A Lattice Story of Proton Spin

Abstract

In this contribution, I summarized the recent Lattice QCD consensuses on the quark helicity, plus the investigations on the gluon helicity and orbital angular momenta. The preliminary nonperturbative normalized and renormalized Ji quark and gluon angular momentum results are also reported and compared with the previous 1-loop perturbative renormalized results.

What we know and what we don’t know about the proton spin after 30 years

“…A first complete study of the AM decomposition was made in REF. 84 , followed by a chiral dynamical simulation 94 . A first study at the physical quark mass appeared in REF.…”

Study of the pseudoscalar glueball in J/ψ radiative decays

“…As shown in Ref. [25,26], the matrix element related to the chiral anomaly can be sizeable, and obviously violated the OZI rule. In other words, the QCD U A (1) anomaly can enhance the coupling of η states to gluons, and this non-perturbative effect results in the violation of the OZI rule when flavor singlet pseudoscalar mesons are involved.…”

Orbital angular momentum of the proton and intrinsic five-quark Fock states

“…Here, a caution is in order: in the present model, contributions to the proton OAM are exclusively due to the intrinsic sea content qq, while LQCD approaches embody also extrinsic quarkantiquark pairs arising from the gluon splitting in the perturbative QCD regime (g → qq). Table III, we report results from two approaches [30,31]. The first remark is that contributions per flavor for light quarks are very different from our values, as well as from those obtained within LCCQM.…”

“…The meson-cloud picture, embodying the Nπ and ∆π Fock components in the proton, leads to a value [23] consistent with ours within 1σ. That is also the case with respect to the LQCD [30,31], albeit with rather large uncertainties. The light cone constituent quark model's outcome is compatible with ours, within 2 − 3σ.…”

“…Lorcé and Pasquini studied the Wigner distributions in the light cone constituent quark model( LCCQM) [28], reaching to a comparable value, L q = 0.126. Lattice QCD calculations is an ongoing long endeavor; see, e.g., [29,30]. Recently, the intrinsic sea flavor content includingū,d,s andc in the nucleon were investigated employing our formalism, within which all the possible five-quark Fock components in the nucleon wave function were taken into account [32,33], coupling between the threeand five-quark components was assumed to be via 3 P 0 quark-antiquark pair creation mech-anism [27], and the coupling strength was fixed by fitting [32,33] the sea flavor asymmetry of the proton [1].…”