Physics Letters B 781 (2018) 270–278
Contents lists available at ScienceDirect
Physics Letters B
www.elsevier.com/locate/physletb
The ε-form of the differential equations for Feynman integrals
in the elliptic case
Luise Adams, Stefan Weinzierl
∗
PRISMA Cluster of Excellence, Institut für Physik, Johannes Gutenberg-Universität Mainz, D-55099 Mainz, Germany
a r t i c l e i n f o a b s t r a c t
Article history:
Received
15 February 2018
Received
in revised form 25 March 2018
Accepted
1 April 2018
Available
online 5 April 2018
Editor:
A. Ringwald
Feynman integrals are easily solved if their system of differential equations is in ε-form. In this letter
we show by the explicit example of the kite integral family that an ε-form can even be achieved, if
the Feynman integrals do not evaluate to multiple polylogarithms. The ε-form is obtained by a (non-
algebraic)
change of basis for the master integrals.
© 2018 The Author(s). Published by Elsevier B.V. This is an open access article under the CC BY license
(http://creativecommons.org/licenses/by/4.0/). Funded by SCOAP
3
.
1. Introduction
Precision calculations in high-energy physics rely on our ability to compute Feynman loop integrals. In recent years, the method
of differential equations [1–10] has become a powerful computational tool for these integrals. This is in particular true, if the system of
differential equations is in ε-form (where ε denotes the regularisation parameter of dimensional regularisation) [7]. The Feynman integrals
under consideration are then easily expressed in terms of iterated integrals of the integration kernels appearing on the right-hand side
of the differential equations. In the past this method has been applied very successfully to Feynman integrals which evaluate to multiple
polylogarithms. In this case, the integration kernels, when written as one-forms are given by
dt
t − z
i
, (1)
where the z
i
’s are (possibly complex) parameters, called the letters of the multiple polylogarithms. Thus, the computation of Feynman
integrals is reduced to finding a transformation, which brings the system of differential equations to ε-form [11–20]. In the case where
the Feynman integrals evaluate to multiple polylogarithms, one considers rational or algebraic transformations.
In
this letter we would like to point out, that the fact that a system of differential equations for Feynman integrals can be brought to
an ε-form is not restricted to Feynman integrals which evaluate to multiple polylogarithms. We do this by giving an explicit example. We
consider the Feynman integrals associated to the kite integral. In this family of Feynman integrals we have an elliptic sub-sector, given by
the sunrise integral. The ε-form is obtained by a suitable change of basis for the master integrals. In contrast to the multiple polylogarithm
case, this transformation is not rational or algebraic, but transcendental. For example, the new first master integral in the sunrise sector is
given as the original sunrise integral divided by a period of the elliptic curve, the latter being a transcendental function of the kinematic
invariants. It is the extension of basis transformations from algebraic/rational transformations to transcendental transformations which
allows us to obtain an ε-form. On the other hand, the change of basis of master integrals is natural: Let us stay with the example of
the first master integral in the sunrise sector: The new first master integral in this sector is nothing than the original integral divided by
the ε
0
-term of the maximal cut for a specific contour, a procedure also often used in the multiple polylogarithm case to obtain master
integrals of uniform weight. Part of the results presented here follow from our earlier work on the sunrise/kite-integrals [21–44], and in
particular from refs. [42,43]. The transition to the ε-form is made possible by a suitable definition of the second master integral in the
sunrise sector. On a technical level, this is the essential new result of this article.
*
Corresponding author.
E-mail
address: weinzierl @uni -mainz .de (S. Weinzierl).
https://doi.org/10.1016/j.physletb.2018.04.002
0370-2693/
© 2018 The Author(s). Published by Elsevier B.V. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/). Funded by
SCOAP
3
.