/usr/include/ThePEG/MatrixElement/ is in libthepeg-dev 1.8.0-3build1.
This file is owned by root:root, with mode 0o755.
..
/usr/include/ThePEG/MatrixElement/Amplitude.h
/usr/include/ThePEG/MatrixElement/ColourLines.h
/usr/include/ThePEG/MatrixElement/ColourLines.xh
/usr/include/ThePEG/MatrixElement/DiagramBase.fh
/usr/include/ThePEG/MatrixElement/DiagramBase.h
/usr/include/ThePEG/MatrixElement/DiagramBase.xh
/usr/include/ThePEG/MatrixElement/ME2to2Base.h
/usr/include/ThePEG/MatrixElement/ME2to2QCD.h
/usr/include/ThePEG/MatrixElement/MEBase.fh
/usr/include/ThePEG/MatrixElement/MEBase.h
/usr/include/ThePEG/MatrixElement/MEGG2GG.h
/usr/include/ThePEG/MatrixElement/MEGG2QQ.h
/usr/include/ThePEG/MatrixElement/MEGroup.fh
/usr/include/ThePEG/MatrixElement/MEGroup.h
/usr/include/ThePEG/MatrixElement/MENCDIS.h
/usr/include/ThePEG/MatrixElement/MEQG2QG.h
/usr/include/ThePEG/MatrixElement/MEQQ2GG.h
/usr/include/ThePEG/MatrixElement/MEQQ2QQ3.h
/usr/include/ThePEG/MatrixElement/MEQQ2qq2.h
/usr/include/ThePEG/MatrixElement/MEQq2Qq4.h
/usr/include/ThePEG/MatrixElement/MEee2gZ2qq.h
/usr/include/ThePEG/MatrixElement/MEqq2qq1.h
/usr/include/ThePEG/MatrixElement/ReweightBase.h
/usr/include/ThePEG/MatrixElement/ReweightConstant.h
/usr/include/ThePEG/MatrixElement/ReweightMinPT.h
/usr/include/ThePEG/MatrixElement/Tree2toNDiagram.h
/usr/include/ThePEG/MatrixElement/Tree2toNDiagram.xh