Last modified 4 years ago Last modified on 07/03/13 10:33:31

Exotic (X,T) Doublet Model (VLQ)


  • Mathieu Buchkremer
    • Université catholique de Louvain (CP3)
  • Giacomo Cacciapaglia
    • Université de Lyon (CNRS/IN2P3)
  • Aldo Deandrea
    • Université de Lyon (CNRS/IN2P3)
  • Luca Panizzi
    • University of Southampton (School of Physics and Astronomy)

Description & Model Implementation

This model is an simplified version of the VLQ FeynRules model, including a single vector-like (X,T) doublet (Q=5/3, 2/3), with non-zero mixings to the first and third generation SM quarks [1] [2], as described in the dedicated study [3]. The pattern of allowed couplings is such that X decays 100% to W bosons, while T decays 50% in Z and 50% in H bosons, in the asymptotic limit where their (degenerate) masses go to infinity. The corresponding FeynRules implementation is based on the model description, which provides a direct connection between experimental observables and the new quark couplings. The model file adds the following interactions to the SM Lagrangian:

The associated electromagnetic and strong interactions are also included, together with the X and T quarks kinetic and mass terms. The user-defined parameters, summarised in Parameters.pdf, consist of:

  • g*, the X and T quarks coupling strength entering single production (set to 0.1 by default in the KAPPA block).
  • M, the X and T quark masses (set to 1 TeV by default in the MASS block).
  • RL, the rate of the X and T quarks decays into light quarks (set to 0.5 by default in the ZETA block).
  • Gvl, the coupling of the X and T quarks to the gauge bosons (set to 1 by default in the GVL block).

Note: the top partners widths are set to 1 GeV by default, but their actual values must be evaluated and adjusted explicitly in MadGraph? (specifying all the corresponding decay channels), for a fixed choice of the above parameters. These parameters are combined internally in FeynRules to match the model-independent prescription presented above. For consistency, all light quarks included in the proton definition are restricted to be massless (5F scheme), except for the top, bottom and tau lepton masses. The default model loads the real 3X3 CKM matrix given in Parameters.pdf as an external parameter. In general, the presence of new top partners induce model-dependent corrections to the mixings between the novel heavy fermions and the lighter quark families, which are not taken into account here.


Below we provide the tarballs containing the main model files and outputs. The UFO model files have been compiled with Feynrules 1.4.9, and are compatible with MadGraph? 1.5.11. The CH model files require the latest updated version of CalcHep?, and have been obtained using version 1.7.178 of FeynRules. To import the model into MadGraph?, it is enough to unzip the UFO directory into the models subdirectory of MadGraph?, and load the model using the "import model XTdoubletVL_UFO" command.


[1] M. Buchkremer, G. Cacciapaglia, A. Deandrea, L. Panizzi, Model Independent Framework for Searches of Top Partners, hep-ph/1305.4172.

[2] G. Cacciapaglia, A. Deandrea, D. Harada, Y. Okada, Bounds and Decays of New Heavy Vector-like Top Partners, hep-ph/1007.2933.

[3] G. Cacciapaglia, A. Deandrea, L. Panizzi, S. Perries, V. Sordini, Heavy Vector-like quark with charge 5/3 at the LHC, hep-ph/1211.4034.

[4] F. del Aguila, M. Perez-Victoria, J. Santiago, Observable contributions of new exotic quarks to quark mixing, hep-ph/0007316.