Nucleon form factors from lattice QCD at the physical point

by Dr Shoichi Sasaki (Department of Physics, Tohoku University)

Room 213, 2F Main Research Building, (RIKEN, 2-1 Hirosawa, Wako, Saitama)

Room 213, 2F Main Research Building,

RIKEN, 2-1 Hirosawa, Wako, Saitama


RIKEN Seminar

Data: May 24 (Fri.), from 14:00
Place: Main Research Building, room 213
Speaker: Prof. Shoichi Sasaki (Tohoku University)
Title: Nucleon form factors from lattice QCD at the physical point

The nucleon vector and axial elastic form factors are good probes to
investigate the internal structure of the nucleon. Although great theoretical
and experimental efforts have been devoted to improving our knowledge of
the nucleon structure, there are several unsolved problems associated with
fundamental properties of the proton and neutron. The proton radius puzzle,
where high-precision measurements of the proton's electric charge radius
from the muonic hydrogen ($?(J\mu H$) Lamb shift disagree with well established?(B
results of both electron-proton scattering and hydrogen spectroscopy, is currently
one of the most intriguing problems in this field. The neutron lifetime puzzle,
where the discrepancy between the results of beam experiments and storage
experiments remains unsolved, is another open question that deserves further
investigation in terms of the nucleon axial-vector coupling ($g_A$).
In this talk, we report our recent results of the nucleon iso-vector form factors
measured on a large-volume lattice $(10.8~{?(J\rm fm})^4$  at the physical point in 2+1?(B
flavor QCD. The configurations are generated with the stout-smeared $O(a)$
improved Wilson quark action and Iwasaki gauge action at $?(J\beta=6/g^2=1.82$,?(B
which corresponds to the lattice spacing of 0.085 fm. The pion mass at the
simulation point is about 135 MeV. A large spatial volume of $(10.8~{?(J\rm fm})^3$?(B
allows us to investigate the form factors at small momentum transfer region.
We obtain the electric and magnetic form factors and their RMS radii which
are evaluated from the slope of the respective form factor at the zero momentum
transfer. We find that our results for the electric RMS charge radius seem
to favor the experimental result of the $?(J\mu H$ spectroscopy within 1-sigma?(B
error, though it is still too early to draw any definitive conclusion. We also
obtain the axial-vector coupling and the axial RMS radius from the axial-vector
form factor. Although the 2% precision of our $g_A$ value is an order-of-magnitude
larger than the experimental one, our result of the axial RMS radius that achieves the
7% precision is comparable with the experimental one. 



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Strangeness Nuclear Physics Laboratory