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Gunnar Bali

Publications and source records attributed to Gunnar Bali.

At least 19 recordsLinked to original sources

Nucleon-sigma terms at $m_\pi=222 ~\rm MeV$ with a variational analysis from lattice QCD

Nucleon sigma terms are important for the decomposition of the nucleon mass and for searches for new physics beyond the Standard Model involving scalar interactions. A persistent tension between lattice QCD and phenomenological determinations may be due to uncontrolled excited-state contamination in lattice QCD analyses. In previous work at $m_\pi=429~\rm MeV$, we showed that this contamination is dominated by $N\sigma$ states and can be strongly suppressed with a variational analysis using $N$ and $N\sigma$ operators. In this talk, we present preliminary results at $m_\pi=222~\rm MeV$, where the $\sigma$ becomes unstable and decays into $\pi\pi$. We investigate whether the same small variational basis can remove the expected $N\pi\pi$ contamination through the overlap of the $N\sigma$ operator with these states.

hep-lat

Future Requirements of Lattice Field Theory Calculations on European High-Performance Computing Facilities

Lattice field theory provides a first-principles framework for studying properties of strongly interacting quantum field theories in elementary particle physics. Researchers in lattice field theory are also among the largest and most efficient users of high- performance computing resources in fundamental science. In this contribution, we outline the computational profile of lattice QCD, from gauge-field generation to large-scale measurements, and discuss the main hardware, software, and human resource requirements needed to sustain progress on current and future European HPC infrastructures.

hep-lat

Nucleon sigma terms with a variational analysis from Lattice QCD

We determine the nucleon-sigma terms from lattice QCD. We find that the dominant excited state contamination in the nucleon three-point function with a scalar current is due to the transition between the nucleon and a S-wave scattering state of a nucleon and a scalar (sigma) meson. In this proof-of-concept study, we analyse a single $N_f=3$ ensemble with the unphysically large pion mass $M_\pi=429$ MeV. Excited state contamination is substantially reduced compared to the standard method when employing nucleon-sigma type interpolating operators within a generalised eigenvector analysis.

hep-lat

Progress on nucleon transition matrix elements with a lattice QCD variational analysis

Nucleon weak matrix elements can be extracted from nucleon correlation functions with lattice QCD simulations. The signal-to-noise ratio prohibits the analysis at large source-sink separations and as a consequence, excited state contamination affects the extraction of the nucleon matrix elements. Chiral perturbation theory (ChPT) suggests that the dominant contamination in some of these channels is due to $Nπ$ states where the pion carries the same momentum of the current. In this talk, we report updates on the variational analysis with $qqq$-operators (nucleon-like) and $(qqq)(\bar{q}q)$-operators (nucleon-pion-like) where we report for the first time some preliminary results of $\langle Nπ| \mathcal{J}| N \rangle $, modulo some kinematic and volume factors, and we compare the results against ChPT. This pilot study is performed on a CLS ensemble with $N_f=3$, $m_π\approx 420~\mathrm{MeV}$, $a\approx 0.1~\mathrm{fm}$ and $T=2L\approx 4.8~\mathrm{fm}$.

hep-lat

Toward $N$ to $Nπ$ matrix elements from lattice QCD

QCD matrix elements of axial and vector currents between nucleons are required for the Monte Carlo reconstruction of the energy of neutrinos that are detected in long baseline oscillation experiments in the quasi-elastic regime. The cleanest approach for determining the axial matrix elements is lattice QCD. However, the extraction of these from the corresponding correlation functions is complicated by very large excited state contributions, that are related to transitions from the nucleon to a nucleon-pion pair. In this pilot study with a pion mass $m_π= 429~ \mathrm{MeV}$, we demonstrate for the first time that these contributions can be removed by including five-(anti)quark operators into the basis of interpolators used to create the nucleon. The same techniques will be needed to compute transition matrix elements between the nucleon and nucleon-pion scattering states that are relevant in the resonance production regime.

hep-lat

Provenance for Lattice QCD workflows

We present a provenance model for the generic workflow of numerical Lattice Quantum Chromodynamics (QCD) calculations, which constitute an important component of particle physics research. These calculations are carried out on the largest supercomputers worldwide with data in the multi-PetaByte range being generated and analyzed. In the Lattice QCD community, a custom metadata standard (QCDml) that includes certain provenance information already exists for one part of the workflow, the so-called generation of configurations. In this paper, we follow the W3C PROV standard and formulate a provenance model that includes both the generation part and the so-called measurement part of the Lattice QCD workflow. We demonstrate the applicability of this model and show how the model can be used to answer some provenance-related research questions. However, many important provenance questions in the Lattice QCD community require extensions of this provenance model. To this end, we propose a multi-layered provenance approach that combines prospective and retrospective elements.

