Hi @Wenjie,
thank you for reaching out on the ROOT forum as chatgtp is not always an answer… especially in a bit more specialised cases as ROOT framework is.
You could indeed start by reviewing the details of the TTrees as was suggested above. What you want here is a sub-branch (or a leaf) and you are now only creating a Branch in your Tree, you can also follow this forum post for more details: TTree: creating a sub-branch of a branch from scratch
Cheers,
Marta