Holger Perlt - Academia.edu (original) (raw)

Papers by Holger Perlt

Proceedings of The 38th International Symposium on Lattice Field Theory — PoS(LATTICE2021)

Lattice calculations of hadronic observables are aggravated by short-distance fluctuations. The g... more 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.

We determine the properties of generalised parton distributions (GPDs) from a lattice QCD calcula... more We determine the properties of generalised parton distributions (GPDs) from a lattice QCD calculation of the off-forward Compton amplitude (OFCA). By extending the Feynman-Hellmann relation to second-order matrix elements at off-forward kinematics, this amplitude can be calculated from lattice propagators computed in the presence of a background field. Using an operator product expansion, we show that the deeply-virtual part of the OFCA can be parameterised in terms of the low-order Mellin moments of the GPDs. We apply this formalism to a numerical investigation for zero-skewness kinematics at two values of the soft momentum transfer, t=−1.1,−2.2;textGeV2t = -1.1, -2.2 \;\text{GeV}^2t=−1.1,−2.2;textGeV2, and a pion mass of mpiapprox470;textMeVm_{\pi}\approx 470\;\text{MeV}mpiapprox470;textMeV. The form factors of the lowest two moments of the nucleon GPDs are determined, including the first lattice QCD determination of the n=4n=4n=4 moments. Hence we demonstrate the viability of this method to calculate the OFCA from first principles, and thereby provide novel ...

Computing the gluon component of momentum in the nucleon is a difficult and computationally expen... more 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. We investigate this renormalisation here by direct computation in the RI mom scheme. A clear statistical signal is obtained in the direct calculation by an adaption of the Feynman-Hellmann technique. A comparison is conducted in order to verify the energy-momentum sum rule of the nucleon.

Proceedings of 37th International Symposium on Lattice Field Theory — PoS(LATTICE2019), 2020

We have initiated a program to compute the Compton amplitude from lattice QCD with the Feynman-He... more We have initiated a program to compute the Compton amplitude from lattice QCD with the Feynman-Hellman method. This amplitude is related to the structure function via a Fredholm integral equation of the first kind. It is known that these types of equations are inherently illposed-they are, e.g., extremely sensitive to perturbations of the system. We discuss two methods which are candidates to handle these problems: the model free inversion based on singular value decomposition and one Bayesian type approach. We apply the Bayesian method to currently available lattice data for the Compton amplitude.

Proceedings of 34th annual International Symposium on Lattice Field Theory — PoS(LATTICE2016), 2017

We investigate implications of the use of the point-split axial vector current derived from a Wil... more We investigate implications of the use of the point-split axial vector current derived from a Wilson like fermionic action. We compute the corresponding renormalization factor nonperturbatively for one beta value. The axial charge gA calculated from this nonlocal current is found to be nearer to the physical value than computed with the local axial vector current-computed both on the same lattice with the same action.

Proceedings of 31st International Symposium on Lattice Field Theory LATTICE 2013 — PoS(LATTICE 2013), 2014

Proceedings of XXIX International Symposium on Lattice Field Theory — PoS(Lattice 2011), 2012

Proceedings of The XXVII International Symposium on Lattice Field Theory — PoS(LAT2009), 2010

Nucleon structure functions can be observed in Deep Inelastic Scattering experiments, but it is a... more Nucleon structure functions can be observed in Deep Inelastic Scattering experiments, but it is an outstanding challenge to confront them with fully non-perturbative QCD results. For this purpose we investigate the product of electromagnetic currents (with large photon momenta) between quark states (of low momenta). By means of an Operator Product Expansion the structure function can be decomposed into matrix elements of local operators, and Wilson coefficients. For consistency both have to be computed non-perturbatively. Here we present precision results for a set of Wilson coefficients. They are evaluated from propagators for numerous quark momenta on the lattice, where the use of chiral fermions suppresses undesired operator mixing. This overdetermines the Wilson coefficients, but reliable results can be extracted by means of a Singular Value Decomposition.

