The art_root_io package contains utilities for interacting with ROOT. Among other facilities, it provides:
RootOutput, an art output module that persists data products to ROOT files (hereafter art/ROOT files),
RootInput, an art input source that can read the art/ROOT files, and
TFileService, an art service that provides a simple interface for making ROOT objects, and persisting them to a bare ROOT file in an organized fashion.