0000000000449519
AUTHOR
Andreas Nyffeler
showing 17 related works from this author
Lattice calculation of the pion transition form factor $\pi^0 \to \gamma^* \gamma^*$
2016
We calculate the $\pi^0\to \gamma^*\gamma^*$ transition form factor ${\cal F}_{\pi^0\gamma^*\gamma^*}(q_1^2,q_2^2)$ in lattice QCD with two flavors of quarks. Our main motivation is to provide the input to calculate the $\pi^0$-pole contribution to hadronic light-by-light scattering in the muon $(g-2)$, $a_\mu^{\rm HLbL;\pi^0}$. We therefore focus on the region where both photons are spacelike up to virtualities of about $1.5~$GeV$^2$, which has so far not been experimentally accessible. Results are obtained in the continuum at the physical pion mass by a combined extrapolation. We reproduce the prediction of the chiral anomaly for real photons with an accuracy of about $8-9\%$. We also com…
Hadronic light-by-light scattering contribution to the muon $g-2$ on the lattice
2018
International Workshop on “Flavour Changing and Conserving Processes, FCCP2017, Anacapri, Italy, 7 Sep 2017 - 9 Sep 2017; The European physical journal / Web of Conferences 179, 01017 (2018). doi:10.1051/epjconf/201817901017
Position-space approach to hadronic light-by-light scattering in the muon $g-2$ on the lattice
2016
The anomalous magnetic moment of the muon currently exhibits a discrepancy of about three standard deviations between the experimental value and recent Standard Model predictions. The theoretical uncertainty is dominated by the hadronic vacuum polarization and the hadronic light-by-light (HLbL) scattering contributions, where the latter has so far only been fully evaluated using different models. To pave the way for a lattice calculation of HLbL, we present an expression for the HLbL contribution to $g-2$ that involves a multidimensional integral over a position-space QED kernel function in the continuum and a lattice QCD four-point correlator. We describe our semi-analytic calculation of t…
Lattice calculation of the pion transition form factor with Nf=2+1 Wilson quarks
2019
We present a lattice QCD calculation of the double-virtual neutral pion transition form factor, with the goal to cover the kinematic range relevant to hadronic light-by-light scattering in the muon $g\ensuremath{-}2$. Several improvements have been made compared to our previous work. First, we take into account the effects of the strange quark by using the ${N}_{f}=2+1$ coordinated lattice simulation gauge ensembles. Second, we have implemented the on-shell $\mathcal{O}(a)$ improvement of the vector current to reduce the discretization effects associated with Wilson quarks. Finally, in order to have access to a wider range of photon virtualities, we have computed the transition form factor …
Developments in the position-space approach to the HLbL contribution to the muon $g-2$ on the lattice
2019
The measurement of the anomalous magnetic moment of the muon and its prediction allow for a high-precision test of the Standard Model (SM). In this proceedings article we present ongoing work combining lattice QCD and continuum QED in order to determine an important SM contribution to the magnetic moment, the hadronic light-by-light contribution. We compute the quark-connected contribution in the Mainz position-space approach and investigate the long-distance part of our data using calculations of the $\pi^0$-pole and charged pion loop contributions.
Lattice calculation of the pion transition form factor π0→γ*γ*
2016
The pion transition form factor for the neutral pion double virtual photon decay is computed in two flavor lattice QCD, extrapolated to the continuum physical point. Implications for the computation of the contribution of hadronic light-by-light scattering to the muon anomalous magnetic moment are discussed.
On the precision of a data-driven estimate of the pseudoscalar-pole contribution to hadronic light-by-light scattering in the muon g-2
2016
The evaluation of the numerically dominant pseudoscalar-pole contribution to hadronic light-by-light scattering in the muon g-2 involves the pseudoscalar-photon transition form factor F_{P gamma^* gamma^*}(-Q_1^2, -Q_2^2) with P = pi^0, eta, eta^\prime and, in general, two off-shell photons with spacelike momenta Q_{1,2}^2. We determine which regions of photon momenta give the main contribution for hadronic light-by-light scattering. Furthermore, we analyze how the precision of future measurements of the single- and double-virtual form factor impacts the precision of a data-driven estimate of this contribution to hadronic light-by-light scattering.
Exploratory studies for the position-space approach to hadronic light-by-light scattering in the muon g - 2
2017
The well-known discrepancy in the muon $g-2$ between experiment and theory demands further theory investigations in view of the upcoming new experiments. One of the leading uncertainties lies in the hadronic light-by-light scattering contribution (HLbL), that we address with our position-space approach. We focus on exploratory studies of the pion-pole contribution in a simple model and the fermion loop without gluon exchanges in the continuum and in infinite volume. These studies provide us with useful information for our planned computation of HLbL in the muon $g-2$ using full QCD.