Proceedings of The XXV International Symposium on Lattice Field Theory — PoS(LATTICE 2007), 2008

J µ J ν £ (where J is the vector current) and comparing it with the expectation values of bilinea... more J µ J ν £ (where J is the vector current) and comparing it with the expectation values of bilinear operators. This will determine the Wilson coefficients in the OPE from lattice data, and so give an alternative to the conventional methods of renormalising lattice structure function calculations. It could also give us access to higher twist quantities such as the longitudinal structure function F L ¤ F 2 ¥ 2xF 1. We use overlap fermions because of their improved chiral properties, which reduces the number of possible operator mixing coefficients.

Proceedings of The 36th Annual International Symposium on Lattice Field Theory — PoS(LATTICE2018), 2019

Early B-physics experiments have left us with a number of puzzles in heavy flavour physics. New l... more Early B-physics experiments have left us with a number of puzzles in heavy flavour physics. New lattice calculations (with a greater understanding of QCD effects in the Standard Model) will be needed to support the increase in experimental precision to be achieved by upcoming experiments such as Belle II. We extend the CSSM/UKQCD/QCDSF studies of SU(3) flavour breaking effects by presenting new results for the decay constants f B and f B s .

Proceedings of The 36th Annual International Symposium on Lattice Field Theory — PoS(LATTICE2018), 2019

The current 3.5σ discrepancy between experimental and Standard Model determinations of the anomal... more 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.

Proceedings of 31st International Symposium on Lattice Field Theory LATTICE 2013 — PoS(LATTICE 2013), 2014

The subtraction of hypercubic lattice corrections, calculated at 1-loop order in lattice perturba... more The subtraction of hypercubic lattice corrections, calculated at 1-loop order in lattice perturbation theory (LPT), is common practice, e.g., for determinations of renormalization constants in lattice hadron physics. Providing such corrections beyond 1-loop order is however very demanding in LPT, and numerical stochastic perturbation theory (NSPT) might be the better candidate for this. Here we report on a first feasibility check of this method and provide (in a parametrization valid for arbitrary lattice couplings) the lattice corrections up to 3-loop order for the SU(3) gluon and ghost propagators in Landau gauge. These propagators are ideal candidates for such a check, as they are available from lattice simulations to high precision and can be combined to a renormalization group invariant product (Minimal MOM coupling) for which a 1-loop LPT correction was found to be insufficient to remove the bulk of the hypercubic lattice artifacts from the data. As a bonus, we also compare our results with the ever popular H(4) method.

Proceedings of The XXVI International Symposium on Lattice Field Theory — PoS(LATTICE 2008), 2009

Lattice calculations could boost our understanding of Deep Inelastic Scattering by evaluating mom... more Lattice calculations could boost our understanding of Deep Inelastic Scattering by evaluating moments of the Nucleon Structure Functions. To this end we study the product of electromagnetic currents between quark states. The Operator Product Expansion (OPE) decomposes it into matrix elements of local operators (depending on the quark momenta) and Wilson coefficients (as functions of the larger photon momenta). For consistency with the matrix elements, we evaluate a set of Wilson coefficients non-perturbatively, based on propagators for numerous momentum sources, on a 24 3 ¢ 48 lattice. The use of overlap quarks suppresses unwanted operator mixing and lattice artifacts. Results for the leading Wilson coefficients are extracted by means of Singular Value Decomposition.

Proceedings of The XXV International Symposium on Lattice Field Theory — PoS(LATTICE 2007), 2008

Results are presented for the lowest moment of the distribution amplitude for the K * vector meso... more Results are presented for the lowest moment of the distribution amplitude for the K * vector meson. Both longitudinal and transverse moments are investigated. We use two flavours of O(a) improved Wilson fermions, together with a non-perturbative renormalisation of the matrix element.

Proceedings of The 32nd International Symposium on Lattice Field Theory — PoS(LATTICE2014), 2015

By introducing an external spin operator to the fermion action, the quark spin fractions of hadro... more By introducing an external spin operator to the fermion action, the quark spin fractions of hadrons are determined from the linear response of the hadron energies using the Feynman-Hellmann (FH) theorem. At our SU(3)-flavour symmetric point, we find that the connected quark spin fractions are universally in the range 55-70% for vector mesons and octet and decuplet baryons. There is an indication that the amount of spin suppression is quite sensitive to the strength of SU(3) breaking. We also present first preliminary results applying the FH technique to calculations of quark-line disconnected contributions to hadronic matrix elements of axial and tensor operators. At the SU(3)-flavour symmetric point we find a small negative contribution to the nucleon spin from disconnected quark diagrams, while the corresponding tensor matrix elements are consistent with zero.

