PD Dr. Oliver Bär
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Forschungsthemen1
Beiträge von Mehrteilchenzuständen zu hadronischen Korrelationsfunktionen mittels chiraler Störungstheorie
Quelle ↗Förderer: DFG Eigene Stelle (Sachbeihilfe) Zeitraum: 02/2017 - 02/2020 Projektleitung: PD Dr. Oliver Bär
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Stand: 26.4.2026, 19:48:44 (Top-K=20, Min-Cosine=0.4)
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Publikationen25
Top 25 nach Zitationen — Quelle: OpenAlex (BAAI/bge-m3 embedded für Matching).
Physical review. D. Particles, fields, gravitation, and cosmology/Physical review. D, Particles, fields, gravitation, and cosmology · 137 Zitationen · DOI
We construct chiral effective Lagrangian for two lattice theories: one with Wilson fermions and the other with Wilson sea fermions and Ginsparg-Wilson valence fermions. For each of these theories we construct the Symanzik action through $\mathcal{O}{(a}^{2}).$ The chiral Lagrangian is then derived, including terms of $\mathcal{O}{(a}^{2}),$ which have not been calculated before. We find that there are only few new terms at this order. Corrections to existing coefficients in the continuum chiral Lagrangian are proportional to ${a}^{2}$ and appear in the Lagrangian at $\mathcal{O}{(a}^{2}{p}^{2})$ or higher. Similarly, O(4) symmetry-breaking terms enter the Symanzik action at $\mathcal{O}{(a}^{2}),$ but contribute to the chiral Lagrangian at $\mathcal{O}{(a}^{2}{p}^{4})$ or higher. We calculate the light meson masses in chiral perturbation theory for both lattice theories. At next-to-leading order, we find that there are no $\mathcal{O}{(a}^{2})$ corrections to the valence-valence meson mass in the mixed theory due to the enhanced chiral symmetry of the valence sector.
Physical review. D. Particles, fields, gravitation, and cosmology/Physical review. D, Particles, fields, gravitation, and cosmology · 101 Zitationen · DOI
We study lattice QCD with staggered sea and Ginsparg-Wilson valence quarks. The Symanzik effective action for this mixed lattice theory, including the lattice spacing contributions of $\mathcal{O}({a}^{2})$, is derived. Using this effective theory we construct the leading-order chiral Lagrangian. The masses and decay constants of pseudoscalars containing two Ginsparg-Wilson valence quarks are computed at one-loop order.
Physical review. D. Particles, fields, gravitation, and cosmology/Physical review. D. Particles and fields · 90 Zitationen · DOI
We discuss simulations with different lattice Dirac operators for sea and valence quarks. A goal of such a ``mixed'' action approach is to probe deeper the chiral regime of QCD by enabling simulations with light valence quarks. This is achieved by using chiral fermions as valence quarks while computationally inexpensive fermions are used in the sea sector. Specifically, we consider Wilson sea quarks and Ginsparg-Wilson valence quarks. The local Symanzik action for this mixed theory is derived to $\mathcal{O}(a),$ and the appropriate low energy chiral effective Lagrangian is constructed, including the leading $\mathcal{O}(a)$ contributions. Using this Lagrangian one can calculate expressions for physical observables and determine the Gasser-Leutwyler coefficients by fitting them to the lattice data.
Physical review. D. Particles, fields, gravitation, and cosmology/Physical review. D, Particles, fields, gravitation, and cosmology · 69 Zitationen · DOI
We construct the next-to-leading-order chiral Lagrangian for scalar and pseudoscalar densities defined using the gradient flow, for flow times much smaller than the square of the pion wavelength. We calculate the chiral condensate and the pion decay constant to this order from operators at positive flow time, and confirm results obtained earlier in the chiral limit. We also calculate the quark-mass dependence of the scales ${t}_{0}$ and ${w}_{0}$ defined from the scalar gluon density and find that nonanalytic terms in the quark mass only enter at next-to-next-to-leading order.
