We calculate the intrinsic quark spin contribution to the total proton spin using overlap valence quarks on three ensembles of 2 + 1-flavor RBC/UKQCD domain-wall configurations with different lattice spacings. The lowest pion mass of the ensembles is around 171 MeV, which is close to the physical point. With overlap fermions and a topological charge derived from the overlap operator, we verify the anomalous Ward identity between nucleon states with momentum transfer. Both the connected and the disconnected insertions of the axial-vector current are calculated. For the disconnected-insertion part, the cluster-decomposition error reduction technique is utilized for the lattice with the largest volume and the error can be reduced by 10%–40%. Nonperturbative renormalization is carried out and the final results are all reported in the MS¯ scheme at 2 GeV. We determine the total quark spin contribution to the nucleon spin to be ΔΣ = 0.405(25)(37), which is consistent with the recent global fitting result of experimental data. The isovector axial coupling we obtain in this study is g3A = 1.254(16)(30), which agrees well with the experimental value of 1.2723(23).
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Funded by SCOAP3.
This work is supported in part by the U.S. DOE Grant No. DE-SC0013065. Y. Y. is supported by the U.S. National Science Foundation under Grant No. PHY 1653405 “CAREER: Constraining Parton Distribution Functions for New-Physics Searches.”This research used resources of the Oak Ridge Leadership Computing Facility at the Oak Ridge National Laboratory, which is supported by the Office of Science of the U.S. Department of Energy under Contract No. DE-AC05-00OR22725. This work used Stampede time under the Extreme Science and Engineering Discovery Environment (XSEDE), which is supported by National Science Foundation Grant No. ACI-1053575. We also thank the National Energy Research Scientific Computing Center (NERSC) for providing HPC resources that have contributed to the research results reported within this paper. We acknowledge the facilities of the USQCD Collaboration used for this research in part, which are funded by the Office of Science of the U.S. Department of Energy.
Liang, Jian; Yang, Yi-Bo; Draper, Terrence; Gong, Ming; and Liu, Keh-Fei, "Quark Spins and Anomalous Ward Identity" (2018). Physics and Astronomy Faculty Publications. 630.