In addition to the above mentioned answers: I wanted to start a job with a simple parameter passed to a second pipeline and found the answer on http://web.archive.org/web/20160209062101/https://dzone.com/refcardz/continuous-delivery-with-jenkins-workflow
So i used:
stage ('Starting ART job') {
build job: 'RunArtInTest', parameters: [[$class: 'StringParameterValue', name: 'systemname', value: systemname]]
}