The following articles may be of interest to you:
Starrett, Cortland. xtUML: Current and Next State of a Modeling Dialect as presented at the 2nd International Workshop on Executable Modeling at Models 2016 in Saint-Malo, France, October 3, 2016.
Håkan Burden, Rogardt Heldal, Toni Siljamaki. “Executable and Translatable UML – How Difficult Can it Be?” In APSEC 2011, 18th Asia-Pacific Software Engineering Conference, Ho Chi Minh City, Vietnam, 5-8 December 2011.
Starr Leon. Time and Synchronization in Executable UML: Rules for Platform Independent Concurrency [Internet]. Version 1. Articulate UML. 2009 May 28.
Starr Leon. What is Executable UML?: And why it’s better than the typical UML [Internet]. Version 1. Articulate UML. 2008 Sep 25.
Brown, Keith. Navigating the Rover with xtUML as presented at 2nd Workshop on Model-Driven Engineering Tools at MODELS 2018. Copenhagen, Denmark. 15 October 2018.
Demystifying UML by Stephen Mellor
Object_Action_Language_Reference_Manual
Using UML for Concurrent Design
Wikipedia Entry on executable UML