27 | | As a generic framework, the four Vector-Like states X (Q=5/3), T (2/3), B (-1/3) and Y (-4/3) are introduced as new spin 1/2 class members. Each class defines a specific top partner with a fixed electric charge, without any assumption on the other quantum numbers. The model files are designed to allow for a straightforward extension to additional vector-like fermions, simply by adding the new particles to the corresponding class (F[5], F[6], F[7] and F[8] for X-, T-, B- and Y-type quarks, respectively). Particles embedded in larger SU(2) representations may also be introduced under new classes, following the same model-independent prescription. |
| 27 | As a generic framework, the four Vector-Like states X (Q=5/3), T (2/3), B (-1/3) and Y (-4/3) are introduced as new spin 1/2 class members. Each class defines a specific top partner with a fixed electric charge, without any assumption on the other quantum numbers. The model file [attachment:VLQ.fr], loaded in Mathematica with [attachment:VLQ.nb], is designed to allow for a straightforward extension to additional vector-like fermions, simply by adding the new particles to the corresponding class (F[5], F[6], F[7] and F[8] for X-, T-, B- and Y-type quarks, respectively). Particles embedded in larger SU(2) representations may also be introduced under new classes, following the same model-independent prescription. |
32 | | The X, T, B and Y electromagnetic and strong currents, as well as the associated kinetic and mass terms, are also implemented for all four quark species. The model-dependent, off-diagonal WQQ' and ZQQ' interactions between top partners have not been included. For convenience, mixings with eventual heavier particles have been neglected as well. |
| 32 | The X, T, B and Y electromagnetic and strong currents, as well as the associated kinetic and mass terms, are also implemented for all four quark species. The model-dependent, off-diagonal WQQ', ZQQ' and HQQ' interactions between two heavy partners have not been included. For convenience, mixings with eventual heavier particles have been neglected as well. |
42 | | All the above parameter classes are combined internally in FeynRules to match the model-independent prescription presented in [1]. The default MadGraph parameter cards use the standard values for the SM input parameters as given in [attachment:Parameters.pdf]. For consistency, all light quarks included in the proton definition are restricted to be massless (5F scheme) using the restriction file [attachment:Massless.rst] when loading the FeynRules model, except for the top, bottom and tau lepton masses. The default model loads the real 3X3 CKM matrix given in [attachment:Parameters.pdf] [2] 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, and the SM electroweak couplings must be modified accordingly. |
| 42 | All the above parameter classes are combined internally in the FeynRules model file [attachment:VLQ.fr] to match the model-independent prescription presented above [1]. The default parameter cards use the standard MadGraph values for the SM input parameters, as given in [attachment:Parameters.pdf]. For consistency, all light quarks included in the proton definition are restricted to be massless (5F scheme) using the restriction file [attachment:Massless.rst] when loading the FeynRules model, except for the top, bottom and tau lepton masses. The default model loads the real 3X3 CKM matrix given in [attachment:Parameters.pdf] [2] 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, and the SM electroweak couplings must be modified accordingly. |
| 43 | |
| 44 | In addition to the above parameterisation, an extension including chromomagnetic couplings between VL and Standard Model quarks is provided in the file VLQ_Chromomagnetic.fr. The new couplings (typically generated at loop level in minimal SM extensions) are implemented in terms of the dimension 6 operator: |
| 45 | [[Image(LagrangianChromo.png)]] |
| 46 | where G denotes the field strength tensor of the gluon, v is the Higgs field vacuum expectation value and gs is the QCD coupling. The KAPPA block is complemented with the additional parameter kappa_g (set to 1 by default), specifying the strength of the new corresponding strong coupling. The new GLUON block includes the new Tqg(g) and Bqg(g) couplings to the three SM quark families, together with the energy scale Lambda (set to 10 TeV by default). These parameters can be adjusted independently from the others described above. |