arXiv subjects

Christian Schwinn

Publications and source records attributed to Christian Schwinn.

34 records · Page 2Linked to original sources

arXiv2008

CSW rules for a massive scalar

We derive the analog of the Cachazo-Svrcek-Witten (CSW) diagrammatic Feynman rules for four dimensional Yang-Mills gauge theory coupled to a massive colored scalar. The mass term is shown to give rise to a new tower of vertices in addition to the CSW vertices for massless scalars in non-supersymmetric theories. The rules are derived directly from an action, once through a canonical transformation within light-cone Yang-Mills and once by the construction of a twistor action. The rules are tested against known results in several examples and are used to simplify the proof of on-shell recursion relations for amplitudes with massive scalars.

hep-th↗

arXiv2007

Recent developments for multi-leg QCD amplitudes with massive particles

We review the extension of modern techniques for the calculation of helicity amplitudes in QCD to massive particles. The focus is on the use of supersymmetric Ward identites that relate amplitudes with massive quarks to those with massive scalars, the application of on-shell recursion relations to amplitudes with massive quarks and an extension of the CSW rules to massive scalars.

hep-ph↗

arXiv2007

Four-fermion production near the W pair production threshold

We perform a dedicated study of the four-fermion production process e- e+ -> mu- nubar_mu u dbar X near the W pair-production threshold in view of the importance of this process for a precise measurement of the W boson mass. Accurate theoretical predictions for this process require a systematic treatment of finite-width effects. We use unstable-particle effective field theory (EFT) to perform an expansion in the coupling constants, GammaW/MW, and the non-relativistic velocity v of the W boson up to next-to-leading order in GammaW/MW ~ alpha_ew ~ v^2. We find that the dominant theoretical uncertainty in MW is currently due to an incomplete treatment of initial-state radiation. The remaining uncertainty of the NLO EFT calculation translates into delta MW ~ 10-15 MeV, and to about 5 MeV with additional input from the NLO four-fermion calculation in the full theory.

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arXiv2007

Effective Theory Approach to W-Pair Production near Threshold

In this talk, I review the effective theory approach to unstable particle production and present results of a calculation of the process e- e+ ->mu- nubar_mu u dbar X near the W-pair production threshold up to next-to-leading order in GammaW/MW ~ alpha ~ v^2. The remaining theoretical uncertainty and the impact on the measurement of the W mass is discussed.

hep-ph↗

arXiv2007

On-shell recursion relations for all Born QCD amplitudes

We consider on-shell recursion relations for all Born QCD amplitudes. This includes amplitudes with several pairs of quarks and massive quarks. We give a detailed description on how to shift the external particles in spinor space and clarify the allowed helicities of the shifted legs. We proof that the corresponding meromorphic functions vanish at z --> infinity. As an application we obtain compact expressions for helicity amplitudes including a pair of massive quarks, one negative helicity gluon and an arbitrary number of positive helicity gluons.

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arXiv2006

Top quark signatures of Higgsless models

In these proceedings I describe the use of tree level unitarity to constrain top quark signatures of Higgsless electroweak models and discuss implications for collider phenomenology.

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arXiv2006

SUSY Ward identities for multi-gluon helicity amplitudes with massive quarks

We use supersymmetric Ward identities to relate multi-gluon helicity amplitudes involving a pair of massive quarks to amplitudes with massive scalars. This allows to use the recent results for scalar amplitudes with an arbitrary number of gluons obtained by on-shell recursion relations to obtain scattering amplitudes involving top quarks.

hep-th↗

arXiv2005

Born amplitudes in QCD from scalar diagrams

We review recent developments for the calculation of Born amplitudes in QCD. This includes the computation of gluon helicity amplitudes from MHV vertices and an approach based on scalar propagators and a set of three- and four-valent vertices. The latter easily generalizes to amplitudes with any number of quark pairs. The quarks may be massless or massive.

hep-th↗

arXiv2005

From Tree Unitarity to Top Quark Physics in 5D Higgsless Models

In five dimensional models of Higgsless electroweak symmetry breaking, tree level unitarity in gauge boson scattering is restored by the exchange of gauge boson Kaluza-Klein modes instead of a Higgs boson. Unitarity of scattering amplitudes involving top quarks requires also the Kaluza-Klein modes of the third family quarks. It is shown that the relevant unitarity cancellations are consistent with gauge symmetry breaking by boundary conditions. These results are used to constrain the couplings of the top quark to Kaluza-Klein modes and the implications for collider phenomenology are discussed.

hep-ph↗

arXiv2005

Unitarity constraints on top quark signatures of Higgsless models

We use conditions for unitarity cancellations to constrain the couplings of the top and bottom quarks to Kaluza-Klein modes in Higgsless models of electroweak symmetry breaking. An example for the mass spectrum of quark resonances in a theory space model is given and the implications for the collider phenomenology in the top sector are discussed, comparing to signatures of Little Higgs and strong electroweak symmetry breaking models.

