View source for One-loop provider

A '''one-loop provider''' (or '''one-loop program''') is a specialised software for generating one-[[Loop|loop]] [[Amplitude|amplitudes]] for [[perturbative quantum field theory]] calculations. They are a special case of [[external amplitude providers]]. One-loop amplitudes contribute to [[next-to-leading order]] (and higher order) [[Cross section|cross sections]] as well as the [[Leading order|leading-order]] cross sections of [[loop-induced]] processes. Most one-loop providers can also compute other amplitudes, but their strength lies in loop amplitudes, which are typically much harder to compute than [[Tree level|tree-level]] amplitudes.

One-loop providers are often used as external software to provide one-loop amplitudes for [[Event generator|event generators]].


== List of one-loop providers ==
* [[OpenLoops]]
* [[GoSam]]
* [[MadGraph5_aMC@NLO]]
* [[VBFNLO]]

== Use from other software ==
The following event generators use or support the use of external one-loop providers:
* [[Herwig]] 7<ref>The Herwig Collaboration: [http://herwig.hepforge.org/tutorials/installation/prerequisite.html#external-amplitude-providers Herwig 7 installation notes]</ref>
* [[Sherpa]]<ref>Sherpa contributors: [http://sherpa.hepforge.org/doc/SHERPA-MC-2.2.0.html#External-one_002dloop-ME Sherpa 2.2.0 online manual]</ref>

== References ==
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