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Register a new userNucleon, $Delta$ and $Omega$ excited states in $N_f=2+1$ lattice QCD
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The energies of the excited states of the Nucleon, $Delta$ and $Omega$ are computed in lattice QCD, using two light quarks and one strange quark on anisotropic lattices. The calculation is performed at three values of the light quark mass, corresponding to pion masses $m_{pi}$ = 392(4), 438(3) and 521(3) MeV. We employ the variational method with a large basis of interpolating operators enabling six energies in each irreducible representation of the lattice to be distinguished clearly. We compare our calculation with the low-lying experimental spectrum, with which we find reasonable agreement in the pattern of states. The need to include operators that couple to the expected multi-hadron states in the spectrum is clearly identified.
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We investigate the excited states of the nucleon using $N_f=2$ twisted mass gauge configurations with pion masses in the range of about 270 MeV to 450 MeV and one ensemble of $N_f=2$ Clover fermions at almost physical pion mass. We use two different sets of variational bases and study the resulting generalized eigenvalue problem. We present results for the two lowest positive and negative parity states.
We compute the axial, scalar, tensor and pseudoscalar isovector couplings of the nucleon as well as the induced tensor and pseudoscalar charges in lattice simulations with $N_f=2$ mass-degenerate non-perturbatively improved Wilson-Sheikholeslami-Wohlert fermions. The simulations are carried out down to a pion mass of 150 MeV and linear spatial lattice extents of up to 4.6 fm at three different lattice spacings ranging from approximately 0.08 fm to 0.06 fm. Possible excited state contamination is carefully investigated and finite volume effects are studied. The couplings, determined at these lattice spacings, are extrapolated to the physical pion mass. In this limit we find agreement with experimental results, where these exist, with the exception of the magnetic moment. A proper continuum limit could not be performed, due to our limited range of lattice constants, but no significant lattice spacing dependence is detected. Upper limits on discretization effects are estimated and these dominate the error budget.
The chirally improved (CI) fermion action allows us to obtain results for pion masses down to 320 MeV on (in lattice units) comparatively small lattices with physical extent of 2.4 fm. We use differently smeared quarks sources to build sets of several interpolators. The variational method then leads to excellent ground state masses for most mesons and baryons. The excited state signals weaken in quality towards smaller quark masses. In particular the excited baryons come out too high.
We compute various (generalized) isovector charges of the octet baryons. These include $g_A$, $g_T$ and $g_S$ as well as the unpolarized, polarized and transversity parton distribution function (PDF) momentum fractions $langle xrangle_{u^+-d^+}$, $langle xrangle_{Delta u^--Delta d^-}$ and $langle xrangle_{delta u^+-delta ^+}$. The simulations are carried out on a subset of the (isospin symmetric) $N_f=2+1$ flavour Coordinated Lattice Simulations (CLS) gauge ensembles with lattice spacings ranging from $aapprox 0.086,$fm down to $aapprox 0.050,$fm. First results on the breaking of flavour symmetry and the low energy constants $F$ and $D$ are presented. While SU(3) flavour symmetry violations are found to be sizeable for $g_A=langle 1rangle_{Delta u^+-Delta d^+}$, these are quite small for $g_T=langle 1rangle_{delta u^--delta d^-}$ and $langle xrangle_{u^+-d^+}$.
We calculate the strange quark content of the nucleon in 2+1-flavor lattice QCD. Chirally symmetric overlap fermion formulation is used to avoid the contamination from up and down quark contents due to an operator mixing between strange and light scalar operators, bar{s}s and bar{u}u+bar{d}d. At a lattice spacing a=0.112(1) fm, we perform calculations at four values of degenerate up and down quark masses, which cover a range of the pion mass M_pi simeq 300-540 MeV. We employ two different methods: one is a direct method where we calculate the strange quark content by directly inserting the strange scalar operator. The other is an indirect method where the quark content is extracted from a derivative of the nucleon mass in terms of the strange quark mass. With these two methods we obtain consistent results with each other. Our best estimate f_{T_s}=0.009(15)(16) is in good agreement with our previous studies in two-flavor QCD.
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