M. Selim Mahbub

Roper Resonance in 2+1 Flavor QCD

Abstract The low-lying even-parity states of the nucleon are explored in lattice QCD using the PACS-CS collaboration 2 + 1 -flavor dynamical-QCD gauge-field configurations made available through the International Lattice Datagrid (ILDG). The established correlation-matrix approach is used, in which various fermion source and sink smearings are utilized to provide an effective basis of interpolating fields to span the space of low-lying energy eigenstates. Of particular interest is the nature of the first excited state of the nucleon, the N 1 2 + Roper resonance of P 11 pion–nucleon scattering. The Roper state of the present analysis approaches the physical mass, displaying significant chiral curvature at the lightest quark mass. These full QCD results, providing the worldʼs first insight into the nucleon mass spectrum in the light-quark regime, are significantly different from those of quenched QCD and provide interesting insights into the dynamics of QCD.

Variational Approach to the Calculation of gA

Benjamin J. Owen, Jack Dragos, Waseem Kamleh, Derek B. Leinweber, M. Selim Mahbub, Benjamin J. Menadue, James M. Zanotti

Searching for low-lying multi-particle thresholds in lattice spectroscopy

Abstract We explore the Euclidean-time tails of odd-parity nucleon correlation functions in a search for the S -wave pion–nucleon scattering-state threshold contribution. The analysis is performed using 2 + 1 flavor 32 3 × 64 PACS-CS gauge configurations available via the ILDG. Correlation matrices composed with various levels of fermion source/sink smearing are used to project low-lying states. The consideration of 25,600 fermion propagators reveals the presence of more than one state in what would normally be regarded as an eigenstate-projected correlation function. This observation is in accord with the scenario where the eigenstates contain a strong mixing of single and multi-particle states but only the single particle component has a strong coupling to the interpolating field. Employing a two-exponential fit to the eigenvector-projected correlation function, we are able to confirm the presence of two eigenstates. The lower-lying eigenstate is consistent with a N π scattering threshold and has a relatively small coupling to the three-quark interpolating field. We discuss the impact of this small scattering-state contamination in the eigenvector projected correlation function on previous results presented in the literature.

Transition of

With the ongoing experimental interest in exploring the excited hadron spectrum, evaluations of the matrix elements describing the formation and decay of such states via radiative processes provide us with an important connection between theory and experiment. In particular, determinations obtained via the lattice allow for a direct comparison of QCD-expectation with experimental observation. Here we present the first light quark determination of the

\rho \rightarrow \pi \gamma

\rho \rightarrow \pi \gamma

in Lattice QCD

transition form factor from lattice QCD using dynamical quarks. Using the PACS-CS 2+1 flavour QCD ensembles we are able to obtain results across a range of masses, to the near physical value of

Parity-expanded variational analysis for nonzero momentum

m_\pi = 157

Probing the nucleon and its excitations in full QCD

MeV. An important aspect of our approach is the use of variational methods to isolate the desired QCD eigenstate. For low-lying states, such techniques facilitate the removal of excited state contributions. In principle the method enables one to consider arbitrary eigenstates. We find our results are in accord with the non-relativistic quark model for heavy masses. In moving towards the light-quark regime we observe an interesting quark mass dependence, contrary to the quark model expectation. Comparison of our light-quark result with experimental determinations highlights a significant discrepancy suggesting that disconnected sea-quark loop contributions may play a significant role in fully describing this process.

The Λ(1405) in Full QCD

Finn M. Stokes, Waseem Kamleh, Derek B. Leinweber, M. Selim Mahbub, Benjamin J. Menadue, and Benjamin J. Owen

Variational approach to the calculation of g A

We present a first look at the application of variational techniques for the extraction of the electromagnetic properties of an excited nucleon system. In particular, we include preliminary results for charge radii and magnetic moments of the proton, its first even-parity excitation and the