Authors

Artur Amorim
University of Porto
artur_at_lip.pt

Model Description

This model is an extension of the Standard Model (SM) that describes in a general way the flavour changing neutral interactions of the top quark with other particles of the SM. The implementation in FeynRules was done in the context of an effective field theory using dimension-six gauge invariant operators. This provides a direct connection between experimental observ-ables and the new anomalous couplings. Because this model just describes interactions of the top quark with other SM particles, there are no additional fields.

Model Implementation

Our lagrangian was derived in the context of an effective field theory using dimension-six gauge invariant operators. In general this lagrangian can be written as

where Cx is a complex number and Ox is a dimension-six gauge invariant operator. In [1, 2] we can find the list of the most relevant and non-redundant operators Ox that are responsible for top anomalous interactions, in partic- ular for flavour changing neutral interactions. Using the results from those papers we arrive at the lagrangian of equation 2 wich is added to the SM lagrangian

This lagrangian is implemented in the file topFCNC.fr (see attachments) wich is then loaded in Mathematica with the notebook topFCNC.nb (see attachements.

The paremeters that appear in LtopFCNC are complex numbers in general and their real and imaginary parts can be set manually by the user. They can be divided in the following blocks :
•the ζ block : set the strenght of the coupling between the top, a gluon and a up or charm quark, as well the chirality of this coupling. The default values of the real and imaginary parts are 0.

• the η block : set the strenght of the coupling between the top, the Higgs boson and a up or charm quark, as well the chirality of this coupling. The default values of the real and imaginary parts are 0.
  
• the X block : set the strenght of the vector coupling between the top, the Z boson and a up or charm quark, as well the chirality of this coupling. The default values of the real and imaginary parts are 0.
  
• the K block : set the strenght of the tensorial coupling between the top, the Z boson and a up or charm quark, as well the chirality of this coupling. The default values of the real and imaginary parts are 0.
  
• the λ block : set the strenght of the coupling between the top, the photon and a up or charm quark, as well the chirality of this coupling. The default values of the real and imaginary parts are 0.
  

This model can be used in Madgraph5 by typing in the command line import model topFCNC UFO

Validation

To validate the UFO model we performed hermicity, mass spectrum tests as well cross-section tests. It was confirmed that the lagrangian is indeed hermitean. Since this lagrangian is just an extension of the SM lagrangian we obtained the same mass mass spectrum of the SM. In cross-section tests we verified that the cross-section is proportional to the modulos squared of the value of the anomalous couplings (exactly one non-zero coupling) and that it does not depend on the chirality of the coupling (with the same numerical value the left-handed coupling had the same cross-section as the right-handed coupling). The samples used for the cross-section tests were generated with MadGraph.

Files

You can find the .fr and .nb files in the attachements. You also have at your disposal a pdf which has a brief description of the parameters as well as their default value (ufo_model_description.pdf). The file topFCNC_UFO.zip is in the attachments must be extracted in the MadGraph folder models in order to it to be called by MadGraph to start event generation.

References

[1] J.A. Aguilar Saavedra, A minimal set of top anomalous couplings, ​arXiv:0811.3842v2 [hep-ph], 2008
[2] J.A. Aguilar Saavedra, A minimal set of top-Higgs anomalous couplings, ​arXiv:0904.2387v2 [hep-ph], 2009