Showing papers by "Huey-Wen Lin published in 2017"

Pion Distribution Amplitude from Lattice QCD

Abstract: We present the first lattice-QCD calculation of the pion distribution amplitude using the large-momentum effective field theory (LaMET) approach, which allows us to extract light cone parton observables from a Euclidean lattice. The mass corrections needed to extract the pion distribution amplitude from this approach are calculated to all orders in ${m}_{\ensuremath{\pi}}^{2}/{P}_{z}^{2}$. We also implement the Wilson-line renormalization which is crucial to remove the power divergences in this approach, and find that it reduces the oscillation at the end points of the distribution amplitude. Our exploratory result at 310-MeV pion mass favors a single-hump form broader than the asymptotic form of the pion distribution amplitude.

Axial Vector Form Factors of the Nucleon from Lattice QCD

Abstract: We present results for the form factors of the isovector axial vector current in the nucleon state using large scale simulations of lattice QCD. The calculations were done using eight ensembles of gauge configurations generated by the MILC collaboration using the HISQ action with 2+1+1 dynamical flavors. These ensembles span three lattice spacings a≈0.06, 0.09, and 0.12 fm and light-quark masses corresponding to the pion masses Mπ≈135, 225, and 310 MeV. High-statistics estimates allow us to quantify systematic uncertainties in the extraction of GA(Q2) and the induced pseudoscalar form factor G˜P(Q2). We perform a simultaneous extrapolation in the lattice spacing, lattice volume and light-quark masses of the axial charge radius rA data to obtain physical estimates. Using the dipole ansatz to fit the Q2 behavior we obtain rA|dipole=0.49(3) fm, which corresponds to MA=1.39(9) GeV, and is consistent with MA=1.35(17) GeV obtained by the miniBooNE collaboration. The estimate obtained using the z-expansion is rA|z-expansion=0.46(6) fm, and the combined result is rA|combined=0.48(4) fm. Analysis of the induced pseudoscalar form factor G˜P(Q2) yields low estimates for gP* and gπNN compared to their phenomenological values. To understand these, we analyze the partially conserved axial current (PCAC) relation by also calculating the pseudoscalar form factor. We find that these low values are due to large deviations in the PCAC relation between the three form factors, and in the pion-pole dominance hypothesis.

Parton distributions and lattice QCD calculations: a community white paper

Abstract: In the framework of quantum chromodynamics (QCD), parton distribution functions (PDFs) quantify how the momentum and spin of a hadron are divided among its quark and gluon constituents. Two main approaches exist to determine PDFs. The first approach, based on QCD factorization theorems, realizes a QCD analysis of a suitable set of hard-scattering measurements, often using a variety of hadronic observables. The second approach, based on first-principle operator definitions of PDFs, uses lattice QCD to compute directly some PDF-related quantities, such as their moments. Motivated by recent progress in both approaches, in this document we present an overview of lattice-QCD and global-analysis techniques used to determine unpolarized and polarized proton PDFs and their moments. We provide benchmark numbers to validate present and future lattice-QCD calculations and we illustrate how they could be used to reduce the PDF uncertainties in current unpolarized and polarized global analyses. This document represents a first step towards establishing a common language between the two communities, to foster dialogue and to further improve our knowledge of PDFs.

Isovector charges of the nucleon from 2 + 1 -flavor QCD with clover fermions

Abstract: We present high-statistics estimates of the isovector charges of the nucleon from four $2+1$-flavor ensembles generated using Wilson-clover fermions with stout smearing and tree-level tadpole improved Symanzik gauge action at lattice spacings $a\ensuremath{\approx}0.127$ and 0.09 fm and with ${M}_{\ensuremath{\pi}}\ensuremath{\approx}280$ and 170 MeV. The truncated solver method with bias correction and the coherent source sequential propagator construction are used to cost-effectively achieve $O(1{0}^{5})$ measurements on each ensemble. Using these data, the analysis of two-point correlation functions is extended to include four states in the fits, and of three-point functions to three states. Control over excited-state contamination in the calculation of the nucleon mass, the mass gaps between excited states, and in the matrix elements is demonstrated by the consistency of estimates using this multistate analysis of the spectral decomposition of the correlation functions and from simulations of the three-point functions at multiple values of the source-sink separation. The results for all three charges, ${g}_{A}$, ${g}_{S}$ and ${g}_{T}$, are in good agreement with calculations done using the clover-on-HISQ lattice formulation with similar values of the lattice parameters.

Kaon Distribution Amplitude from Lattice QCD and the Flavor SU(3) Symmetry

Abstract: We present the first lattice-QCD calculation of the kaon distribution amplitude using the large-momentum effective theory (LaMET) approach. The momentum-smearing technique has been implemented to improve signals at large meson momenta. We subtract the power divergence due to Wilson line to high precision using multiple lattice spacings. The kaon structure clearly shows an asymmetry of the distribution amplitude around $x=1/2$, a clear sign of its skewness. We also study the leading SU(3) flavor symmetry breaking relations for the pion, kaon and eta meson distribution amplitudes, and the results are consistent with the prediction from chiral perturbation theory.

