Reply to the Comment of X. Ji on "Do gluons carry half of the nucleon momentum?" [PRL 103:062001 (2009)]

Understand

We affirm that the proper momentum defined in [PRL 103:062001 (2009)] does respect exact gauge symmetry and is as measurable as the kinetic momentum.

• The physical part of the gauge field is also as measurable as the electromagnetic field.
• The Comment of Ji [arXiv:0910.5022] is due to a misunderstanding of our work, and a typical confusion of our gauge-invariant formalism with the specific Coulomb-gauge calculation.