Nucleon form factors with Nf=2 dynamical twisted mass fermions

TL;DR

This work demonstrates that $N_f=2$ twisted mass fermions can reliably compute nucleon isovector electromagnetic and axial form factors on lattices near $a\approx 0.09$ fm, across $m_\pi$ from about 260 to 470 MeV. By evaluating connected isovector diagrams and employing $Z_V=1$ and $Z_A\approx0.76$, along with smearing techniques and HB$χ$PT (and SSE) for chiral extrapolations, the authors extract $G_E(Q^2)$, $G_M(Q^2)$, $G_A(Q^2)$ and $G_p(Q^2)$, perform dipole fits to obtain radii and moments, and compare with domain-wall fermion results. The findings indicate a flatter $Q^2$-dependence than experiment, a weaker pion-mass dependence of the Dirac radius than predicted, and small finite-volume effects, with $g_A$ at the physical point around $1.1$–$1.13$ within extrapolation uncertainties. Overall, the results corroborate the viability of $N_f=2$ twisted mass fermions for nucleon structure studies and align with contemporary DWF benchmarks, informing future multi-spacing analyses to control cut-off effects.

Abstract

We present results on the electromagnetic and axial nucleon form factors using two degenerate flavors of twisted mass fermions on lattices of spatial size 2.1 fm and 2.7 fm and a lattice spacing of about 0.09 fm. We consider pion masses in the range of 260-470 MeV. We chirally extrapolate results on the nucleon axial ch arge, the isovector Dirac and Pauli root mean squared radii and magnetic moment to the physical point and co mpare to experiment.