used in the case of the two-loop sunrise graph with arbitrary masses for recovering the second-order differential equation for the scalar amplitude in d=2 dimensions, as well as a chained set of equations for all the coefficients of the expansions in (d−2). The shift from d≈2 to d≈4 dimensions is then discussed.

Two-loop QCD corrections to the helicity amplitudes for H → 3 partons

Many search strategies for the Standard Model Higgs boson apply specific selection criteria on hadronic jets observed in association with the Higgs boson decay products, either in the form of a jet veto, or by defining event samples according to jet multiplicity. To improve the theoretical description of Higgs-boson-plus-jet production (and the closely related Higgs boson transverse momentum distribution), we derive the two-loop QCD corrections to the helicity amplitudes for the processes $H \to ggg$ and $H\to q \bar q g$ in an effective theory with infinite top quark mass. The helicity amplitudes are extracted from the coefficients appearing in the general tensorial structure for each process. The coefficients are derived from the Feynman graph amplitudes by means of projectors within the conventional dimensional regularization scheme. The infrared pole structure of our result agrees with the expectation from infrared factorization and the finite parts of the amplitudes are expressed in terms of one- and two-dimensional harmonic polylogarithms.

Ponzano–Regge model revisited: III. Feynman diagrams and effective field theory

We study the no-gravity limit GN → 0 of the Ponzano–Regge amplitudes with massive particles and show that we recover in this limit Feynman graph amplitudes (with the Hadamard propagator) expressed as an Abelian spin foam model. We show how the GN expansion of the Ponzano–Regge amplitudes can be resummed. This leads to the conclusion that the effective dynamics of quantum particles coupled to quantum 3D gravity can be expressed in terms of an effective new non-commutative field theory which respects the principles of doubly special relativity. We discuss the construction of Lorentzian spin foam models including Feynman propagators.

Two-loop QCD helicity amplitudes for e+e−→3 jets

We compute the two-loop QCD helicity amplitudes for the process e +e −→q q ̄ g . The amplitudes are extracted in a scheme-independent manner from the coefficients appearing in the general tensorial structure for this process. The tensor coefficients are derived from the Feynman graph amplitudes by means of projectors, within the conventional dimensional regularization scheme. The actual calculation of the loop integrals is then performed by reducing all of them to a small set of known master integrals. The infrared pole structure of the renormalized helicity amplitudes agrees with the prediction made by Catani using an infrared factorization formula. We use this formula to structure our results for the finite part into terms arising from the expansion of the pole coefficients and a genuine finite remainder, which is independent of the scheme used to define the helicity amplitudes. The analytic result for the finite parts of the amplitudes is expressed in terms of one- and two-dimensional harmonic polylogarithms.

Algebraic reduction of one-loop Feynman graph amplitudes

An algorithm for the reduction of one-loop n-point tensor integrals to basic integrals is proposed. We transform tensor integrals to scalar integrals with shifted dimension [A.I. Davydychev, Phys. Lett. B 263 (1991) 107] and reduce these by recurrence relations to integrals in generic dimension [O.V. Tarasov, Phys. Rev. D 54 (1996) 6479]. Also the integration-by-parts method [F.V. Tkachov, Phys. Lett. B 100 (1981) 65; K.G. Chetyrkin, F.V. Tkachov, Nucl. Phys. B 192 (1981) 159] is used to reduce indices (powers of scalar propagators) of the scalar diagrams. The obtained recurrence relations for one-loop integrals are explicitly evaluated for 5- and 6-point functions. In the latter case the corresponding Gram determinant vanishes identically for d=4, which greatly simplifies the application of the recurrence relations.

Differential equations for Feynman graph amplitudes

It is by now well established that, by means of the integration by part identities, all the integrals occurring in the evaluation of a Feynman graph of given topology can be expressed in terms of a few independent master integrals. It is shown in this paper that the integration by part identities can be further used for obtaining a linear system of first-order differential equations for the master integrals themselves. The equations can then be used for the numerical evaluation of the amplitudes as well as for investigating their analytic properties, such as the asymptotic and threshold behaviours and the corresponding expansions (and for analytic integration purposes, when possible). The new method is illustrated through its somewhat detailed application to the case of the one-loop self-mass amplitude, by explicitly working out expansions and quadrature formulas, both in arbitrary continuous dimensionn and in then→4 limit. It is then shortly discussed which features of the new method are expected to work in the more general case of multi-point, multi-loop amplitudes.

A rigorous derivation of the gaussian representation for propagators in scalar field theories

Using the well-known α-representation for Feynman graph amplitudes in scalar ∅ 3 and ∅ 4 theories, we show that the α-parameter can everywhere on a given one-particle irreducible graph G with an infinite number of loops be taken equal to some constant α∼ 1 l where l is the number of propagators of G (once an integration over an over-all scale for all the α's is performed). This is done for any number of euclidean dimensions ( d<6 for ∅ 3 and d<4 for ∅ 4), except the one where such a theory is renormalizable.