arXiv subjects

H. Stüben

Publications and source records attributed to H. Stüben.

At least 37 records · Page 2Linked to original sources

arXiv2022

The Compton Amplitude, lattice QCD and the Feynman-Hellmann approach

A major objective of lattice QCD is the computation of hadronic matrix elements. The standard method is to use three-point and four-point correlation functions. An alternative approach, requiring only the computation of two-point correlation functions is to use the Feynman-Hellmann theorem. In this talk we develop this method up to second order in perturbation theory, in a context appropriate for lattice QCD. This encompasses the Compton Amplitude (which forms the basis for deep inelastic scattering) and hadron scattering. Some numerical results are presented showing results indicating what this approach might achieve.

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arXiv2021

State mixing and masses of the $π^0$, $η$ and $η^\prime$ mesons from $n_f=1+1+1$ lattice QCD+QED

We present a lattice analysis of the light pseudoscalar mesons with consideration for the mixing between the flavour-neutral states $π^0$, $η$ and $η^\prime$. We extract the masses and flavour compositions of the pseudoscalar meson nonet in $n_f=1+1+1$ lattice QCD+QED around an SU(3)-flavour symmetric point, and observe flavour-symmetry features of the extracted data, along with preliminary extrapolation results for the flavour compositions at the physical point. A key result of this work is the observed mass splitting between the $π^0$ and $η$ on our ensembles, which is found to exhibit behaviour that is simply related to the corresponding flavour compositions.

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arXiv2021

Advances in lattice hadron physics calculations using the gradient flow

Lattice calculations of hadronic observables are aggravated by short-distance fluctuations. The gradient flow, which can be viewed as a particular realisation of the coarse-graining step of momentum space RG transformations, proves a powerful tool for evolving the lattice gauge field to successively longer length scales for any initial coupling. Already at small flow times we find the signal-to-noise ratio of two- and three-point functions significantly enhanced and the projection onto the ground state largely improved, while the effect on the hadronic observables considered here to be negligible. A further benefit is that far fewer conjugate gradient iterations are needed for the Wilson-Dirac inverter to converge. Additionally, we find the renormalisation constants of quark bilinears to be significantly closer to unity.

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arXiv2021

Tensor Charges and their Impact on Physics Beyond the Standard Model

The nucleon tensor charge, $g_T$, is an important quantity in the search for beyond the Standard Model tensor interactions in neutron and nuclear $β$-decays as well as the contribution of the quark electric dipole moment (EDM) to the neutron EDM. We present results from the QCDSF/UKQCD/CSSM collaboration for the tensor charge, $g_T$, using lattice QCD methods and the Feynman-Hellmann theorem. We use a flavour symmetry breaking method to systematically approach the physical quark mass using ensembles that span three lattice spacings.

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arXiv2021

Patterns of flavour symmetry breaking in hadron matrix elements involving u, d and s quarks

Using an SU(3)-flavour symmetry breaking expansion between the strange and light quark masses, we determine how this constrains the extrapolation of baryon octet matrix elements and form factors. In particular we can construct certain combinations, which fan out from the symmetric point (when all the quark masses are degenerate) to the point where the light and strange quarks take their physical values. As a further example we consider the vector amplitude at zero momentum transfer for flavour changing currents.

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arXiv2021

Charged multi-hadron systems in lattice QCD+QED

Systems with the quantum numbers of up to twelve charged and neutral pseudoscalar mesons, as well as one-, two-, and three-nucleon systems, are studied using dynamical lattice quantum chromodynamics and quantum electrodynamics (QCD+QED) calculations and effective field theory. QED effects on hadronic interactions are determined by comparing systems of charged and neutral hadrons after tuning the quark masses to remove strong isospin breaking effects. A non-relativistic effective field theory, which perturbatively includes finite-volume Coulomb effects, is analyzed for systems of multiple charged hadrons and found to accurately reproduce the lattice QCD+QED results. QED effects on charged multi-hadron systems beyond Coulomb photon exchange are determined by comparing the two- and three-body interaction parameters extracted from the lattice QCD+QED results for charged and neutral multi-hadron systems.

