M. Mahbub

Isolating the Λ(1405) in lattice QCD.

The odd-parity ground state of the Λ baryon lies surprisingly low in mass. At 1405 MeV, it lies lower than the odd-parity ground-state nucleon, even though it has a valence strange quark. Using the PACS-CS (2+1)-flavor full-QCD ensembles, we employ a variational analysis using source and sink smearing to isolate this elusive state. For the first time we reproduce the correct level ordering with respect to nearby scattering thresholds. With a partially quenched strange quark to produce the appropriate kaon mass, we find a low-lying, odd-parity mass trend consistent with the experimental value.

Isolating Excited States of the Nucleon in Lattice QCD

We discuss a robust projection method for the extraction of excited-state masses of the nucleon from a matrix of correlation functions. To illustrate the algorithm in practice, we present results for the positive parity excited states of the nucleon in quenched QCD. Using eigenvectors obtained via the variational method, we construct an eigenstate-projected correlation function amenable to standard analysis techniques. The method displays its utility when comparing results from the fit of the projected correlation function with those obtained from the eigenvalues of the variational method. Standard nucleon interpolators are considered, with 2x2 and 3x3 correlation matrix analyses presented using various combinations of source-smeared, sink-smeared, and smeared-smeared correlation functions. Using these new robust methods, we observe a systematic dependency of the extracted nucleon excited-state masses on source- and sink-smearing levels. To the best of our knowledge, this is the first clear indication that a correlation matrix of standard nucleon interpolators is insufficient to isolate the eigenstates of QCD.

Positive-parity excited states of the nucleon in quenched lattice QCD

Positive-parity spin-

Low-lying odd-parity states of the nucleon in lattice QCD

\frac{1}{2}

Ordering of spin- excitations of the nucleon in lattice QCD

excitations of the nucleon are explored in lattice QCD. The variational method is used in this investigation and several correlation matrices are employed. As our focus is on the utility and methodology of the variational approach, we work in the quenched approximation to QCD. Various sweeps of Gaussian fermion-field smearing are applied at the source and at the sink of

Structure and flow of the nucleon eigenstates in lattice QCD

{\ensuremath{\chi}}_{1}{\overline{\ensuremath{\chi}}}_{1}

Accessing high momentum states in lattice QCD

and

Isolating the Roper Resonance in Lattice QCD

{\ensuremath{\chi}}_{1}{\ensuremath{\chi}}_{2}

Extracting Low‐Lying Lambda Resonances Using Correlation Matrix Techniques

correlation functions to obtain a large basis of operators. Using several different approaches for constructing basis interpolators, we demonstrate how improving the basis can split what otherwise might be interpreted as a single state into multiple eigenstates. Consistency of the extracted excited energy states are explored over various dimensions of the correlation matrices. The use of large correlation matrices is emphasized for the reliable extraction of the excited eigenstates of QCD.

Nucleon excitations in 2+1 flavor QCD

M. Selim Mahbub, Waseem Kamleh, Derek B. Leinweber, Peter J. Moran, and Anthony G. Williams