Do Gluons Carry Half of the Nucleon Momentum?

Abstract

We examine the conventional picture that gluons carry about half of the nucleon momentum in the asymptotic limit. We show that this large fraction is due to an unsuitable definition of the gluon momentum in an interacting theory. If defined in a gauge-invariant and consistent way, the asymptotic gluon momentum fraction is computed to be only about one-fifth. This result suggests that the asymptotic limit of the nucleon spin structure should also be reexamined. A possible experimental test of our finding is discussed in terms of novel parton distribution functions.

Glue Spin of the Proton

“…(15). The a (fm) S G (p = 0) S G (p = 2 GeV) 0.112(3) 0.075(4) 0.047(7) 0.084(2) 0.100 (7) 0.084 (10) extrapolated results at the physical point of the pion mass, for different momenta on two ensembles, are listed in Table 2 and plotted in Fig. 4.…”

The nucleon spin decomposition: News and experimental implications

“…There are essentially four kinds of proton spin decompositions into quark/gluon and spin/OAM contributions [7,8], referred to as the Jaffe-Manohar [5], Chen et al [1,2], Ji [6] and Wakamatsu [9,10] decompositions, and given by…”

“…The question of how to decompose the proton spin into measurable quark/gluon and spin/orbital angular momentum (OAM) contributions has been revived half a decade ago by Chen et al [1,2]. They challenged the textbook knowledge [3][4][5][6] by providing a gauge-invariant decomposition of the gluon angular momentum into spin and OAM contributions.…”

Orbital angular momentum and generalized transverse momentum distribution

“…A different perspective is provided in [28], as the gluon spin operator in [5,6] is shown to be equal to that defined from the first moment of ∆g(x, Q 2 ) in the IMF limit, thus it can be used for the lattice calculation of ∆G. This idea is further developed in [29], and an attempt to calculate ∆G in lattice QCD has been carried out very recently [30,31].…”

“…To fully understand the proton spin structure, one needs to find the spin and OAM operators whose matrix elements give the partonic contributions that are both measurable in experiments and calculable in theory, especially, in lattice QCD. In this work, we address the question of orbital contributions, and show that the OAM operators in the gauge-invariant sum rule for nucleon spin of Chen et al [5,6], when boosted to the infinite momentum frame (IMF), are equal to those derived from generalized transverse momentum distributions (GTMD) that can be measured in hard exclusive processes. The operators in [5,6] can be defined locally on the lattice, and therefore our work paves the way for a well-defined non-perturbative calculation of the measurable quark and gluon OAM.…”