hep-lat

Sigma terms of the baryon octet in $N_\mathrm{f} = 2+1$ QCD with Wilson quarks

A lot of progress has been made in the direct determination of nucleon sigma terms. Using similar methods, we consider the sigma terms of the other octet baryons as well. These are determined on CLS gauge field ensembles employing the Lüscher-Weisz gluon action and the Sheikholeslami-Wohlert fermion action with $N_\mathrm{f} = 2 + 1$. The ensembles have pion masses ranging from ${410}\,\mathrm{MeV}$ down to the physical value and lattice spacings covering a range between ${0.098}\,\mathrm{fm}$ and ${0.039}\,\mathrm{fm}$. We present some preliminary results for the pion and strange sigma terms and compare to indirect determinations. To do so, we discuss multi-state fits to tackle the well-known problem of excited state contamination comparing the ratio and summation methods also including priors.

hep-lat

Lattice gauge ensembles and data management

The generation of ensembles of gauge configurations is a considerable expense. The preservation and curation of these ensembles constitutes a valuable shared resource for the lattice field theory community. The organizers of Lattice 2022 dedicated a parallel session to the presentation of gauge ensembles and their generation, plans for ensemble publication and data management/storage activities of different collaborations. A summary of the twelve contributions is presented here.

hep-lat

Determination of $m_c$ from $N_f = 2+1$ QCD with Wilson fermions

We present preliminary results for the charm quark mass in the $N_f=4$ RGI scheme. These were obtained using $N_f=2+1$ CLS ensembles with $\mathcal{O}(a)$ non-perturbatively improved Wilson fermions. We employed five different lattice spacings, ranging down to $a\lesssim 0.04$ fm and realized approximately physical pion and kaon masses, with ensembles spread out along three different trajectories in the quark mass plane, enabling a thorough study of the dependence on the lattice spacing and the light and strange sea quark masses. We sketch our analysis strategy and find that the dominant errors at present are due to the renormalization and scale setting uncertainties.

hep-lat

Towards the determination of sigma terms for the baryon octet on $N_\mathrm{f} = 2+1$ CLS ensembles

A lot of progress has been made in the determination of nucleon sigma terms. In this work we consider the sigma terms of the other octet baryons as well. These are determined on CLS gauge field ensembles employing the Lüscher-Weisz gluon action and the Sheikholeslami-Wohlert fermion action with $N_\mathrm{f} = 2 + 1$. The ensembles have pion masses ranging from ${410}\,\mathrm{MeV}$ down to the physical value and lattice spacings covering a range between ${0.09}\,\mathrm{fm}$ and ${0.04}\,\mathrm{fm}$. We present some preliminary results for $a\approx 0.06$ fm along a trajectory where the sum of the sea quark masses is kept constant, focusing on the quark mass dependence. We discuss multi-state fits to tackle the well-known problem of excited state contamination and detail how we analyse connected and disconnected contributions.

hep-lat

Nucleon Charges and Sigma Terms from $N_{f}=2+1$ QCD

We report on recent progress of our analysis of the nucleon sigma terms, as well as the singlet scalar, axial and tensor nucleon charges. These are determined employing the CLS gauge ensembles, which are generated using the Lüscher-Weisz gluon action and the non-perturbatively improved Sheikholeslami-Wohlert fermion action with $N_{f}=2+1$ dynamical fermions. For the ensembles analysed thus far, the pion masses range from 200 MeV up to 410 MeV, and the lattice spacings take five values between 0.09~fm and 0.04~fm. We have employed a variety of methods to determine the relevant correlation functions, including the sequential source method for connected contributions and the truncated solver method for disconnected contributions.

hep-lat

$η$ and $η^\prime$ masses and decay constants

We present preliminary results for the masses and decay constants of the $η$ and $η^\prime$ mesons using CLS $N_f = 2+1$ ensembles. One of the major challenges in these calculations are the large statistical fluctuations due to disconnected quark loops. We tackle these by employing a combination of noise reduction techniques which are tuned to minimize the statistical error at a fixed cost. On the analysis side we carefully assess excited states contributions by using a direct fit approach.

