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Search Results: 1 - 10 of 343158 matches for " Christian S.;Williams "
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Beyond the rainbow: effects from pion back-coupling
Fischer, Christian S.;Williams, Richard
High Energy Physics - Phenomenology , 2008, DOI: 10.1103/PhysRevD.78.074006
Abstract: We investigate hadronic unquenching effects in light quarks and mesons. To this end we take into account the back-coupling of the pion onto the quark propagator within the non-perturbative continuum framework of Schwinger-Dyson equations (SDE) and Bethe-Salpeter equations (BSE). We improve on a previous approach by explicitly solving both the coupled system of DSEs and BSEs in the complex plane and the normalisation problem for Bethe-Salpeter kernels depending on the total momentum of the meson. As a result of our study we find considerable unquenching effects in the spectrum of light pseudoscalar, vector and axial-vector mesons.
Pion-cloud effects in the BSE description of mesons
Williams, Richard;Fischer, Christian S.
High Energy Physics - Phenomenology , 2008,
Abstract: We investigate the effect of including hadronic resonance contributions in the description of light quarks and mesons. To this end we take into account the back-coupling of the pion onto the quark propagator within the non-perturbative continuum framework of Schwinger-Dyson equations (SDE) and Bethe-Salpeter equations (BSE), in essence describing the so-called \emph{pion-cloud}. As a result of our study we find that an unquenching of this form provides for considerable effects in the spectrum of light mesons.
Probing the gluon self-interaction in light mesons
Christian S. Fischer,Richard Williams
Physics , 2009, DOI: 10.1103/PhysRevLett.103.122001
Abstract: We investigate masses and decay constants of light mesons from a coupled system of Dyson--Schwinger and Bethe--Salpeter equations. We explicitly take into account dominant non-Abelian contributions to the dressed quark-gluon vertex stemming from the gluon self-interaction. We construct the corresponding Bethe-Salpeter kernel that satisfies the axial-vector Ward-Takahashi identity. Our numerical treatment fully includes all momentum dependencies with all equations solved completely in the complex plane. This approach goes well beyond the rainbow-ladder approximation and permits us to investigate the influence of the gluon self-interaction on the properties of mesons. As a first result we find indications of a nonperturbative cancellation of the gluon self-interaction contributions and pion cloud effects in the mass of the rho-meson.
Bethe-Salpeter equations: mesons beyond the rainbow-ladder truncation
Richard Williams,Christian S. Fischer
Physics , 2009,
Abstract: We investigate masses of light mesons from a coupled system of Dyson--Schwinger (DSE) and Bethe--Salpeter equations (BSE), taking into account dominant non-Abelian, sub-leading Abelian, and dominant pion cloud contributions to the dressed quark-gluon vertex. The axial-vector Ward-Takahashi identity preserving Bethe-Salpeter kernel is constructed and the spectrum of light mesons calculated. Our model goes significantly beyond the rainbow-ladder. We find that sub-leading Abelian corrections are further dynamically suppressed, and that our results supersede early qualitative predictions from simple truncation schemes.
Pion-cloud effects in the BSE description of mesons
Richard Williams,Christian S. Fischer
Physics , 2008,
Abstract: We investigate the effect of including hadronic resonance contributions in the description of light quarks and mesons. To this end we take into account the back-coupling of the pion onto the quark propagator within the non-perturbative continuum framework of Schwinger-Dyson equations (SDE) and Bethe-Salpeter equations (BSE), in essence describing the so-called \emph{pion-cloud}. As a result of our study we find that an unquenching of this form provides for considerable effects in the spectrum of light mesons.
Beyond the rainbow: effects from pion back-coupling
Christian S. Fischer,Richard Williams
Physics , 2008, DOI: 10.1103/PhysRevD.78.074006
Abstract: We investigate hadronic unquenching effects in light quarks and mesons. To this end we take into account the back-coupling of the pion onto the quark propagator within the non-perturbative continuum framework of Schwinger-Dyson equations (SDE) and Bethe-Salpeter equations (BSE). We improve on a previous approach by explicitly solving both the coupled system of DSEs and BSEs in the complex plane and the normalisation problem for Bethe-Salpeter kernels depending on the total momentum of the meson. As a result of our study we find considerable unquenching effects in the spectrum of light pseudoscalar, vector and axial-vector mesons.
Finite volume effects and dynamical chiral symmetry breaking in QED3
Goecke, Tobias;Fischer, Christian S.;Williams, Richard
High Energy Physics - Phenomenology , 2008, DOI: 10.1103/PhysRevB.79.064513
Abstract: We investigate the impact of finite volume effects on the critical number of flavours, N_f^c, for chiral symmetry restoration in QED3. To this end we solve a set of coupled Dyson-Schwinger equations on a torus. For order parameters such as the anomalous dimension of the fermion wave function or the chiral condensate we find substantial evidence for a large dependence on the volume. We observe a shift in N_f^c from values in the range of 3.61 \le N_f^c \le 3.84 in the infinite volume/continuum limit down to values below N_f \le 1.5 at finite volumes in agreement with earlier results of Gusynin and Reenders in a simpler truncation scheme. These findings explain discrepancies in N_f^c between continuum and lattice studies.
The role of vector meson dominance and running masses in the hadronic contributions to the muon $g-2$
Goecke, Tobias;Fischer, Christian S.;Williams, Richard
High Energy Physics - Phenomenology , 2013,
Abstract: We summarize our recent results for the quark loop part of the light-by-light scattering contribution as well as the hadronic vacuum polarisation contributions to the anomalous magnetic moment of the muon. In particular we focus on the role played by the momentum dependence of the quark- and quark-photon vertex dressing functions. We give a detailed comparison of the Dyson-Schwinger description of this contribution to the corresponding picture emerging from hadronic models in particular the extended Nambu--Jona-Lasonio model (ENJL). We find that the details of the momentum dependence are important on a quantitative level. Especially the transverse parts of the quark-photon-vertex, which serve as a dynamical extension of simple vector meson dominance models, do not yield the large suppression of the light-by-light contribution found in the ENJL model if realistic dressings are taken into account.
Non-contact interactions and the hadronic light-by-light contribution to the muon g-2
Williams, Richard;Fischer, Christian S.;Goecke, Tobias
High Energy Physics - Phenomenology , 2013,
Abstract: We summarise recent results for the quark loop part of the light-by-light scattering contribution to the muons anomalous magnetic moment. In particular we focus on the impact of a momentum dependent quark and quark-photon vertex. We compare the Dyson-Schwinger description with that of the extended Nambu--Jona-Lasinio model (ENJL) and find important quantitative differences. In particular the transverse parts of the quark-photon-vertex, which serve as a dynamical extension of simple vector meson dominance models, do not yield the large suppression as found in the ENJL model.
On Gribov's supercriticality picture of quark confinement
Fischer, Christian S.;Nickel, Dominik;Williams, Richard
High Energy Physics - Phenomenology , 2008, DOI: 10.1140/epjc/s10052-008-0821-1
Abstract: Some years ago Gribov developed the so-called supercritical light quark confinement scenario. Based on physical arguments he conjectured a drastic change in the analytical properties of the quark propagator when the back-reaction of Goldstone bosons (pions) is considered. We investigate this scenario and provide numerical solutions for the quark propagator in the complex plane with and without the pion back-reaction. We find no evidence for the scenario Gribov advocated. As an aside we present a novel method to solve the quark Dyson-Schwinger equation in the complex plane and discuss new characteristics of dynamical chiral symmetry breaking in our truncation scheme.
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