arXiv subjects

Nicolas Greiner

Publications and source records attributed to Nicolas Greiner.

33 records · Page 2Linked to original sources

arXiv2013

Effective Field Theory of Precision Electroweak Physics at One Loop

The one loop effects of two dimension-six operators on gauge boson self energies are computed within an effective field theory framework. These self energies are translated into effects on precision electroweak observables, and bounds are obtained on the operator coefficients. The effective field theory framework allows for the divergences that arise in the loop calculations to be properly handled, and for unambiguous bounds on the coefficients to be obtained. We find that the coefficients are only weakly bounded, in contrast to previous calculations that obtained much stronger bounds. We argue that the results of these previous calculations are specious.

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arXiv2012

Susy-QCD corrections to neutralino pair production in association with a jet

We present the NLO Susy-QCD corrections to the production of a pair of the lightest neutralinos plus one jet at the LHC, appearing as a monojet signature in combination with missing energy. We fully include all non-resonant diagrams, i.e. we do not assume that production and decay factorise. We derive a parameter point based on the p19MSSM which is compatible with current experimental bounds and show distributions based on missing transverse energy and jet observables. Our results are produced with the program GOSAM for automated one-loop calculations in combination with MadDipole/MadGraph for the real radiation part.

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arXiv2012

Constraints on Non-standard Top Quark Couplings

We study non-standard top quark couplings in the effective field theory approach. All nine dimension-six operators that generate anomalous couplings between the electroweak gauge bosons and the third-generation quarks are included. We calculate their contributions at tree level and one loop to all major precision electroweak observables. The calculations are compared with data to obtain constraints on eight of these operators.

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arXiv2012

Effective Field Theory: A Modern Approach to Anomalous Couplings

We advocate an effective field theory approach to anomalous couplings. The effective field theory approach is the natural way to extend the standard model such that the gauge symmetries are respected. It is general enough to capture any physics beyond the standard model, yet also provides guidance as to the most likely place to see the effects of new physics. The effective field theory approach also clarifies that one need not be concerned with the violation of unitarity in scattering processes at high energy. We apply these ideas to pair production of electroweak vector bosons.

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arXiv2012

Automated One-Loop Calculations with GoSam

We present the program package GoSam which is designed for the automated calculation of one-loop amplitudes for multi-particle processes in renormalisable quantum field theories. The amplitudes, which are generated in terms of Feynman diagrams, can be reduced using either D-dimensional integrand-level decomposition or tensor reduction. GoSam can be used to calculate one-loop QCD and/or electroweak corrections to Standard Model processes and offers the flexibility to link model files for theories Beyond the Standard Model. A standard interface to programs calculating real radiation is also implemented. We demonstrate the flexibility of the program by presenting examples of processes with up to six external legs attached to the loop.

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arXiv2011

Automation of One-Loop Calculations with GoSam: Present Status and Future Outlook

In this presentation, we describe the GoSam (Golem/Samurai) framework for the automated computation of multi-particle scattering amplitudes at the one-loop level. The amplitudes are generated analytically in terms of Feynman diagrams, and can be evaluated using either D-dimensional integrand reduction or tensor decomposition. GoSam can be used to compute one-loop corrections to Standard Model (QCD and EW) processes, and it is ready to link generic model files for theories Beyond SM. We show the main features of GoSam through its application to several examples of different complexity.

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arXiv2011

Effective Field Theory for Nonstandard Top Quark Couplings

We present an effective-field-theory calculation of the effect of a dimension-six operator involving the top quark on precision electroweak data via a top-quark loop. We demonstrate the renormalizability, in the modern sense, of the effective field theory. We use the oblique parameter U to bound the coefficient of the operator, and compare with the bound derived from top-quark decay.

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arXiv2011

NLO QCD corrections to the production of two bottom-antibottom pairs at the LHC

We report the results of a computation of the full next-to-leading order QCD corrections to the production of two $b\bar{b}$ pairs at the LHC. This calculation at the parton level provides predictions for well separated $b$-jets. The results show that the next-to-leading order corrections lead to an enhancement of the cross-section for the central scale choice by roughly 50% with respect to the leading order result. The theoretical uncertainty estimated by variation of the renormalization and factorization scales is strongly reduced by the inclusion of next-to-leading order corrections.

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arXiv2010

Modern Feynman Diagrammatic One-Loop Calculations

In this talk we present techniques for calculating one-loop amplitudes for multi-leg processes using Feynman diagrammatic methods in a semi-algebraic context. Our approach combines the advantages of the different methods allowing for a fast evaluation of the amplitude while monitoring the numerical stability of the calculation. In phase space regions close to singular kinematics we use a method avoiding spurious Gram determinants in the calculation. As an application of our approach we report on the status of the calculation of the amplitude for the process $pp\to b\bar{b}b\bar{b}+X$.

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arXiv2010

Recent Progress in the Golem Project

We report on the current status of the Golem project which aims at the construction of a general one-loop evaluator for matrix elements. We construct the one-loop matrix elements from Feynman diagrams in a highly automated way and provide a library for the reduction and numerically stable evaluation of the tensor integrals involved in this approach. Furthermore, we present applications to physics processes relevant for the LHC.

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arXiv2010

NLO QCD corrections to 4 b-quark production

In this talk we present the recent calculation of the NLO QCD corrections to the production of four b-quarks induced by initial state quarks at the CERN LHC. We describe the details of the calculation using the GOLEM approach for the virtual corrections and MadEvent/MadDipole for the real emission part and present some results.

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arXiv2010

Integrated dipoles with MadDipole in the MadGraph framework

Heading towards a full automation of next-to-leading order (NLO) QCD corrections, one important ingredient is the analytical integration over the one-particle phase space of the unresolved particle that is necessary when adding the subtraction terms to the virtual corrections. We present the implementation of these integrated dipoles in the MadGraph framework. The result is a package that allows an automated calculation for the NLO real emission parts of an arbitrary process.

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arXiv2010

NLO Cross Sections for the LHC using GOLEM: Status and Prospects

In this talk we review the GOLEM approach to one-loop calculations and present an automated implementation of this technique. This method is based on Feynman diagrams and an advanced reduction of one-loop tensor integrals which avoids numerical instabilities. We have extended our one-loop integral library golem95 with an automated one-loop matrix element generator to compute the virtual corrections of the process $q\bar{q}\to b\bar{b}b\bar{b}$. The implementation of the virtual matrix element has been interfaced with tree-level Monte Carlo programs to provide the full result for the above process.

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arXiv2008

Muon Excess in Cosmic Rays and at CDF: Signs for a Hidden Sector?

In this letter we discuss a certain class of Hidden Valley models where the dynamics in the hidden sector are close to a strongly coupled conformal fixed point. We show that these models can explain the excess in cosmic ray muon events with high muon multiplicities that has been reported from cosmic ray studies with the ALEPH and DELPHI detectors. We further point out that these models can also at least partially account for the excess in multi-muon events that was found by the CDF experiment. Finally we discuss possible signatures of these models at the LHC.

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arXiv2008

Automation of the Dipole Subtraction Method in MadGraph/MadEvent

We present the implementation of the dipole subtraction formalism for the real radiation contributions to any next-to-leading order QCD process in the MadGraph/MadEvent framework. Both massless and massive dipoles are considered. Starting from a specific (n+1)-particle process the package provides a Fortran code for all possible dipoles to all Born processes that constitute the subtraction term to the (n+1)-particle process. The output files are given in the usual "MadGraph StandAlone" style using helicity amplitudes.

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