Physical review. D. Particles, fields, gravitation, and cosmology/Physical review. D, Particles, fields, gravitation, and cosmology · 68 Zitationen · DOI
We point out a caveat in the proof for automatic $\mathrm{O}(a)$ improvement in twisted mass lattice QCD at maximal twist angle. With the definition for the twist angle previously given by Frezzotti and Rossi, automatic $\mathrm{O}(a)$ improvement can fail unless the quark mass satisfies ${m}_{q}\ensuremath{\gg}{a}^{2}{\ensuremath{\Lambda}}_{\mathrm{Q}\mathrm{C}\mathrm{D}}^{3}$. We propose a different definition for the twist angle which does not require a restriction on the quark mass for automatic $\mathrm{O}(a)$ improvement. In order to illustrate explicitly automatic $\mathrm{O}(a)$ improvement we compute the pion mass in the corresponding chiral effective theory. We consider different definitions for maximal twist and show explicitly the absence or presence of the leading $\mathrm{O}(a)$ effect, depending on the size of the quark mass.
Physical review. D/Physical review. D. · 55 Zitationen · DOI
The nucleon-pion-state contribution to QCD two- and three-point functions used in lattice calculations of the nucleon axial form factors are studied in chiral perturbation theory. For small quark masses this contribution is expected to be the dominant excited-state contamination at large time separations. To leading order in chiral perturbation theory the results depend on only two experimentally known low-energy constants and the nucleon-pion-state contribution to the form factors can be estimated. The nucleon-pion-state contribution to the axial form factor ${G}_{\mathrm{A}}({Q}^{2})$ is at the 5 percent level for a source-sink separation of 2 fm and shows almost no dependence on the momentum transfer ${Q}^{2}$. In contrast, for the induced pseudoscalar form factor ${\stackrel{\texttildelow{}}{G}}_{\mathrm{P}}({Q}^{2})$ the nucleon-pion-state contribution shows a rather strong dependence on ${Q}^{2}$ and leads to a 10 to 40 percent underestimation of ${\stackrel{\texttildelow{}}{G}}_{\mathrm{P}}({Q}^{2})$ at small momentum transfers. The chiral perturbation theory results can be used to analytically remove the nucleon-pion-state contribution from lattice data. Performing this removal for lattice data generated by the PACS collaboration we find agreement with experimental data and the predictions of the pion-pole dominance model. The removal works surprisingly well even for source-sink separations as small as 1.3 fm.
Physical review. D/Physical review. D. · 41 Zitationen · DOI
We employ leading-order covariant chiral perturbation theory to compute the nucleon-pion-state contribution to the 3-point correlation functions one typically measures in lattice QCD to extract the isovector nucleon charges ${g}_{A}$, ${g}_{T}$ and ${g}_{S}$. We estimate the impact of the nucleon-pion-state contribution on both the plateau and the summation method for lattice simulations with physical pion masses. The nucleon-pion-state contribution results in an overestimation of all charges with both methods. The overestimation is roughly equal for the axial and the tensor charge, and about 50% larger for the scalar charge.
Nuclear Physics B · 37 Zitationen · DOI
Physical review. D. Particles, fields, gravitation, and cosmology/Physical review. D, Particles, fields, gravitation, and cosmology · 35 Zitationen · DOI
We study the nucleon-pion-state contribution to the QCD two-point function of the standard nucleon interpolating fields. For sufficiently small quark masses these two-particle states are expected to have a smaller total energy than the single-particle excited states. We calculate the nucleon-pion-state contribution to leading order in chiral perturbation theory. Both parity channels are considered. We find the nucleon-pion-state contribution to be small, contributing at the few percent level to the effective mass in the positive parity channel.
Journal of High Energy Physics · 32 Zitationen · DOI
Nuclear Physics B · 31 Zitationen · DOI
Physical review. D/Physical review. D. · 30 Zitationen · DOI
The nucleon-pion-state contribution in the QCD three-point function of the pseudoscalar density is calculated to leading order in chiral perturbation theory. It predicts a nucleon-pion-state contamination in lattice estimates for the pseudoscalar form factor ${G}_{\mathrm{P}}({Q}^{2})$ determined with the plateau method. Depending on the momentum transfer ${Q}^{2}$ the contamination varies between $\ensuremath{-}20%$ and $+50%$ for a source-sink separation of 2 fm. The nucleon-pion-state contamination also causes violations in the generalized Goldberger-Treiman relation among the pseudoscalar and the axial nucleon form factors, the dominant source being the nucleon-pion-state contamination in the induced pseudoscalar form factor ${\stackrel{\texttildelow{}}{G}}_{\mathrm{P}}({Q}^{2})$. Comparing the chiral perturbation theory predictions with lattice results of the PACS Collaboration we find reasonable agreement even for source-sink separations as small as 1.3 fm.