Gaussian-weighted Parton Quasi-distribution

Abstract: We propose a revised definition of quasi-distributions within the framework of large-momentum effective theory (LaMET) that improves convergence towards the large-momentum limit. Since the definition of quasi-distributions is not unique, each choice goes along with a specific matching function, we can use this freedom to optimize convergence towards the large-momentum limit. As an illustration, we study quasi-distributions with a Gaussian weighting factor that naturally suppresses long-range correlations, which are plagued by artifacts. This choice has the advantage that the matching functions can be trivially obtained from the known ones. We apply the Gaussian weighting to the previously published results for the nonperturbatively renormalized unpolarized quark distribution, and find that the unphysical oscillatory behavior is significantly reduced.

Operator classification for nonlocal quark bilinear on lattice

Abstract: We address the operator-mixing pattern of a class of nonlocal quark bilinear operators that involve a straight Wilson line in a spatial direction. In recent years, this type of nonlocal operator has received a lot of attention in the context of the lattice QCD calculations needed to extract the Bjorken-x dependence of the parton distribution functions (PDFs). The mixing is complicated by the breaking of symmetries on the lattice. Analyzing the behavior of the operators under the symmetries of the lattice action and the equation of motion, we classify the structure of O(a) relevant operators. Going beyond the symmetry point of view, we also investigate the perturbative structure of the O(a) operators due to lattice discretization effects. Since we need a large hadron momentum in the quasi-PDF approach to reduce higher-twist effects, the O(a)-improvement operator will be necessary to obtain a better determination of PDFs, and the analysis provided in this work gives a basis for the improvement.

Symmetry Properties of Nonlocal Quark Bilinear Operators on a Lattice

Abstract: Using symmetry properties, we determine the mixing pattern of a class of nonlocal quark bilinear operators containing a straight Wilson line along a spatial direction. We confirm the previous study that mixing among the lowest dimensional operators, which have mass dimension equals three, can occur if chiral symmetry is broken in the lattice action. For higher dimensional operators, we find that the dimension three operators will always mix with dimension four operators even if chiral symmetry is preserved. Also, the number of dimension four operators involved in the mixing is large hence it is impractical to remove the mixing by the improvement procedure. Our result is important to determining the Bjorken-$x$ dependence parton distribution functions using the quasi-distribution method on a Euclidean lattice. The requirement of using large hadron momentum in this approach makes the control of errors from dimension four operators even more important.

The PDFLattice2017 workshop: A summary report

Abstract: The workshop on Parton Distributions and Lattice Calculations in the LHC era (PDFLattice2017) was hosted at Balliol College, Oxford (UK), from 22$^{\rm nd}$ to 24$^{\rm th}$ March 2017. The workshop brought together the lattice-QCD and the global-fit physicists who devote their efforts to determine the parton distribution functions (PDFs) of the proton. The goals were to make the two communities more familiar between each other, review developments from both sides, and set precision targets for lattice calculations so that they can contribute, together with the forthcoming experimental input, to the next generation of PDF determinations. This contribution summarises the relevant outcome of the workshop, in anticipation of a thorough white paper.

The PDFLattice2017 workshop: a summary report

Abstract: The workshop on Parton Distributions and Lattice Calculations in the LHC era (PDFLattice2017) was hosted at Balliol College, Oxford (UK), from 22$^{\rm nd}$ to 24$^{\rm th}$ March 2017. The workshop brought together the lattice-QCD and the global-fit physicists who devote their efforts to determine the parton distribution functions (PDFs) of the proton. The goals were to make the two communities more familiar between each other, review developments from both sides, and set precision targets for lattice calculations so that they can contribute, together with the forthcoming experimental input, to the next generation of PDF determinations. This contribution summarises the relevant outcome of the workshop, in anticipation of a thorough white paper.

Nucleon Vector and Axial-Vector Form Factors

Abstract: We present a status report on the calculation of the isovector axial-vector and vector form factors of the nucleon using (i) eight ensembles of N_f = 2 + 1 + 1 HISQ fermions at lattice spacings a = 0.12, 0.09, and 0.06 fm and three values of light quark masses corresponding to M_\pi = 310, 220, 130 MeV generated by the MILC collaboration and (ii) four ensembles of clover fermions generated by the JLAB/W&M collaboration. In both cases, the clover action is used for valence quarks. Analysis of the Q^2 dependence of the form factors is carried out using the z−expansion and the dipole ansatz. The results for the charge radii and dipole masses are compared and found to be consistent between the two fit ansatz and between the two lattice formulations.

Some Statistics on Women in Lattice QCD

Abstract: We present a sampling of analyses concerning the gender ratio of plenary speakers during the years 2000--2016 and make comparisons with other conferences, such as the APS April meeting. We hope this will invite discussion of ideas for how to make our field more accessible to women and minorities. We are preparing for an in-depth survey of the lattice field and welcome any ideas or suggestions. To leave post-conference comments and read about news affecting women in our field, see our Facebook page: https://www.facebook.com/WLQCD