The electromagnetic structure of hadrons provide important information about the internal quark structure of hadrons and the underlying dynamics of nonperturbative QCD. Recently, there has been a considerable interest in the study of electromagnetic structure of hadrons using lattice QCD. There are two methods available to study electromagnetic structure on the lattice: the form-factor method, which involves the evaluation of three-point functions, and the background-field method, where two-point functions are calculated in the presence of an external field. A particular advantage of the background-field technique is that one can get the magnetic form factor at zero momentum transfer directly, without an extrapolation in Q2.
In order to perform a full lattice calculation of the magnetic moment, one requires both connected and disconnected contributions to be evaluated. This will allow one to report both isoscalar and isovector combinations of the nucleon form factors — in contrast to just the isovector term accessed in connected-only studies. Further, there has been a considerable interest, both experimentally and theoretically, in the study of the strangeness content of the nucleon. Numerous experiments have been performed, where the most comprehensive knowledge to date is available at Q2 = 0.1 GeV2. Ongoing and future measurements at Jefferson Lab and Mainz will continue to map the knowledge of strangeness form factors over a wider range of Q2.
The recent efforts of Leinweber et al., based on quenched simulations, have received considerable attention, and it would be interesting to probe the features of this result in a dynamical simulation. An ab initio calculation of the strangeness content necessarily requires the evaluation of a disconnected insertion. To date, disconnected contributions to the nucleon matrix elements have been primarily based on the calculation of 3-point functions, and have been found to be generally quite noisy.
One group has recently studied baryon magnetic moments using external-field technology in the quenched approximation. Here we wish to extend upon this investigation to dynamical gauge configurations on realistic volumes to be generated by Edwards et al. While for the connected diagram this is a straightforward task, a naive extension of the external-field method to study disconnected diagrams would suggest coming at the cost of generating new dynamical configurations. Here we propose a novel technique to couple the external field to the sea quarks through an appropriate reweighting of the untouched gauge fields, eliminating the need to generate a new ensemble of gauge configurations. This approach is similar to that of Allton et al. used to study QCD at finite density.
We will calculate magnetic moments of baryons both for octet and decuplet baryon sectors. This will be quite useful to constrain various models. Moreover, an accurate determination of the flavor structure in the valence sector alone, when combined with experimental measurements, can yield important information on the disconnected diagrams and thereby the strangeness content.
The primary focus of our disconnected study is to compute the scalar and vector channels from which one can get ss content, the πN σ-term, and the strangeness magnetic moment. If we can demonstrate success of the reweighting technique for the disconnected insertions, we will reuse the same matrix inversions to investigate further disconnected contributions, such as the quark spin content, orbital angular momentum and neutron electric dipole moment. |