Nuclear Physics B · 30 Zitationen · DOI
Nuclear Physics B - Proceedings Supplements · 29 Zitationen · DOI
Physical review. D. Particles, fields, gravitation, and cosmology/Physical review. D, Particles, fields, gravitation, and cosmology · 25 Zitationen · DOI
The pion masses and the pion decay constant are calculated to 1-loop order in SU(2) twisted mass Wilson chiral perturbation theory, assuming a large pion mass splitting and tuning to maximal twist. Taking the large mass splitting at leading order in the chiral expansion leads to significant modifications in the chiral logarithms. For example, the result for the charged pion mass contains a chiral logarithm that involves the neutral pion mass instead of the charged one. Similar modifications appear in the results for the neutral pion mass and the decay constant. These new results are used in fits to lattice data obtained recently by the European twisted mass collaboration. The data can be fitted well, in general better than with the standard chiral perturbation theory expressions that ignore the mass splitting. The impact on the extraction of low-energy couplings is briefly discussed.
Erratum: Chiral perturbation theory for gradient flow observables [Phys. Rev. D 89, 034505 (2014)]
2014Physical review. D. Particles, fields, gravitation, and cosmology/Physical review. D, Particles, fields, gravitation, and cosmology · 22 Zitationen · DOI
Excited-state contribution to the axial-vector and pseudoscalar correlators with two extra pions
2013Physical review. D. Particles, fields, gravitation, and cosmology/Physical review. D, Particles, fields, gravitation, and cosmology · 22 Zitationen · DOI
We study multiparticle-state contributions to the QCD two-point functions of the axial-vector and pseudoscalar quark bilinears in a finite spatial volume. For sufficiently small quark masses one expects three-meson states with two additional pions at rest to have the lowest total energy after the ground state. We calculate this three-meson-state contribution using chiral perturbation theory. We find it to be strongly suppressed and too small to be seen in present-day lattice simulations.
Physical review. D. Particles, fields, gravitation, and cosmology/Physical review. D, Particles, fields, gravitation, and cosmology · 21 Zitationen · DOI
We compute the scattering amplitude for pion scattering in Wilson chiral perturbation theory for two degenerate quark flavors. We consider two different regimes where the quark mass $m$ is of order (i) $a{\ensuremath{\Lambda}}_{\mathrm{QCD}}^{2}$ and (ii) ${a}^{2}{\ensuremath{\Lambda}}_{\mathrm{QCD}}^{3}$. Analytic expressions for the scattering lengths in all three isospin channels are given. As a result of the $O({a}^{2})$ terms the $I=0$ and $I=2$ scattering lengths do not vanish in the chiral limit. Moreover, additional chiral logarithms proportional to ${a}^{2}\mathrm{ln} {M}_{\ensuremath{\pi}}^{2}$ are present in the one-loop results for regime (ii). These contributions significantly modify the familiar results from continuum chiral perturbation theory.
EPJ Web of Conferences · 20 Zitationen · DOI
Multi-particle states with additional pions are expected to be a non-negligible source of the excited-state contamination in lattice simulations at the physical point. It is shown that baryon chiral perturbation theory (ChPT) can be employed to calculate the contamination due to two-particle nucleon-pion states in various nucleon observables. Results to leading order are presented for the nucleon axial, tensor and scalar charge and three Mellin moments of parton distribution functions: the average quark momentum fraction, the helicity and the transversity moment. Taking into account experimental and phenomenological results for the charges and moments the impact of the nucleon-pionstates on lattice estimates for these observables can be estimated. The nucleon-pion-state contribution leads to an overestimation of all charges and moments obtained with the plateau method. The overestimation is at the 5-10% level for source-sink separations of about 2 fm. Existing lattice data is not in conflict with the ChPT predictions, but the comparison suggests that significantly larger source-sink separations are needed to compute the charges and moments with few-percent precision.
