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A position space method for the nucleon magnetic moment in lattice QCD

Research paper by Constantia Alexandrou, Martha Constantinou, Giannis Koutsou, Konstantin Ottnad, Marcus Petschlies

Indexed on: 24 May '16Published on: 24 May '16Published in: High Energy Physics - Lattice



Abstract

The extraction of the magnetic form factor of the nucleon at zero momentum transfer is usually performed by adopting a parametrization for its momentum dependence and fitting the results obtained at finite momenta. We present a position space method that allows to remove the momentum prefactor in the form factor decomposition and hence compute the magnetic form factor directly at zero momentum without the need to assume a functional form for its momentum dependence. The method is explored on one ensemble using $N_f=2+1+1$ Wilson twisted mass fermions with a light quark mass corresponding to $M_\pi\approx373\mathrm{GeV}$ and a lattice spacing of $a\approx0.082\mathrm{fm}$. For the isovector magnetic form factor we obtain $G_M^\mathrm{isov}=4.45(17)_\mathrm{stat}(07)_\mathrm{sys}$ as our final result, closer to the experimental value than the results obtained from a standard dipole fit ansatz. In addition, we obtain estimates for the magnetic moment of the proton and the neutron, leading to $G_M^\mathrm{p}=2.73(9)_\mathrm{stat}(4)_\mathrm{sys}$ and $G_M^\mathrm{n}=-1.72(6)_\mathrm{stat}(3)_\mathrm{sys}$, respectively.