Proton mass decompositions in the NNLO QCD

Abstract

Proton matrix elements of the QCD energy-momentum tensor are expressed by the gravitational form factors. The forward values of the gravitational form factors allow for a decomposition of the proton mass into the contributions from quarks and gluons, and the further subdivisions into the contributions from quark masses and from the QCD trace anomaly may be considered. We present the most recent evaluations of these mass decompositions, using a recent quantitative evaluation of the forward values of relevant gravitational form factors at the next-to-next-to-leading order (NNLO) QCD. We also calculate the renormalization scale dependence of each component within these decompositions. The results indicate several shortcomings of those decompositions. We propose a new decomposition according to the twist of the relevant operators, i.e., classifying the effects via the partonic motion according to asymptotic freedom and the parton correlation by non-perturbative QCD interactions. We demonstrate the improvements by our new decomposition. We also present the results for the pion, which exhibit quite different parton-correlation behaviors from the proton.

Figures (4)

• Figure 1: The ratios $M_j/M$ ($j=q, g, m, a$) in Ji's four-term decomposition of the proton mass, (\ref{['4term']}), in the ($\overline{\rm MS}$) NNLO QCD with the $n_f=3$ active flavors: (a) at $\mu=2$ GeV; (b) as functions of $\mu$ with the dashed line indicating the case (a).
• Figure 2: Each term, divided by $M$, of the new four-term decomposition of the proton mass, (\ref{['new4term']}), in the ($\overline{\rm MS}$) NNLO QCD with the $n_f=3$ active flavors: (a) at $\mu=2$ GeV; (b) as functions of $\mu$ with the dashed line indicating the case (a).
• Figure 3: Same as Fig. \ref{['piechart']}, but for the pion case.
• Figure 4: Same as Fig. \ref{['fig2']}, but for the pion case.