Hadron Structure

Pion electromagnetic form factor from full lattice QCD
Jonna Koponen, Francis Bursa, Christine Davies, Rachel Dowdall
Mon, 16:30, Seminar Room C -- Parallels 2C (Slides)

We present the first calculation of the pion electromagnetic form factor at physical light quark masses. This form factor parameterises the deviations from the behaviour of a point-like particle when a photon hits the pion. These deviations result from the internal structure of the pion and can thus be calculated in QCD. We use three sets (different lattice spacings) of \(n_f = 2+1+1\) lattice configurations generated by the MILC collaboration. The Highly Improved Staggered Quark formalism (HISQ) is used for all of the sea and valence quarks. Using lattice configurations with u/d quark masses very close to the physical value is a big advantage, as we avoid the chiral extrapolation. We study the shape of the vector (\(f_+\)) form factor in the \(q^2\) range from 0 to -0.35 GeV\(^2\) and extract the mean square radius, \(\langle r^2_v \rangle\). The shape of the vector form factor and the resulting radius is compared with experiment. We also discuss the scalar form factor and radius extracted from that, which is not directly accessible to experiment.

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The scalar radius of the pion in two-flavor Wilson lattice QCD
Vera Guelpers, Georg von Hippel, Hartmut Wittig
Mon, 16:50, Seminar Room C -- Parallels 2C (Slides)

We present an update of our calculation of the scalar charge radius of the pion using \(N_f=2\) dynamical flavors of non-perturbatively \(O(a)\)-improved Wilson fermions, extending the calculation to a wider range of pion masses for a fixed lattice spacing \(a\). We find that the disconnected contribution to the scalar radius is not negligible especially for smaller pion masses, and is required in order to obtain the behavior expected from next-to-leading order (NLO) Chiral Perturbation Theory (\(\chi\)PT). The low energy constant \(\bar\ell_4\) is determined from a fit to NLO \(\chi\)PT.

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Lattice study of the Boer-Mulders transverse momentum distribution in the pion
Michael Engelhardt
Mon, 17:10, Seminar Room C -- Parallels 2C (Slides)

The Boer-Mulders transverse momentum-dependent parton distribution (TMD) characterizes polarized quark transverse momentum in an unpolarized hadron. Techniques previously developed for lattice calculations of nucleon TMDs are applied to the pion. These techniques are based on the evaluation of matrix elements of quark bilocal operators containing a staple-shaped Wilson connection. Results for the Boer-Mulders transverse momentum shift in the pion, obtained at a pion mass of 518 MeV, are presented and compared to corresponding results in the nucleon.

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Lattice study of quark distribution amplitudes in the pion and its excitations
Ekaterina Mastropas, David Richards
Mon, 17:30, Seminar Room C -- Parallels 2C (Slides)

Lattice QCD serves as a computational framework capable of predicting the spectrum of hadronic excitations from first principles. Our desire to describe the wealth of existing experimental data on the spectrum and to predict the outcomes of future experiments poses numerous challenges. Thus, obtaining an accurate resolution of excited states using methods of LQCD is complicated due to the faster decay of excited state correlation functions in Euclidean space in comparison with those of ground states, which we overcome through the use of anisotropic lattices with a finer temporal than spatial discretization. The aim of this project is to go beyond the spectrum to discern the structure of the states through the computation of the quark distribution amplitudes for both the ground and excited pion states on improved anisotropic lattices developed by the Hadron Spectrum Collaboration. Application of variational method allows us to extract the excited-state spectrum. When combined with undergoing parallel perturbative study of renormalization coefficients for quark bilinear operators, this work will enable us to explore the internal structure of the excited states, and to investigate the approach to a quark and gluon description of hadrons when probed at high-momentum transfers.

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Electric Polarizability of hadrons with nHYP-Clover fermions
Michael Lujan, Andrei Alexandru, Walter Freeman, Frank Lee
Mon, 18:10, Seminar Room C -- Parallels 2C (Slides)

Electric polarizability is an important parameter for the internal structure of hadrons. It quantifies the ability of the electric field to deform them. For the nucleon, it is very sensitive to the quark mass and is expected to diverge in the chiral limit. Previous studies of polarizabilities have been done at relatively heavy pion masses, leaving the chiral region largely unexplored. Here we present a study using two flavors of dynamical nHYP-clover fermions with two different dynamical pion masses (~ 250 and 300 MeV) and several partially quenched valence masses. In addition, we also study the volume dependence using elongated lattices where the elongation is in the direction of the electric field.