hep-lat

Nucleon generalized form factors from lattice QCD with nearly physical quark masses

We determine generalized form factors of the nucleon from lattice simulations with $N_f = 2$ mass-degenerate non-perturbatively improved Wilson-Sheikholeslami-Wohlert fermions down to a pion mass of 150 MeV. We also present the resulting isovector quark angular momentum. Possible excited-state contaminations are investigated with correlated simultaneous fits.

hep-lat

Topology and glueballs in SU(7) Yang-Mills with open boundary conditions

It is well known that the topology of gauge configurations generated in a Markov Monte-Carlo chain freezes as the continuum limit is approached. The corresponding autocorrelation time increases exponentially with the inverse lattice spacing, affecting the ergodicity of the simulation. In SU(N) gauge theories for large N this problem sets in at much coarser lattice spacings than for N=3. This means that its systematics can be studied on lattices that are smaller in terms of the number of lattice sites. It has been shown that using open boundary conditions in time allows instantons to be created and destroyed, restoring topological mobility and ergodicity. However, with open boundary conditions translational invariance is lost and the influence of spurious states propagating from the boundary into the bulk on physical correlators needs to be carefully evaluated. Moreover, while the total topological charge can be changed, the mobility of instantons across the lattice is still reduced. We consider SU(7) Yang-Mills theory and analyse its topological content in the periodic and open boundary condition cases. We also investigate scalar and pseudo-scalar glueball correlation functions.

hep-lat

QCD spectroscopy and quark mass renormalisation in external magnetic fields with Wilson fermions

We study the change of the QCD spectrum of low-lying mesons in the presence of an external magnetic field using Wilson fermions in the quenched approximation. Motivated by qualitative differences observed in the spectra of overlap and Wilson fermions for large magnetic fields, we investigate the dependence of the additive quark mass renormalisation on the magnetic field. We provide evidence that the magnetic field changes the critical quark mass both in the free case and on our quenched ensemble. The associated change of the bare quark mass with the magnetic field affects the spectrum and is relevant for the magnetic field dependence of a number of related quantities. We derive Ward identities for lattice and continuum QCD+QED from which we can extract the current quark masses. We also report on a first test of the tuning of the quark masses with the magnetic field using the current quark masses, and show that this tuning resolves the qualitative discrepancy between the Wilson and overlap spectra.

hep-lat

Hadronic vacuum polarization and muon g-2 from magnetic susceptibilities on the lattice

We present and test a new method to compute the hadronic vacuum polarization function in lattice simulations. This can then be used, e.g., to determine the leading hadronic contribution to the anomalous magnetic moment of the muon. The method is based on computing susceptibilities with respect to external electromagnetic plane wave fields and allows for a precision determination of both the connected and the disconnected contributions to the vacuum polarization. We demonstrate that the statistical errors obtained with our method are much smaller than those quoted in previous lattice studies, primarily due to a very effective suppression of the errors of the disconnected terms. These turn out to vanish within small errors, enabling us to quote an upper limit. We also comment on the accuracy of the vacuum polarization function determined from present experimental R-ratio data.

hep-lat

(Approximate) Low-Mode Averaging with a new Multigrid Eigensolver

We present a multigrid based eigensolver for computing low-modes of the Hermitian Wilson Dirac operator. For the non-Hermitian case multigrid methods have already replaced conventional Krylov subspace solvers in many lattice QCD computations. Since the $γ_5$-preserving aggregation based interpolation used in our multigrid method is valid for both, the Hermitian and the non-Hermitian case, inversions of very ill-conditioned shifted systems with the Hermitian operator become feasible. This enables the use of multigrid within shift-and-invert type eigensolvers. We show numerical results from our MPI-C implementation of a Rayleigh quotient iteration with multigrid. For state-of-the-art lattice sizes and moderate numbers of desired low-modes we achieve speed-ups of an order of magnitude and more over PARPACK. We show results and develop strategies how to make use of our eigensolver for calculating disconnected contributions to hadronic quantities that are noisy and still computationally challenging. Here, we explore the possible benefits, using our eigensolver for low-mode averaging and related methods with high and low accuracy eigenvectors. We develop a low-mode averaging type method using only a few of the smallest eigenvectors with low accuracy. This allows us to avoid expensive exact eigensolves, still benefitting from reduced statistical errors.

hep-lat