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Register a new userAxial charges of excited nucleons from CI-fermions
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We report lattice QCD results on the axial charges of ground and excited nucleon states of both parities. This is the first study of these quantities with approximately chiral (CI) fermions. Two energy levels in the range of the negative parity resonances N*(1535) and N*(1650) are observed and we determine the axial charge for both. We obtain a small axial charge for one of them, which is consistent with the chiral symmetry restoration in this state as well as with the small axial charge of the N*(1535) predicted within the quark model. This result agrees with the findings of Takahashi et al. obtained with Wilson quarks which violate chiral symmetry for finite lattice spacing. At the same time for the other observed negative parity state we obtain a large axial charge, that is close to the axial charge of the nucleon. This is in disagreement both with the quark model prediction as well as with the chiral restoration but allows for an interpretation as an s-wave {pi} N state.
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Complete flavour decompositions of the scalar, axial and tensor charges of the proton, deuteron, diproton and $^3$He at SU(3)-symmetric values of the quark masses corresponding to a pion mass $m_pisim806$ MeV are determined using lattice QCD. At the physical quark masses, the scalar charges constrain mean-field models of nuclei and the low-energy interactions of nuclei with potential dark matter candidates. The axial and tensor charges of nuclei constrain their spin content, integrated transversity and the quark contributions to their electric dipole moments. External fields are used to directly access the quark-line connected matrix elements of quark bilinear operators, and a combination of stochastic estimation techniques is used to determine the disconnected sea-quark contributions. Significant nuclear modifications are found, with particularly large, O(10%), effects in the scalar charges. Typically, these nuclear effects reduce the effective charge of the nucleon (quenching), although in some cases an enhancement is not excluded. Given the size of the nuclear modifications of the scalar charges resolved here, contributions from correlated multi-nucleon effects should be quantified in the analysis of dark matter direct-detection experiments using nuclear targets.
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We report on recent results of the QCDSF/UKQCD Collaboration on investigations of baryon structure using configurations generated with N_f=2+1 dynamical flavours of O(a) improved Wilson fermions. With the strange quark mass as an additional dynamical degree of freedom in our simulations we avoid the need for a partially quenched approximation when investigating the properties of particles containing a strange quark, e.g. the hyperons. In particular, we will focus on the nucleon and hyperon axial charges and quark momentum fractions.
We report on lattice QCD calculations of the nucleon isovector axial, scalar, and tensor charges. Our calculations are performed on two 2+1-flavor ensembles generated using a 2-HEX-smeared Wilson-clover action at the physical pion mass and lattice spacings $aapprox$ 0.116 and 0.093 fm. We use a wide range of source-sink separations - eight values ranging from roughly 0.4 to 1.4 fm on the coarse ensemble and three values from 0.9 to 1.5 fm on the fine ensemble - which allows us to perform an extensive study of excited-state effects using different analysis and fit strategies. To determine the renormalization factors, we use the nonperturbative Rome-Southampton approach and compare RI-MOM and RI-SMOM intermediate schemes to estimate the systematic uncertainties. Our final results are computed in the MS-bar scheme at scale 2 GeV. The tensor and axial charges have uncertainties of roughly 4%, $g_T=0.972(41)$ and $g_A=1.265(49)$. The resulting scalar charge, $g_S=0.927(303)$, has a much larger uncertainty due to a stronger dependence on the choice of intermediate renormalization scheme and on the lattice spacing.
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