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Sea Contributions to the Electric Polarizability of Hadrons using Reweighting
Walter Freeman, Andrei Alexandru, Michael Lujan, Frank Lee
Mon, 18:30, Seminar Room C -- Parallels 2C (Slides)

One of the most challenging aspects of a physically-complete calculation of electric polarizability using the background field method is to capture the interaction of the sea quarks with the background electric field. One way to do this is via reweighting in the sea quark electric charge. However, the standard stochastic estimate of the weight factor is quite noisy. We instead use a perturbative estimator whose variance can be reduced by several techniques to create a power series for the weight factor in terms of the background field. We present results from this approach for the sea contributions to the nucleon and neutral pion polarizability, and discuss the cause of the large stochastic fluctuations in the estimator.

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Looking at the gluon moment of the nucleon with dynamical twisted mass fermions
Christian Wiese, Constantia Alexandrou, Vincent Drach, Kyriakos Hadjiyiannakou, Karl Jansen, Bartosz Kostrzewa
Tue, 14:00, Seminar Room B -- Parallels 3B (Slides)

To understand the structure of hadrons it is important to know the parton distribution function (PDF) of their constituents, i.e. quarks and gluons. In our work we aim at computing the first moment of the gluon PDF \(_g\) for the nucleon. We follow the approach of using the Feynman-Hellmann theorem, as suggested in arXiv:1205.6410, to compute the proton-gluon three-point function in order to extract the gluon moment. In this talk preliminary results computed on \(24^3 \times 48\) lattices will be presented employing \(N_f=2+1+1\) maximally twisted mass fermions. We will also report on an attempt to compute the three-point function with the direct method on similar lattices.

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Moments of structure functions for \(N_f=2\) near the physical point.
Sara Collins, Gunnar Bali, Benjamin Glässle, Johannes Najjar, Meinulf Göckeler, Rudolf Roedl, Andreas Schäfer, Wolfgang Soeldner, Andre Sternbeck
Tue, 14:20, Seminar Room B -- Parallels 3B (Slides)

We report on our on-going study of the lower moments of iso-vector polarised and unpolarised structure functions, \(g_A\) and \(\langle x\rangle_{u-d}\), for \(N_f=2\) non-perturbatively improved clover fermions. With pion masses which go down to about \(160\)~MeV, we systematically investigate finite volume and excited state contributions.

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Nucleon generalized form factors from lattice QCD near the physical quark mass
Andre Sternbeck, Gunnar Bali, Sara Collins, Benjamin Glässle, Johannes Najjar, Meinulf Göckeler, Rudolf Rödl, Andreas Schäfer, Wolfgang Söldner, Philipp Wein
Tue, 14:40, Seminar Room B -- Parallels 3B (Slides)

We present new \(N_f=2\) data for nucleon (generalized) form factors, varying volume, lattice spacing and pion mass, down to below 160 MeV. We will also include an update of our (direct) calculation of the nucleon sigma term for a range of pion mass values.

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Nucleon structure with twisted mass fermions
Constantia Alexandrou, Martha Constantinou, Vincent Drach, Karl Jansen, Christos Kallidonis, Giannis Koutsou
Tue, 15:00, Seminar Room B -- Parallels 3B (Slides)

We present results on the nucleon form factors, momentum fraction and helicity moment for Nf=2 and Nf=2+1+1 twisted mass fermions for a number of lattice sizes and lattice spacings. The implications of these results on the spin content of the nucleon are discussed. We also plan to show preliminary results for a new Nf=2 ensemble at the physical pion mass.