Proceedings of 34th annual International Symposium on Lattice Field Theory — PoS(LATTICE2016), 2017

The determination of hadronic form factors at large momentum transfers has been a challenging pro... more The determination of hadronic form factors at large momentum transfers has been a challenging problem in lattice QCD simulations. Here we show how the Feynman-Hellmann method may be extended to non-forward matrix elements to calculate hadronic form factors in lattice QCD at much higher momenta than previously accessible. We are able to determine the electromagnetic form factors of the pion and nucleon up to approximately 6 GeV 2 , with results for G E /G M in the proton agreeing well with experimental results.

Proceedings of The 33rd International Symposium on Lattice Field Theory — PoS(LATTICE 2015), 2016

Physical Review Letters, 2015

We compute the electric dipole moment d n of the neutron from a fully dynamical simulation of lat... more We compute the electric dipole moment d n of the neutron from a fully dynamical simulation of lattice QCD with 2 + 1 flavors of clover fermions and nonvanishing θ term. The latter is rotated into a pseudoscalar density in the fermionic action using the axial anomaly. To make the action real, the vacuum angle θ is taken to be purely imaginary. The physical value of d n is obtained by analytic continuation. We find d n = −3.9(2)(9) × 10 −16 θ e cm, which, when combined with the experimental limit on d n , leads to the upper bound |θ| 7.4 × 10 −11 .

Nuclear Physics B - Proceedings Supplements, 1996

We review our progress on the lattice calculation of low moments of both the unpolarised and pola... more We review our progress on the lattice calculation of low moments of both the unpolarised and polarised nucleon structure functions.

Lepton Scattering, Hadrons and QCD, 2001

In this talk we highlight recent lattice calculations of the nucleon form factors and structure f... more In this talk we highlight recent lattice calculations of the nucleon form factors and structure functions.

Proceedings of The 38th International Symposium on Lattice Field Theory — PoS(LATTICE2021)

Lattice calculations of hadronic observables are aggravated by short-distance fluctuations. The g... more 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.

We determine the properties of generalised parton distributions (GPDs) from a lattice QCD calcula... more We determine the properties of generalised parton distributions (GPDs) from a lattice QCD calculation of the off-forward Compton amplitude (OFCA). By extending the Feynman-Hellmann relation to second-order matrix elements at off-forward kinematics, this amplitude can be calculated from lattice propagators computed in the presence of a background field. Using an operator product expansion, we show that the deeply-virtual part of the OFCA can be parameterised in terms of the low-order Mellin moments of the GPDs. We apply this formalism to a numerical investigation for zero-skewness kinematics at two values of the soft momentum transfer, t=−1.1,−2.2;textGeV2t = -1.1, -2.2 \;\text{GeV}^2t=−1.1,−2.2;textGeV2, and a pion mass of mpiapprox470;textMeVm_{\pi}\approx 470\;\text{MeV}mpiapprox470;textMeV. The form factors of the lowest two moments of the nucleon GPDs are determined, including the first lattice QCD determination of the n=4n=4n=4 moments. Hence we demonstrate the viability of this method to calculate the OFCA from first principles, and thereby provide novel ...

Computing the gluon component of momentum in the nucleon is a difficult and computationally expen... more 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. We investigate this renormalisation here by direct computation in the RI mom scheme. A clear statistical signal is obtained in the direct calculation by an adaption of the Feynman-Hellmann technique. A comparison is conducted in order to verify the energy-momentum sum rule of the nucleon.

Proceedings of 37th International Symposium on Lattice Field Theory — PoS(LATTICE2019), 2020

We have initiated a program to compute the Compton amplitude from lattice QCD with the Feynman-He... more We have initiated a program to compute the Compton amplitude from lattice QCD with the Feynman-Hellman method. This amplitude is related to the structure function via a Fredholm integral equation of the first kind. It is known that these types of equations are inherently illposed-they are, e.g., extremely sensitive to perturbations of the system. We discuss two methods which are candidates to handle these problems: the model free inversion based on singular value decomposition and one Bayesian type approach. We apply the Bayesian method to currently available lattice data for the Compton amplitude.