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arXiv2005

Scalar diagrammatic rules for Born amplitudes in QCD

We show that all Born amplitudes in QCD can be calculated from scalar propagators and a set of three- and four-valent vertices. In particular, our approach includes amplitudes with any number of quark pairs. The quarks may be massless or massive. The proof of the formalism is given entirely within quantum field theory.

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arXiv2005

Top quark couplings at ILC: six and eight fermion final states

I discuss the calculation of cross sections for processes with six and eight fermions in the final state, contributing to single-top production and associated top-Higgs production at a linear collider. I describe the schemes for the treatment of finite decay widths implemented in the matrix element generator O'Mega and give a numerical comparison for single-top production. In the case of single top production, after reducing vector boson fusion backgrounds by appropriate cuts, the effect of including the full six fermion final state amounts to 2-5%. In associated top-Higgs production, non-resonant electroweak backgrounds are of a similar magnitude while QCD backgrounds are much larger.

hep-ph↗

arXiv2004

Consistent Dyson summation of Higgs propagators in nonlinear parameterizations revisited

As we demonstrate in a process independent way, in a nonlinear parameterization of the scalar sector of the standard model the Dyson summation of the Higgs self energy can be performed without violating the Ward Identities. This implies also the Goldstone boson equivalence theorem, in the limited range of its validity in effective field theories. This proves an earlier conjecture of Valencia and Willenbrock. Furthermore, the full Higgs propagator is independent of the gauge parameters. These results are consistent with the extension of the `gauge flip' formalism for the construction of gauge invariant classes of Feynman diagrams to loop diagrams. In a nonlinear parameterization of a 2-Higgs doublet model, the consistent Dyson summation is possible for all neutral Higgs bosons, but not for the charged scalars. Explicit examples of the equivalence theorem are discussed both in the minimal standard model and a two-Higgs doublet model.

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arXiv2004

Unitarity, BRST Symmetry and Ward Identities in Orbifold Gauge Theories

We discuss the use of BRST symmetry and the resulting Ward identities for orbifold gauge theories as consistency checks in an arbitrary number of dimensions. We verify that both the usual orbifold symmetry breaking and the recently proposed Higgsless symmetry breaking are consistent with the nilpotency of the BRST transformation. Imposing the Ward identities resulting from the BRST symmetry on the 4-point functions of theory, we obtain relations on the coupling constants that are shown to be equivalent to the conditions for tree level unitarity. We present the complete set of these sum rules also for inelastic scattering and discuss applications to 6-dimensional models and to incomplete matter multiplets on orbifold fixed points.

hep-ph↗

arXiv2004

Higgsless fermion masses and unitarity

We discuss the consistency of fermion mass generation by boundary conditions and brane localized terms in higher dimensional Higgsless models of gauge symmetry breaking. The sum rules imposed by tree-level unitarity and Ward Identities are applied to check the consistency of mass generation by orbifold projections and more general boundary conditions consistent with the variational principle. We find that the sum rules are satisfied for boundary conditions corresponding to brane localized mass and kinetic terms consistent with the reduced gauge symmetry on the brane.

hep-ph↗

arXiv2003

Gauge checks, consistency of approximation schemes and numerical evaluation of realistic scattering amplitudes

We discuss both theoretical tools to verify gauge invariance in numerical calculations of cross sections and the consistency of approximation schemes used in realistic calculations. A finite set of Ward Identities for 4 point scattering amplitudes is determined, that is sufficient to verify the correct implementation of Feynman rules of a spontaneously broken gauge theory in a model independent way. These identities have been implemented in the matrix element generator O'Mega and have been used to verify the implementation of the complete Standard Model in R_ξgauge. The consistency of approximation schemes in tree level calculations is discussed in the last part of this work. We determine the gauge invariance classes of spontaneously broken gauge theories, providing a new proof for the formalism of gauge and flavor flips. The schemes for finite width effects that have been implemented in O'Mega are reviewed. As a comparison with existing calculations, we study the consistency of these schemes in the process e^-e^+\to e^- \bar ν_e u\bar d. The violations of gauge invariance caused by the introduction of running coupling constants are analyzed.

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