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Calculating the \(x\) Dependence of Nucleon Parton Distribution Functions
Huey-Wen Lin
Tue, 15:20, Seminar Room B -- Parallels 3B (Slides)

We present a first direct lattice-QCD calculation of the \(x\) dependence of nucleon structure functions. By taking a nucleon with a large momentum boost, we are able to connect light-cone quantities to lattice-QCD nonlocal but time-independent matrix elements. Since the largest attainable momentum is limited, we correct for the sizable leading momentum dependence. In this talk, we present an exploratory study of the quark density, helicity and transversity distributions using \(N_f=2+1+1\) HISQ lattice gauge ensembles and clover valence fermions at pion mass 310 MeV. We demonstrate that using this methodology, our distributions reproduce the first moments determined by conventional lattice methods using the same parameters. Even at this pion mass, we are able to make significant observations and predictions about the nucleon sea flavor structure.

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Nucleon transversity generalized form factors with twisted mass fermions
Martha Constantinou, Constantia Alexandrou, Karl Jansen, Giannis Koutsou, Haralambos (Haris) Panagopoulos
Tue, 15:40, Seminar Room B -- Parallels 3B (Slides)

We compare results extracted using the summation and plateau methods for a number of connected three-point functions associated with nucleon matrix elements. We present results on the nucleon tensor form factors and first moment of the transversity generalized parton distribution for Nf=2+1+1 twisted mass fermions. We also plan to add a new ensemble at the physical pion mass.

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The quark contents of the nucleon and their implication for dark matter search
Vincent Drach, Constantia Alexandrou, Martha Constantinou, Karl Jansen, Giannis Koutsou, Alejandro Vaquero
Tue, 16:20, Seminar Room B -- Parallels 4B (Slides)

We present results concerning the light and strange quark contents of the Nucleon using \(N_f=2+1+1\) flavours of maximally twisted mass fermions. The corresponding sigma-terms are casting light on the origin of the nucleon mass and their values are important to interpret experimental data from direct dark matter searches. We provide our results of the sigma terms and of the strangeness parameter including a detailed and comprehensive analysis of systematic uncertainties arising in our computations.

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Strange and Charm Spin in the Nucleon with Overlap Fermion
Ming Gong
Tue, 16:40, Seminar Room B -- Parallels 4B (Slides)

The strange and charm spin contributions to the nucleon spin are calculated with overlap fermion on DWF configurations. This work is done by calculating the disconnected three point correlation functions through the anomalous Ward identity. The quark loop of the pseuscalar density is calculated with low-mode average and grid-noise estimate for the high modes. The overlap operator is used for the topological charge density.

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Wave functions of the Nucleon and the \(N^*(1535)\)
Rainer Schiel, Vladimir Braun, Sara Collins, Meinulf Göckeler, Andreas Schäfer, Andre Sternbeck, Philipp Wein
Tue, 17:00, Seminar Room B -- Parallels 4B (Slides)

The wave functions of the nucleon and its parity partner, the N*(1535), in the infinite momentum frame can be computed from Lattice QCD. We have used Nf = 2 flavors of dynamical Clover fermions to determine the normalization constants of the leading and next-to-leading twist wave functions as well as moments of the leading twist wave functions, also known as distribution amplitudes. Here, we present our latest results which include data from lattices with almost physical pion mass and a variational method approach to improve the signal of the N* (1535) state.

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Neutron and proton EDM with \(N_f=2+1\) domain-wall fermion
Eigo Shintani, Tom Blum, Taku Izubuchi, Amarjit Soni
Tue, 17:20, Seminar Room B -- Parallels 4B (Slides)

We will present the recent calculation of neutron and proton EDM and EDM form factor with Nf=2+1 domain-wall fermion including the \(\theta\)-term. Applying the all-mode-averaging technique in this measurement enables us to have a solid estimate of EDM from lattice QCD. We also check the excited state contamination and variance of transfer momentum of EDM form factor. We will discuss the chiral behavior and future plans.

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Neutron Electric Dipole Moments from Beyond the Standard Model Physics
Tanmoy Bhattacharya, Vincenzo Cirigliano, Rajan Gupta
Tue, 17:40, Seminar Room B -- Parallels 4B (Slides)

Neutron Electric Dipole Moments (nEDM), a generic feature of CP-violation, is predicted to be very small in the Standard Model, but can be much larger in most extensions of the model. In this talk, I will discuss the classification of the CP violating operators up to dimension 6 that can give rise to nEDM, and describe their mixing and their renormalization structure in both dimensional and cutoff regularizations in general terms. Finally I will describe how to connect the dimension 5 operators, in particular, the Chromoelectric Dipole Moment of the quarks, between MSbar scheme and a Regularization Independent prescription, in the chiral limit.

