35 research outputs found
Kira - A Feynman Integral Reduction Program
In this article, we present a new implementation of the Laporta algorithm to
reduce scalar multi-loop integrals---appearing in quantum field theoretic
calculations---to a set of master integrals. We extend existing approaches by
using an additional algorithm based on modular arithmetic to remove linearly
dependent equations from the system of equations arising from
integration-by-parts and Lorentz identities. Furthermore, the algebraic
manipulations required in the back substitution are optimized. We describe in
detail the implementation as well as the usage of the program. In addition, we
show benchmarks for concrete examples and compare the performance to Reduze 2
and FIRE 5.
In our benchmarks we find that Kira is highly competitive with these existing
tools.Comment: 37 pages, 3 figure
Numerical integration of massive two-loop Mellin-Barnes integrals in Minkowskian regions
Mellin-Barnes (MB) techniques applied to integrals emerging in particle
physics perturbative calculations are summarized. New versions of AMBRE
packages which construct planar and nonplanar MB representations are shortly
discussed. The numerical package MBnumerics.m is presented for the first time
which is able to calculate with a high precision multidimensional MB integrals
in Minkowskian regions. Examples are given for massive vertex integrals which
include threshold effects and several scale parameters.Comment: Proceedings for 13th DESY Workshop on Elementary Particle Physics:
Loops and Legs in Quantum Field Theory (LL2016), final PoS versio
New prospects for the numerical calculation of Mellin-Barnes integrals in Minkowskian kinematics
During the last several years remarkable progress has been made in numerical
calculations of dimensionally regulated multi-loop Feynman diagrams using
Mellin-Barnes (MB) representations. The bottlenecks were non-planar diagrams
and Minkowskian kinematics. The method has been proved to work in highly
non-trivial physical application (two-loop electroweak bosonic corrections to
the decay), and cross-checked with the sector decomposition
(SD) approach. In fact, both approaches have their pros and cons. In
calculation of multidimensional integrals, depending on masses and scales
involved, they are complementary. A powerful top-bottom approach to the
numerical integration of multidimensional MB integrals is automatized in the
MB-suite AMBRE/MB/ MBtools/MBnumerics/CUBA. Key elements are a dedicated use of
the Cheng-Wu theorem for non-planar topologies and of shifts and deformations
of the integration contours. An alternative bottom-up approach starting with
complex 1-dimensional MB-integrals, based on the exploration of steepest
descent integration contours in Minkowskian kinematics, is also discussed.
Short and long term prospects of the MB-method for multi-loop applications to
LHC- and LC-physics are discussed.Comment: Presented at the Epiphany Cracow conference 2017, refs adde
Complete electroweak two-loop corrections to Z boson production and decay
This article presents results for the last unknown two-loop contributions to
the -boson partial widths and -peak cross-section. These are the
so-called bosonic electroweak two-loop corrections, where "bosonic" refers to
diagrams without closed fermion loops. Together with the corresponding results
for the -pole asymmetries , which have been presented earlier,
this completes the theoretical description of -boson precision observables
at full two-loop precision within the Standard Model. The calculation has been
achieved through a combination of different methods: (a) numerical integration
of Mellin-Barnes representations with contour rotations and contour shifts to
improve convergence; (b) sector decomposition with numerical integration over
Feynman parameters; (c) dispersion relations for sub-loop insertions. Numerical
results are presented in the form of simple parameterization formulae for the
total width, , partial decay widths
,
branching ratios and the hadronic peak cross-section,
. Theoretical intrinsic uncertainties from missing higher
orders are also discussed.Comment: 10 page
The two-loop electroweak bosonic corrections to
The prediction of the effective electroweak mixing angle in the Standard Model at two-loop accuracy has now been completed
by the first calculation of the bosonic two-loop corrections to the vertex. Numerical predictions are presented in the form of a fitting
formula as function of and , . For central input values, we obtain a relative correction of
, amounting
to about a quarter of the fermionic corrections, and corresponding to
. The integration of the
corresponding two-loop vertex Feynman integrals with up to three dimensionless
parameters in Minkowskian kinematics has been performed with two approaches:
(i) Sector decomposition, implemented in the packages FIESTA 3 and SecDec 3,
and (ii) Mellin-Barnes representations, implemented in AMBRE 3/MB and the new
package MBnumerics.Comment: 14 pp; v2: some explanations and Tab.2 added, version published in
PL
Integral Reduction with Kira 2.0 and Finite Field Methods
We present the new version 2.0 of the Feynman integral reduction program Kira
and describe the new features. The primary new feature is the reconstruction of
the final coefficients in integration-by-parts reductions by means of finite
field methods with the help of FireFly. This procedure can be parallelized on
computer clusters with MPI. Furthermore, the support for user-provided systems
of equations has been significantly improved. This mode provides the
flexibility to integrate Kira into projects that employ specialized reduction
formulas, direct reduction of amplitudes, or to problems involving linear
system of equations not limited to relations among standard Feynman integrals.
We show examples from state-of-the-art Feynman integral reduction problems and
provide benchmarks of the new features, demonstrating significantly reduced
main memory usage and improved performance w.r.t. previous versions of Kira
Electroweak bosonic 2-loop corrections to the Z-pole precision observables
Precision measurements of properties of the electroweak W - and Z-bosons provide strong constraints on the Standard Model (SM) and extensions thereof. This sensitivity crucially depends on the availability of accurate theoretical predictions for these quantities, including higher-order radiative corrections. This contribution gives a brief overview of available calculations and compares the estimate of theory uncertainties with the projected experimental precision of future e(+)e(-) collider proposals. Then it is shown how numerical Mellin-Barnes integrals can be used to evaluate higher-order loop integrals that depend on three or more independent mass and momentum scales. As a concrete physical application, the complete two-loop corrections to Z -> b (b) over bar are considered