Proceedings of 34th annual International Symposium on Lattice Field Theory — PoS(LATTICE2016), 2017

We investigate implications of the use of the point-split axial vector current derived from a Wil... more We investigate implications of the use of the point-split axial vector current derived from a Wilson like fermionic action. We compute the corresponding renormalization factor nonperturbatively for one beta value. The axial charge gA calculated from this nonlocal current is found to be nearer to the physical value than computed with the local axial vector current-computed both on the same lattice with the same action.

Proceedings of 31st International Symposium on Lattice Field Theory LATTICE 2013 — PoS(LATTICE 2013), 2014

Proceedings of XXIX International Symposium on Lattice Field Theory — PoS(Lattice 2011), 2012

Proceedings of The XXVII International Symposium on Lattice Field Theory — PoS(LAT2009), 2010

Nucleon structure functions can be observed in Deep Inelastic Scattering experiments, but it is a... more Nucleon structure functions can be observed in Deep Inelastic Scattering experiments, but it is an outstanding challenge to confront them with fully non-perturbative QCD results. For this purpose we investigate the product of electromagnetic currents (with large photon momenta) between quark states (of low momenta). By means of an Operator Product Expansion the structure function can be decomposed into matrix elements of local operators, and Wilson coefficients. For consistency both have to be computed non-perturbatively. Here we present precision results for a set of Wilson coefficients. They are evaluated from propagators for numerous quark momenta on the lattice, where the use of chiral fermions suppresses undesired operator mixing. This overdetermines the Wilson coefficients, but reliable results can be extracted by means of a Singular Value Decomposition.

Proceedings of The XXV International Symposium on Lattice Field Theory — PoS(LATTICE 2007), 2008

J µ J ν £ (where J is the vector current) and comparing it with the expectation values of bilinea... more J µ J ν £ (where J is the vector current) and comparing it with the expectation values of bilinear operators. This will determine the Wilson coefficients in the OPE from lattice data, and so give an alternative to the conventional methods of renormalising lattice structure function calculations. It could also give us access to higher twist quantities such as the longitudinal structure function F L ¤ F 2 ¥ 2xF 1. We use overlap fermions because of their improved chiral properties, which reduces the number of possible operator mixing coefficients.

Proceedings of The 36th Annual International Symposium on Lattice Field Theory — PoS(LATTICE2018), 2019

Early B-physics experiments have left us with a number of puzzles in heavy flavour physics. New l... more Early B-physics experiments have left us with a number of puzzles in heavy flavour physics. New lattice calculations (with a greater understanding of QCD effects in the Standard Model) will be needed to support the increase in experimental precision to be achieved by upcoming experiments such as Belle II. We extend the CSSM/UKQCD/QCDSF studies of SU(3) flavour breaking effects by presenting new results for the decay constants f B and f B s .

Proceedings of The 36th Annual International Symposium on Lattice Field Theory — PoS(LATTICE2018), 2019

The current 3.5σ discrepancy between experimental and Standard Model determinations of the anomal... more 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.

Proceedings of 31st International Symposium on Lattice Field Theory LATTICE 2013 — PoS(LATTICE 2013), 2014

The subtraction of hypercubic lattice corrections, calculated at 1-loop order in lattice perturba... more The subtraction of hypercubic lattice corrections, calculated at 1-loop order in lattice perturbation theory (LPT), is common practice, e.g., for determinations of renormalization constants in lattice hadron physics. Providing such corrections beyond 1-loop order is however very demanding in LPT, and numerical stochastic perturbation theory (NSPT) might be the better candidate for this. Here we report on a first feasibility check of this method and provide (in a parametrization valid for arbitrary lattice couplings) the lattice corrections up to 3-loop order for the SU(3) gluon and ghost propagators in Landau gauge. These propagators are ideal candidates for such a check, as they are available from lattice simulations to high precision and can be combined to a renormalization group invariant product (Minimal MOM coupling) for which a 1-loop LPT correction was found to be insufficient to remove the bulk of the hypercubic lattice artifacts from the data. As a bonus, we also compare our results with the ever popular H(4) method.