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Computation of disconnected contributions to nucleon observables
Alejandro Vaquero, Constantia Alexandrou, Vincent Drach, Kyriacos Hadjiyiannakou, Karl Jansen, Giannis Koutsou
Thu, 14:00, Seminar Room B -- Parallels 7B (Slides)

We compare several methods for computing disconnected fermion loops contributing to nucleon three-point functions. We obtain high statistics results using \(N_f=2+1+1\) twisted mass fermions for which some of the methods developed are optimal. The computations are performed using code implemented for GPUs, which will be pesented in the dedicated code-development session.

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Nucleon structure from stochastic estimators
Johannes Najjar, Gunnar S. Bali, Sara Collins, Benjamin Glässle, Meinulf Göckeler, Rudolf Rödl, Andreas Schäfer, Andre Sternbeck, Wolfgang Söldner
Thu, 14:20, Seminar Room B -- Parallels 7B (Slides)

Using stochastic estimators for connected meson and baryon three-point functions has successfully been tried in the past years. Compared to the standard sequential source method we trade the freedom to compute the current-to-sink propagator independent of the hadron sink for additional stochastic noise in our observables. In the case of the nucleon we can use this freedom to compute many different sink-momentum/polarization combinations, which grants access to more virtualities. We will present preliminary results on the scalar, electro-magnetic and axial form factors of the nucleon in \(N_f=2+1\) lattice QCD.

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A high-statistics study of the nucleon axial charge and quark momentum fraction
Benjamin Jäger, Stefano Capitani, Thomas Rae, Michele Della Morte, Georg von Hippel, Bastian Knippschild, Harvey Meyer, Hartmut Wittig
Thu, 14:40, Seminar Room B -- Parallels 7B (Slides)

We present updated results for the nucleon axial charge [arXiv:1205.0180] using non-perturbatively O(a) improved Wilson fermions with two dynamical quarks on the CLS ensembles. This update includes a substantial increase of statistics (up to 4000 measurements) and four more chiral ensembles with pion masses down to 195 MeV. We show that excited states can be controlled by using the summed operator insertion method. In addition, we present preliminary results for quantities involving derivatives, such as the average momentum fraction carried by a quark in a nucleon.

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A high-statistics study of nucleon electromagnetic form factors
Thomas Rae, Stefano Capitani, Michele Della Morte, Georg von Hippel, Benjamin Jäger, Bastian Knippschild, Harvey Meyer, Hartmut Wittig
Thu, 15:00, Seminar Room B -- Parallels 7B (Slides)

We present updated results for the nucleon electromagnetic form factors, which include a significant increase in statistics for all ensembles (to up to 4000 measurements), as well as the addition of an ensemble with a near-physical pion mass (\(m_\pi\sim 195\) MeV). Our previous form factor calculations [arXiv:1211.1282] indicated a significant systematic effect due to excited state contaminations, similar to those observed for the nucleon axial charge [arXiv:1205.0180]. The new data allows us to perform a thorough study of the potential systematic effects encountered in the lattice extraction, through a comparison of the `plateau fit' method (using several different time separations between the operators at the source and sink (the largest \(\sim 1.1\) fm)) and the summed operator insertion method (which provides a mechanism to suppress the contamination). Any systematic effects will impact the \(Q^2\)-dependence of the form factors and derived quantities, both of which will be discussed. The measurements are calculated using the CLS ensembles with non-perturbatively \(O(a)\) improved Wilson fermions in \(N_f=2\) QCD.

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Nucleon axial charge in 2+1-flavor dynamical DWF lattice QCD (for RBC and UKQCD Collaborations)
Shigemi Ohta
Thu, 15:20, Seminar Room B -- Parallels 7B (Slides)

