Hi @Jean_Letang,
Welcome to the Geant4 forum, and thanks for your question!
I actually wondered the same thing when trying something similar myself [1]. I tried replacing the physics list in the Hadr09 example with one of the built-in physics lists, but I ran into a segmentation fault.
From what I’ve seen, it looks like the ConstructProcess() method in the physics list depends on a Run Manager, so trying to use it without one may cause the crash.
I’m also curious if there’s a way to use a physics list in this case, or, if not, whether there’s a specific reason why it cannot work.
Maybe @ribon, as the original author of the example could help clarify this.
Best,
Alvaro