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arXiv2020

Lattice QCD evaluation of the Compton amplitude employing the Feynman-Hellmann theorem

The forward Compton amplitude describes the process of virtual photon scattering from a hadron and provides an essential ingredient for the understanding of hadron structure. As a physical amplitude, the Compton tensor naturally includes all target mass corrections and higher twist effects at a fixed virtuality, $Q^2$. By making use of the second-order Feynman-Hellmann theorem, the nucleon Compton tensor is calculated in lattice QCD at an unphysical quark mass across a range of photon momenta $3 \lesssim Q^2 \lesssim 7$ GeV$^2$. This allows for the $Q^2$ dependence of the low moments of the nucleon structure functions to be studied in a lattice calculation for the first time. The results demonstrate that a systematic investigation of power corrections and the approach to parton asymptotics is now within reach.

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arXiv2020

Electromagnetic contribution to $Σ$-$Λ$ mixing using lattice QCD+QED

Mixing in the $Σ^0$-$Λ^0$ system is a direct consequence of broken isospin symmetry and is a measure of both isospin-symmetry breaking as well as general SU(3)-flavour symmetry breaking. In this work we present a new scheme for calculating the extent of $Σ^0$-$Λ^0$ mixing using simulations in lattice QCD+QED and perform several extrapolations that compare well with various past determinations. Our scheme allows us to easily contrast the QCD-only mixing case with the full QCD+QED mixing.

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arXiv2020

Determining the glue component of the nucleon

Computing the gluon component of momentum in the nucleon is a difficult and computationally expensive problem, as the matrix element involves a quark-line-disconnected gluon operator which suffers from ultra-violet fluctuations. But also necessary for a successful determination is the non-perturbative renormalisation of this operator. As a first step we investigate here this renormalisation in the RI-MOM scheme. Using quenched QCD as an example, a statistical signal is obtained in a direct calculation using an adaption of the Feynman-Hellmann technique.

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arXiv2020

Scaling and higher twist in the nucleon Compton amplitude

The partonic structure of hadrons plays an important role in a vast array of high-energy and nuclear physics experiments. It also underpins the theoretical understanding of hadron structure. Recent developments in lattice QCD offer new opportunities for reliably studying partonic structure from first principles. Here we report on the use of the Feynman-Hellmann theorem to study the forward Compton amplitude in the unphysical region. We demonstrate how this amplitude provides direct constraint on hadronic inelastic structure functions. The use of external momentum transfer allows us to study the $Q^2$ evolution to explore the onset of asymptotic scaling and reveal higher-twist effects in partonic structure.

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arXiv2019

Patterns of flavour symmetry breaking in hadron matrix elements involving u, d and s quarks

By considering a flavour expansion about the SU(3)-flavour symmetric point, we investigate how flavour-blindness constrains octet baryon matrix elements after SU(3) is broken by the mass difference between quarks. Similarly to hadron masses we find the expansions to be constrained along a mass trajectory where the singlet quark mass is held constant, which provides invaluable insight into the mechanism of flavour symmetry breaking and proves beneficial for extrapolations to the physical point. Expansions are given up to third order in the expansion parameters. Considering higher orders would give no further constraints on the expansion parameters. The relation of the expansion coefficients to the quark-line-connected and quark-line disconnected terms in the 3-point correlation functions is also given. As we consider Wilson clover-like fermions, the addition of improvement coefficients is also discussed and shown to be included in the formalism developed here. As an example of the method we investigate this numerically via a lattice calculation of the flavour-conserving matrix elements of the vector first class form factors.

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arXiv2019

Isospin splittings in the decuplet baryon spectrum from dynamical QCD+QED

We report a new analysis of the isospin splittings within the decuplet baryon spectrum. Our numerical results are based upon five ensembles of dynamical QCD+QED lattices. The analysis is carried out within a flavour-breaking expansion which encodes the effects of breaking the quark masses and electromagnetic charges away from an approximate SU(3) symmetric point. The results display total isospin splittings within the approximate SU(2) multiplets that are compatible with phenomenological estimates. Further, new insight is gained into these splittings by separating the contributions arising from strong and electromagnetic effects. We also present an update of earlier results on the octet baryon spectrum.