We report the current status of nucleon isovector axial charge, (\(g_A\)), and vector charge, (\(g_V\)), calculated using recent RBC/UKQCD 2+1-flavor dynamical domain-wall fermions (DWF) lattice QCD ensembles: with Iwasaki gauge action at lattice cutoff, (\(a^{-1}\)), of about 1.7 GeV, linear lattice extent, (L), of about 2.8 fm, pion mass, (\(m_\pi\)), of about 420 and 330 MeV, and with Iwasaki+DSDR gauge action at (\(a^{-1}\)) of about 1.4 GeV, (L) of 4.6 fm, (\(m_\pi\)) of 250 and 170 MeV. The calculations have been refined with enhanced statistics through successful application of the all-mode-averaging (AMA) technique. Precision agreement seen in the charge ratio, (\(g_A/g_V\)), of 1.17(2) for 420-MeV and 1.18(4) for 250-MeV ensembles that share the finite-size scaling parameter (\(m_\pi L\)) of about 5.8 further corroborate our scaling conjecture. We also present study on the dependence on source-sink separation that excludes excited-state contamination in these and earlier calculations of ours that share similarly careful planning and execution. We will also report the isovector quark momentum and helicity fraction moments of nucleon structure functions.

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Nucleon form factors with 2+1 flavors of domain wall fermions and All-Mode-Averaging
Meifeng Lin
Thu, 15:40, Seminar Room B -- Parallels 7B (Slides)

We report recent progress in the calculations of the isovector nucleon electromagnetic and axial form factors using 2+1 flavors of domain wall fermions at pion masses of 170 MeV and 250 MeV. The lattice size is fixed at \(32^3\times 64\) with a lattice cutoff scale of 1.37(1) GeV. For the 170-MeV ensemble, we employed the All-Mode-Averaging (AMA) technique, which led to roughly a factor of 20 improvement in computational efficiency and has reduced the statistical errors in our results significantly. We were also able to do calculations at different source-sink separations without much additional cost by reusing the low eigen-modes stored for the AMA calculations. We will present results for the isovector form factors and their derived quantities, including the Dirac and Pauli radii, anomalous magnetic moment, and the axial radius, and discuss the effects of possible excited-state contaminations. Prospects and future plans will also be briefly discussed.

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Nucleon form factors with light Wilson quarks
Jeremy Green, Michael Engelhardt, Stefan Krieg, John Negele, Andrew Pochinsky, Sergey Syritsyn
Thu, 16:30, Seminar Room B -- Parallels 8B (Slides)

Observables including nucleon isovector Dirac and Pauli form factors will be presented from calculations using 2+1 flavors of Wilson quarks. These include an ensemble with pion mass 149 MeV and box size 5.6 fm, which nearly eliminates the uncertainty associated with extrapolation to the physical pion mass. The results show agreement with experiment for the vector form factors, which occurs only when excited-state contributions are reduced.

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Probing the nucleon and its excitations in full QCD
Ben Owen, Waseem Kamleh, Derek Leinweber, Selim Mahbub, Ben Menadue
Thu, 16:50, Seminar Room B -- Parallels 8B (Slides)

In this presentation we discuss how correlation matrix techniques enable precision calculations of the form factors of hadronic ground state properties and new access to the corresponding properties in hadronic excitations. In particular, we will focus on the nucleon and present results for the electromagnetic form factors and axial charges of the ground state nucleon and its excitations using the PACS-CS (2+1)-flavor full-QCD ensembles.

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SU(3) flavour breaking and baryon structure
James Zanotti
Thu, 17:10, Seminar Room B -- Parallels 8B (Slides)

The QCDSF Collaboration has recently proposed a new method for performing nf=2+1 simulations along a quark mass trajectory that starts at the SU(3)-flavour symmetric limit and moves towards the physical point while keeping the singlet quark mass fixed. In this talk, I will report on how baryon octet matrix elements change as we move along this trajectory.

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Sigma-terms and axial charge for hyperons and charmed baryons
Kyriakos Hadjiyiannakou, Constantia Alexandrou, Karl Jansen, Christos Kallidonis, Alejandro Vaquero
Thu, 17:30, Seminar Room B -- Parallels 8B (Slides)

We discuss methods for the stochastic evaluation of baryon three-point functions using the nucleon as a test case. We present results on the sigma-terms and axial charge of hyperons and charmed baryons using \(N_f=2+1+1\) twisted mass fermions.