Physical review. D. Particles, fields, gravitation, and cosmology/Physical review. D, Particles, fields, gravitation, and cosmology · 19 Zitationen · DOI
In this paper we present a proof for automatic $O(a)$ improvement in twisted mass lattice QCD at maximal twist, which uses only the symmetries of the leading part in the Symanzik effective action. In the process of the proof we clarify that the twist angle is dynamically determined by vacuum expectation values in the Symanzik theory. For maximal twist according to this definition, we show that scaling violations of all quantities which have nonzero values in the continuum limit are even in $a$. In addition, using Wilson chiral perturbation theory, we investigate this definition for maximal twist and compare it to other definitions which were already employed in actual simulations.
Physical review. D/Physical review. D. · 17 Zitationen · DOI
The RQCD Collaboration proposed a projection method to remove the excited-state contamination in lattice QCD calculations of nuclear form factors. The effectiveness of this method in removing the two-particle nucleon-pion-state contamination is examined using chiral perturbation theory. It is shown that the projection method has practically no impact in the calculation of the axial and induced pseudoscalar form factors. In the pseudoscalar form factor the projection method strongly enhances the nucleon-pion-state contamination. The generalized Goldberger-Treiman relation is satisfied even though large nucleon-pion-state contaminations are present in individual form factors. Therefore, the projection method is not a solution to the excited-state problem in nucleon form factor calculations.
Nuclear Physics B · 16 Zitationen · DOI
Physical review. D/Physical review. D. · 15 Zitationen · DOI
The nucleon-pion-state contribution to QCD two-point and three-point functions relevant for lattice calculations of the nucleon electromagnetic form factors are studied in chiral perturbation theory. To leading order, the results depend on a few experimentally known low-energy constants only, and the nucleon-pion-state contribution to the form factors can be estimated. The nucleon-pion-state contribution to the electric form factor ${G}_{\mathrm{E}}({Q}^{2})$ is at the $+5%$ level for a source-sink separation of 2 fm, and it increases with increasing momentum transfer ${Q}^{2}$. For the magnetic form factor, the nucleon-pion-state contribution leads to an underestimation of ${G}_{\mathrm{M}}({Q}^{2})$ by about 5% that decreases with increasing ${Q}^{2}$. For smaller source-sink separations that are accessible in present-day lattice simulations, the impact is larger, although the chiral perturbation theory results may not be applicable for such small time separations. Still, a comparison with lattice data at $t\ensuremath{\approx}1.6\text{ }\text{ }\mathrm{fm}$ works reasonably well.
Nucleon-pion-state contribution in lattice calculations of moments of parton distribution functions
2017Physical review. D/Physical review. D. · 15 Zitationen · DOI
We employ chiral perturbation theory to calculate the nucleon-pion-state contribution to the 3-point correlation functions measured in lattice QCD to compute various moments of parton distribution functions (quark momentum fraction, helicity and transversity moment). We estimate the impact of the nucleon-pion-state contribution on the plateau method for lattice simulations with a physical pion mass. The nucleon-pion-state contribution results in an overestimation of all three moments. The overestimation is at the 5--20% level for source-sink separations of about 1.5 fm.
Physical review. D. Particles, fields, gravitation, and cosmology/Physical review. D, Particles, fields, gravitation, and cosmology · 15 Zitationen · DOI
We consider $2+1$ flavor Wilson chiral perturbation theory including the lattice spacing contributions of $\mathrm{O}({a}^{2})$. We adopt a power counting appropriate for the unquenched lattice simulations carried out by the CP-PACS/JLQCD Collaboration and compute the pseudoscalar meson masses to one loop. These expression are required to perform the chiral extrapolation of the CP-PACS/JLQCD lattice data.
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Stammdaten
Identität, Organisation und Kontakt aus HU-FIS.
- Name
- PD Dr. Oliver Bär
- Titel
- PD Dr.
- Fakultät
- Mathematisch-Naturwissenschaftliche Fakultät
- Institut
- Institut für Physik
- Arbeitsgruppe
- Theoretische Physik (Phänomenologie der Elementarteilchenphysik jenseits des Standardmodells)
- Telefon
- +49 30 2093-82383
- HU-FIS-Profil
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- 26.4.2026, 01:02:13