Proceedings of The XXVI International Symposium on Lattice Field Theory — PoS(LATTICE 2008), 2009

Lattice calculations could boost our understanding of Deep Inelastic Scattering by evaluating mom... more Lattice calculations could boost our understanding of Deep Inelastic Scattering by evaluating moments of the Nucleon Structure Functions. To this end we study the product of electromagnetic currents between quark states. The Operator Product Expansion (OPE) decomposes it into matrix elements of local operators (depending on the quark momenta) and Wilson coefficients (as functions of the larger photon momenta). For consistency with the matrix elements, we evaluate a set of Wilson coefficients non-perturbatively, based on propagators for numerous momentum sources, on a 24 3 ¢ 48 lattice. The use of overlap quarks suppresses unwanted operator mixing and lattice artifacts. Results for the leading Wilson coefficients are extracted by means of Singular Value Decomposition.

Proceedings of The XXV International Symposium on Lattice Field Theory — PoS(LATTICE 2007), 2008

Results are presented for the lowest moment of the distribution amplitude for the K * vector meso... more Results are presented for the lowest moment of the distribution amplitude for the K * vector meson. Both longitudinal and transverse moments are investigated. We use two flavours of O(a) improved Wilson fermions, together with a non-perturbative renormalisation of the matrix element.

Proceedings of The 32nd International Symposium on Lattice Field Theory — PoS(LATTICE2014), 2015

By introducing an external spin operator to the fermion action, the quark spin fractions of hadro... more By introducing an external spin operator to the fermion action, the quark spin fractions of hadrons are determined from the linear response of the hadron energies using the Feynman-Hellmann (FH) theorem. At our SU(3)-flavour symmetric point, we find that the connected quark spin fractions are universally in the range 55-70% for vector mesons and octet and decuplet baryons. There is an indication that the amount of spin suppression is quite sensitive to the strength of SU(3) breaking. We also present first preliminary results applying the FH technique to calculations of quark-line disconnected contributions to hadronic matrix elements of axial and tensor operators. At the SU(3)-flavour symmetric point we find a small negative contribution to the nucleon spin from disconnected quark diagrams, while the corresponding tensor matrix elements are consistent with zero.

Proceedings of 34th annual International Symposium on Lattice Field Theory — PoS(LATTICE2016), 2017

The determination of hadronic form factors at large momentum transfers has been a challenging pro... more The determination of hadronic form factors at large momentum transfers has been a challenging problem in lattice QCD simulations. Here we show how the Feynman-Hellmann method may be extended to non-forward matrix elements to calculate hadronic form factors in lattice QCD at much higher momenta than previously accessible. We are able to determine the electromagnetic form factors of the pion and nucleon up to approximately 6 GeV 2 , with results for G E /G M in the proton agreeing well with experimental results.

Proceedings of The 33rd International Symposium on Lattice Field Theory — PoS(LATTICE 2015), 2016

Physical Review Letters, 2015

We compute the electric dipole moment d n of the neutron from a fully dynamical simulation of lat... more We compute the electric dipole moment d n of the neutron from a fully dynamical simulation of lattice QCD with 2 + 1 flavors of clover fermions and nonvanishing θ term. The latter is rotated into a pseudoscalar density in the fermionic action using the axial anomaly. To make the action real, the vacuum angle θ is taken to be purely imaginary. The physical value of d n is obtained by analytic continuation. We find d n = −3.9(2)(9) × 10 −16 θ e cm, which, when combined with the experimental limit on d n , leads to the upper bound |θ| 7.4 × 10 −11 .

Nuclear Physics B - Proceedings Supplements, 1996

We review our progress on the lattice calculation of low moments of both the unpolarised and pola... more We review our progress on the lattice calculation of low moments of both the unpolarised and polarised nucleon structure functions.

Lepton Scattering, Hadrons and QCD, 2001

In this talk we highlight recent lattice calculations of the nucleon form factors and structure f... more In this talk we highlight recent lattice calculations of the nucleon form factors and structure functions.