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Electromagnetic Structure of the \(\Lambda(1405)\)
Ben Menadue, Derek Leinweber, Waseem Kamleh, Selim Mahbub, Ben Owen
Thu, 17:50, Seminar Room B -- Parallels 8B (Slides)

The electromagnetic form factors of a hadron can be used to provide insight into its internal structure and quark distribution. Continuing to build on our successful technique to isolate the otherwise-elusive \(\Lambda(1405)\) using correlation matrix techniques and multiple source and sink smearings, we present calculations of quark sector contributions to the electric and magnetic form factors of the \(\Lambda(1405)\). We use the PACS-CS (2+1)-flavour full-QCD ensembles available through the ILDG. Our preliminary results are consistent with the development of an important \(\bar{K}N\) molecular-type contribution to the as the pion mass approaches the physical value.

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Determination of Delta resonance parameters from lattice QCD
Marcus Petschlies, Constantia Alexandrou, John Negele, Alexei Strelchenko, Antonios Tsapalis
Thu, 18:10, Seminar Room B -- Parallels 8B (Slides)

We discuss a method suitable for extracting the resonance parameters of unstable baryons in lattice QCD. The method is applied to the strong decay of the Delta to a pion-nucleon state, for which case we extract the pion-nucleon - Delta coupling constant and Delta decay width.

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Tests of the vacuum polarization fits for the muon \(g-2\)
Santiago Peris, Maarten Golterman, Kim Maltman
Fri, 14:00, Seminar Room B -- Parallels 9B (Slides)

Using accurate experimental spectral data, supplemented by a well motivated (and phenomenologically successful) parameterization for the high-s region not covered by the data, we construct a physically constrained model of the isospin-one vector channel polarization function. At low euclidean \(Q^2\), the model is strongly dominated by the measured experimental data, and allows us to explore the systematic error associated with the \(Q^2\) fit in existing lattice determinations of the hadronic vacuum polarization contribution to \((g-2)_\mu\). We find that a final error in this quantity of the order of a few percent is not presently possible, given the lack of precise information about momentum of the order of the muon mass.

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Leading-order hadronic contribution to the anomalous magnetic moment of the muon from \(N_f=2+1+1\) twisted mass fermions
Grit Hotzel, Florian Burger, Xu Feng, Karl Jansen, Marcus Petschlies, Dru Renner
Fri, 14:20, Seminar Room B -- Parallels 9B (Slides)

We present results for the leading anomalous QCD correction to the magnetic moment of the muon including the first two generations of quarks as dynamical degrees of freedom. Several light quark masses are examined in order to yield a controlled extrapolation to the physical pion mass. We analyse ensembles for three different lattice spacings and several volumes in order to investigate lattice artefacts and finite-size effects, respectively. We also plan to provide preliminary results for this quantity for two flavours of mass-degenerate quarks at the physical value of the pion mass.

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First-order hadronic contributions to muon \(g-2\) from HEX-smeared clover fermions
Eric Gregory, Zoltan Fodor, Christian Hoelbling, Stefan Krieg, Laurent Lellouch, Rehan Malek, Craig McNeile, Kalman Szabo
Fri, 14:40, Seminar Room B -- Parallels 9B (Slides)

We present preliminary lattice results for the leading-order hadronic contribution to the muon anomalous magnetic moment, calculated with hex-smeared clover fermions. In our analysis we include 2+1-flavor ensembles with pions at the physical mass.

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The hadronic vacuum polarization with twisted boundary conditions
Christopher Aubin, Thomas Blum, Maarten Golterman, Santiago Peris
Fri, 15:00, Seminar Room B -- Parallels 9B (Slides)

The leading-order hadronic contribution to the anomalous magnetic moment is given by a weighted integral over the subtracted hadronic vacuum polarization. This integral is dominated by euclidean momenta of order the muon mass which are not available on current lattice volumes with periodic boundary conditions. Twisted boundary conditions can in principle help access momenta of any size even in a finite volume. We investigate the implementation of twisted boundary conditions both numerically (using all-mode averaging for improved statistics) and analytically, and present our initial results.

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Computing the Adler function from vacuum polarization
Hanno Horch, Gregorio Herdoiza, Andreas Jüttner, Michele Della Morte, Benjamin Jäger, Hartmut Wittig
Fri, 15:20, Seminar Room B -- Parallels 9B (Slides)

We use lattice data for the hadronic vacuum polarization tensor to study the associated Ward identities and to determine the Adler function. The vacuum polarization tensor is computed from a combination of point-split and local vector currents, using two flavours of Wilson fermions. Partially twisted boundary conditions are employed to obtain a fine momentum resolution. The approach towards the continuum Ward identities can be monitored using data computed for three values of the lattice spacing. We determine the Adler function from the derivative of the vacuum polarization tensor. A large range of momenta is considered to analyze the size of cutoff effects.

