Lattice study of the nucleon excited states with domain wall fermions

Abstract

We present results of our numerical calculation of the mass spectrum for isospin one-half and spin one-half nonstrange baryons, i.e., the ground and excited states of the nucleon, in quenched lattice QCD. We use a new lattice discretization scheme for fermions, domain wall fermions, which possess almost exact chiral symmetry at nonzero lattice spacing. We make a systematic investigation of the negative-parity N* spectrum by using two distinct interpolating operators at ␤ϭ6/g 2 ϭ6.0 on a 16 3 ϫ32ϫ16 lattice. The mass estimates extracted from the two operators are consistent with each other. The observed large mass splitting between this state, N*(1535), and the positive-parity ground state, the nucleon N(939), is well reproduced by our calculations. We have also calculated the mass of the first positive-parity excited state and find that it is heavier than the negative-parity excited state for the quark masses studied.

Nucleon form factors with2+1flavor dynamical domain-wall fermions

“…The projection operator P 4 = (1 + γ 4 )/2 eliminates the contributions from the opposite-parity state for p 2 = 0 [42,43]. We use the standard proton operator,…”

Excited baryons from the FLIC fermion action

“…2. In addition to the FLIC and Wilson results from the present analysis, also shown are the DWF results [4], and results from an earlier analysis with Wilson fermions together with the operator product expansion [1]. The most striking feature of the data is the relatively large excitation energy of the N ′ , some 1 GeV above the nucleon.…”

“…1 we show the N and N * ( 1 2 − ) masses as a function of m 2 π for the new simulations with the FLIC action. For comparison, we also show results from earlier simulations with Wilson [6] and domain wall fermions (DWF) [4], and a nonperturbatively improved clover action [3] with different source smearing and volumes.…”

“…The most striking feature of the data is the relatively large excitation energy of the N ′ , some 1 GeV above the nucleon. It has been speculated that the χ p + 2 field may have overlap with the lowest 1 2 + excited state [4], however, there is little evidence that this state is the N * (1440). It is likely that the χ 2 interpolating field simply does not have good overlap with either the nucleon or the N * (1440), but rather a (combination of) excited 1 2 + state(s).…”

“…Another challenge is presented by the odd parity Λ * (1405), whose anomalously small mass has been interpreted as an indication of strong coupled channel effects, and a weak overlap with a three-valence quark state. While model studies have provided some understanding of the level orderings and mass splittings in the baryons spectrum, it is hoped that lattice QCD [1,2,3,4] will provide a definitive resolution of some of the outstanding issues.…”