Gluon gravitational form factors of the nucleon and the pion from lattice QCD

Abstract

A future Electron-Ion Collider will enable the gluon contributions to the gravitational form factors of the proton to be constrained experimentally for the first time. Here, the first calculation of these form factors from lattice Quantum Chromodynamics is presented. The calculations use a largerthan-physical value of the light quark mass corresponding to mπ ∼ 450 MeV. All three form factors, which encode the momentum-dependence of the lowest moment of the spin independent gluon generalised parton distributions and are related to different components of the energy-momentum tensor, are resolved. In particular, the gluon D-term form factor, related to the pressure distribution inside the nucleon, is determined for the first time. The gluon contributions to the two gravitational form factors of the pion are also determined, and are compared to existing lattice determinations of the quark contributions to the gravitational form factors and to phenomenology. arXiv:1810.04626v1 [hep-lat]

Mechanical properties of particles

“…• New lattice measurements of the pressure and shear force distributions (for quarks and gluons) in the pion and in the nucleon were reported in Refs. [6,7]. We note however that the results for the shear force distribution s(r) obtained in these papers seems violate the stability constraint 2 3 s(r) + p(r) > 0.…”

Complete flavor decomposition of the spin and momentum fraction of the proton using lattice QCD simulations at physical pion mass

“…60as suggested in Ref. [32]. Since our action is not smeared, one would need to apply reweighting in the calculation of both the Z factors and matrix elements.…”

“…Results on the gluon momentum fraction using dynamical gauge field configurations appeared only recently. They used mostly simulations with larger than physical pion mass relying on chiral extrapolations to obtain final results [32][33][34]. A first attempt to fully decompose the nucleon spin was carried out by χQCD [35] in the quenched approximation, followed by a study of the gluon spin [36] using 2 þ 1 dynamical fermions on four lattice spacings and four volumes including an ensemble with physical values for the quark masses.…”

Near threshold J/ψ and ϒ photoproduction at JLab and RHIC

“…Very recently, the GlueX collaboration at JLab has reported new data for the threshold cross section which significantly differ from the Cornell data [2]. Meanwhile, there have been theory developments on the renormalization of the trace anomaly [7,8] as well as the first lattice calculation of the gluon 'D-term' gravitational form factors [9]. In view of these, we feel it is necessary to revise the calculations and fits in [1].…”