A lattice calculation of the hadronic vacuum polarization contribution to (g - 2)μ
2017
We present results of calculations of the hadronic vacuum polarisation contribution to the muon anomalous magnetic moment. Specifically, we focus on controlling the infrared regime of the vacuum polarisation function. Our results are corrected for finite-size effects by combining the Gounaris-Sakurai parameterisation of the timelike pion form factor with the Lüscher formalism. The impact of quark-disconnected diagrams and the precision of the scale determination is discussed and included in our final result in two-flavour QCD, which carries an overall uncertainty of 6%. We present preliminary results computed on ensembles with Nf = 2 + 1 dynamical flavours and discuss how the long-distance …
Direct calculation of hadronic light-by-light scattering
2015
We report calculations of hadronic light-by-light scattering amplitudes via lattice QCD evaluation of Euclidean four-point functions of vector currents. These initial results include only the fully quark-connected contribution. Particular attention is given to the case of forward scattering, which can be related via dispersion relations to the $\gamma^* \gamma^* \to$ hadrons cross section, and thus allows lattice data to be compared with phenomenology. We also present a strategy for computing the hadronic light-by-light contribution to the muon anomalous magnetic moment.
The anomalous magnetic moment of the muon in the Standard Model
2020
We are very grateful to the Fermilab Directorate and the Fermilab Theoretical Physics Department for their financial and logistical support of the first workshop of the Muon g -2 Theory Initiative (held near Fermilab in June 2017) [123], which was crucial for its success, and indeed for the successful start of the Initiative. Financial support for this workshop was also provided by the Fermilab Distinguished Scholars program, the Universities Research Association through a URA Visiting Scholar award, the Riken Brookhaven Research Center, and the Japan Society for the Promotion of Science under Grant No. KAKEHNHI-17H02906. We thank Shoji Hashimoto, Toru Iijima, Takashi Kaneko, and Shohei Nis…
Hadronic light-by-light scattering in the anomalous magnetic moment of the muon
2018
15th International Workshop on Tau Lepton Physics, Amsterdam, The Netherlands, 24 Sep 2018 - 28 Sep 2018; SciPost physics 1, 031 (2019). doi:10.21468/SciPostPhysProc.1.031
Precision of a data-driven estimate of hadronic light-by-light scattering in the muong−2: Pseudoscalar-pole contribution
2016
Within a dispersive approach to hadronic light-by-light scattering in the muon g-2, the evaluation of the numerically dominant pseudoscalar-pole contribution involves the pseudoscalar-photon transition form factor FPγ*γ*(-Q12,-Q22) with P=π0,η,η′ and, in general, two off-shell photons with spacelike momenta Q1,22. We show that for π0(η,η′), the region of photon momenta below about 1(1.5) GeV gives the main contribution to hadronic light-by-light scattering. We then discuss how the precision of current and future measurements of the single- and double-virtual transition form factor in different momentum regions impacts the precision of a data-driven estimate of this contribution to hadronic …
Lattice calculation of the pion transition form factor with Nf=2+1 Wilson quarks
2019
We present a lattice QCD calculation of the double-virtual neutral pion transition form factor, with the goal to cover the kinematic range relevant to hadronic light-by-light scattering in the muon g−2. Several improvements have been made compared to our previous work. First, we take into account the effects of the strange quark by using the Nf=2+1 coordinated lattice simulation gauge ensembles. Second, we have implemented the on-shell O(a) improvement of the vector current to reduce the discretization effects associated with Wilson quarks. Finally, in order to have access to a wider range of photon virtualities, we have computed the transition form factor in a moving frame as well as in th…
Hadronic light-by-light scattering contribution to the muon $g-2$ from lattice QCD: semi-analytical calculation of the QED kernel
2023
Hadronic light-by-light scattering is one of the virtual processes that causes the gyromagnetic factor $g$ of the muon to deviate from the value of two predicted by Dirac's theory. This process makes one of the largest contributions to the uncertainty of the Standard Model prediction for the muon $(g-2)$. Lattice QCD allows for a first-principles approach to computing this non-perturbative effect. In order to avoid power-law finite-size artifacts generated by virtual photons in lattice simulations, we follow a coordinate-space approach involving a weighted integral over the vertices of the QCD four-point function of the electromagnetic current carried by the quarks. Here we present in detai…
Lattice calculation of the pion transition form factor $��^0 \to ��^* ��^*$
2016
We calculate the $��^0\to ��^*��^*$ transition form factor ${\cal F}_{��^0��^*��^*}(q_1^2,q_2^2)$ in lattice QCD with two flavors of quarks. Our main motivation is to provide the input to calculate the $��^0$-pole contribution to hadronic light-by-light scattering in the muon $(g-2)$, $a_��^{\rm HLbL;��^0}$. We therefore focus on the region where both photons are spacelike up to virtualities of about $1.5~$GeV$^2$, which has so far not been experimentally accessible. Results are obtained in the continuum at the physical pion mass by a combined extrapolation. We reproduce the prediction of the chiral anomaly for real photons with an accuracy of about $8-9\%$. We also compare to various recen…
Lattice calculation of the pion transition form factor with $N_f=2+1$ Wilson quarks
2019
Physical review / D D 100(3), 034520 (2019). doi:10.1103/PhysRevD.100.034520