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arXiv2019

Anomalous magnetic moment of the muon with dynamical QCD+QED

The current $3.5σ$ discrepancy between experimental and Standard Model determinations of the anomalous magnetic moment of the muon $a_μ=(g-2)/2$ can only be extended to the discovery $5σ$ regime through a reduction of both experimental and theoretical uncertainties. On the theory side, this means a determination of the hadronic vacuum polarisation (HVP) contribution to better than 0.5%, a level of precision that demands the inclusion of QCD + QED effects to properly understand how the behaviour of quarks are modified when their electric charges are turned on. The QCDSF collaboration has generated an ensemble of configurations with dynamical QCD and QED fields with the specific aim of studying flavour breaking effects arising from differences in the quark masses and charges in physical quantities. Here we study these effects in a calculation of HVP around the SU(3) symmetric point. Furthermore, by performing partially-quenched simulations we are able to cover a larger range of quark masses and charges on these configurations and then fit the results to an SU(3) flavour breaking expansion. Subsequently, this allows for an extrapolation to the physical point.

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arXiv2019

The strange quark contribution to the spin of the nucleon

Quark line disconnected matrix elements of an operator, such as the axial current, are difficult to compute on the lattice. The standard method uses a stochastic estimator of the operator, which is generally very noisy. We discuss and develop further our alternative approach using the Feynman-Hellmann theorem which involves only evaluating two-point correlation functions. This is applied to computing the contribution of the quark spin to the nucleon and in particular for the strange quark. In this process we also pay particular attention to the development of an SU(3) flavour breaking expansion for singlet operators.

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arXiv2017

Electromagnetic form factors at large momenta from lattice QCD

Accessing hadronic form factors at large momentum transfers has traditionally presented a challenge for lattice QCD simulations. Here we demonstrate how a novel implementation of the Feynman-Hellmann method can be employed to calculate hadronic form factors in lattice QCD at momenta much higher than previously accessible. Our simulations are performed on a single set of gauge configurations with three flavours of degenerate mass quarks corresponding to $m_π\approx 470 \text{ MeV}$. We are able to determine the electromagnetic form factors of the pion and nucleon up to approximately $6 \text{ GeV}^2$, with results for $G_E/G_M$ in the proton agreeing well with experimental results.

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arXiv2017

Magnetic form factors of the octet baryons from lattice QCD and chiral extrapolation

We present a 2+1--flavor lattice QCD calculation of the electromagnetic Dirac and Pauli form factors of the octet baryons. The magnetic Sachs form factor is extrapolated at six fixed values of $Q^2$ to the physical pseudoscalar masses and infinite volume using a formulation based on heavy baryon chiral perturbation theory with finite-range regularization. We properly account for omitted disconnected quark contractions using a partially-quenched effective field theory formalism. The results compare well with the experimental form factors of the nucleon and the magnetic moments of the octet baryons.

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arXiv2017

Electric form factors of the octet baryons from lattice QCD and chiral extrapolation

We apply a formalism inspired by heavy baryon chiral perturbation theory with finite-range regularization to dynamical $2+1-$flavor CSSM/QCDSF/UKQCD Collaboration lattice QCD simulation results for the electric form factors of the octet baryons. The electric form factor of each octet baryon is extrapolated to the physical pseudoscalar masses, after finite-volume corrections have been applied, at six fixed values of $Q^2$ in the range 0.2-1.3 GeV$^2$. The extrapolated lattice results accurately reproduce the experimental form factors of the nucleon at the physical point, indicating that omitted disconnected quark loop contributions are small. Furthermore, using the results of a recent lattice study of the magnetic form factors, we determine the ratio $μ_p G^p_E/G^p_M$. This quantity decreases with $Q^2$ in a way qualitatively consistent with recent experimental results.

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arXiv2016

Flavour breaking effects in the pseudoscalar meson decay constants

The SU(3) flavour symmetry breaking expansion in up, down and strange quark masses is extended from hadron masses to meson decay constants. This allows a determination of the ratio of kaon to pion decay constants in QCD. Furthermore when using partially quenched valence quarks the expansion is such that SU(2) isospin breaking effects can also be determined. It is found that the lowest order SU(3) flavour symmetry breaking expansion (or Gell-Mann-Okubo expansion) works very well. Simulations are performed for 2+1 flavours of clover fermions at four lattice spacings.

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