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Adler function and hadronic vacuum polarization from lattice vector correlators in the time-momentum representation
Anthony Francis, Harvey Meyer, Hartmut Wittig, Benjamin Jäger
Fri, 15:40, Seminar Room B -- Parallels 9B (Slides)

We study a representation of the hadronic vacuum polarization based on the time-momentum representation of the vector correlator. This representation suggests a way to compute the hadronic vacuum polarization and the associated Adler function for any value of virtuality, irrespective of the flavor structure of the current. We present results on both of these phenomenologically important functions, derived from local-conserved two-point lattice vector correlation functions, computed on a subset of light two-flavor ensembles made available to us through the CLS effort. In addition we comment on the finite-size effects of our particular representation of the Adler function based on theoretical arguments.

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Finite volume renormalization scheme for fermionic operators
Kostas Orginos, Christopher Monahan
Poster Session

We present a proposal for a new finite volume renormalization scheme. This scheme, much like the Schrodinger functional method, allows for a non-perturbative determination of the running of operators using a step scaling approach and is based on the Wilson flow applied to both fermion and gauge fields. We give some preliminary results for quark bilinears in the quenched approximation and for matching to the MS-bar scheme.

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Fitting the lattice vacuum polarisation function to perturbation theory
Gregorio Herdoiza, Michele Della Morte, Hanno Horch, Benjamin Jäger, Andreas Jüttner, Hartmut Wittig
Poster Session

The hadronic vacuum polarisation function computed with two flavours of improved Wilson fermions is studied in the large momentum region. Three values of the lattice spacing are used to monitor the size of discretisation effects. A comparison to perturbative QCD expressions including the contributions from the Operator Product Expansion is performed. The dominant systematic effects present in this matching to perturbation theory are explored.

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First and second moments of the disconnected sea partons from overlap fermion on DWF configurations
Mingyang Sun, Ming Gong, Keh-Fei Liu
Poster Session

We present a calculation of \(\langle x\rangle\) and \(\langle x^2 \rangle\) of the proton for the disconnected insertion. We adopt overlap fermion on 2 + 1 flavor domain-wall fermion configurations on the \(24^3 \times 64\) lattice. Smeared sources are used for nucleon propagator. Low-mode average togather with an even-odd grid source and time-dilution with stochastic noise for the high modes are utilized to calculate the quark loop.

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Fitting strategies to extract the axial charge of the nucleon from lattice QCD
Jiayu Hua, Georg von Hippel, Benjamin Jäger, Harvey Meyer, Thomas Rae, Hartmut Wittig
Poster Session

We report on our fit methods for the nucleon axial charge \(g_A\) in QCD with two flavours of dynamical quarks. The plateau method, the summation method and a new "midpoint" method are used to investigate contributions from excited states which affect the determination of \(g_A\). We also present a method to perform correlated fits when the standard estimator for the inverse of the covariance matrix becomes unstable.

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Pion structure from lattice QCD
Narjes Javadi-Motaghi, Gunnar Bali, Sara Collins, Benjamin Glaessle, Meinulf Göckeler, Wolfgang Söldner, Andre Sternbeck
Poster Session

We report on lattice calculations of the lowest moment of pion structure functions and (generalized) form factors at several values of momentum transfer. We use Nf=2 flavors of O(a) improved Wilson-Clover fermions. Our preliminary results are for pion masses down to 160 MeV.

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Time-like pion form factor in lattice QCD
Xu Feng, Sinya Aoki, Shoji Hashimoto, Takashi Kaneko
Poster Session

We present a lattice QCD calculation of the time-like pion form factor with 2+1-flavors of overlap fermions. In a finite box, the virtual-photon-to-two-pion amplitude is altered by a finite-size effect. Using the method proposed by Meyer and its extension to various moving frames, we can determine these finite-size corrections properly and thus relate the finite-volume calculation of the two-pion decay amplitude to the wanted time-like pion form factor in the infinite volume.

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