Chen Yang's Candidacy Talk

Title:  Is Trace Anomaly a Distinct Quantum Gluonic Contribution to the Nucleon Mass?

Abstract:
The nucleon mass structure and energy-momentum tensor (EMT) in quantum chromodynamics (QCD) were first studied in Ji's pioneering work, identifying the classical contributions of quarks and gluons together with a purely quantum component from the trace anomaly or the scalar gluon condensate, providing the mass generation scale. This picture has recently been critically examined in the literature, proposing to further divide the trace anomaly into the quark and gluon parts, which are combined with the classical pieces to form two-term decompositions of the QCD EMT and nucleon mass.

In this talk, I revisit these new ideas and point out that they have mixed the quark and gluon, classical and quantum, tensor and scalar contributions with fundamentally different physical origins. We argue that alternative decompositions through the EMT trace fail to exhibit an energy interpretation. The above arguments are further demonstrated by carefully studying the standard renormalization of the QCD EMT, where we show that these separations of the trace anomaly break the Lorentz symmetry as it fail to use the irreducible representations as the operator basis, and attempt to trace the ultraviolet origins of an infrared quantity. We conclude that the trace anomaly contribution is intrinsically a distinct, purely quantum gluonic component in the QCD EMT and nucleon mass.