Changes between Version 3 and Version 4 of StandardModel


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Timestamp:
04/06/12 16:33:19 (8 years ago)
Author:
trac
Comment:

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  • StandardModel

    v3 v4  
    1 = Standard Model interactions =
    21
    3 The Standard Model of particles and interactions, based on the SU(3),,c,, x SU(2),,L,, x U(1),,Y,, gauge symmetry has been available since the first versions of both MadGraph and more recently of MadEvent. There is, however, one important differences w.r.t. the previous version of the package, regarding how the couplings of the models are handled. As was already mentioned in the previous section, the task of computing from the parameters in the Lagrangian (primary parameters) all the secondary parameters (masses, widths and dependent parameters) needed by MadGraph is left to an external program, the SM Calculator. The output of the SM Calculator is a parameter card, param_card.dat, which contains the numerical values of the main couplings (primary and secondary) of a specific model. The parameter card has a format compliant with the SUSY Les Houches Accord.
    42
    5 A simple example is given by the EW parameters that characterize the gauge SU(2),,L,, x U(1),,Y,, interactions and its breaking: in the Standard Model there are five relevant parameters, α,,em,,, G,,F,,, sin θ,,W,,, m,,Z,,, m,,W,, of which only three are independent at tree level. Various schemes differing by the choice of the parameters considered independent are used in the literature. In the SM Calculator, the default is to take G,,F,,, m,,Z,,, m,,W,, as inputs and derive  α,,em,,, sin θ,,W,,, but other choices are available. As a result a consistent and unique set of values of the couplings appearing in the Feynman rules is derived and used for the computation of the amplitudes.
    6  
     3== Standard Model interactions ==
    74
    8 {{{
    9 #!html
    10 Another sometimes important feature of our SM implementation, is the possibility of distinguishing between the kinematic mass (pole mass) for the quarks and that entering in the Yukawa coupling definition ( <span style="text-decoration: overline">MS</span> mass). For the latter, the user can choose to evolve the mass to the scale corresponding to the Higgs mass, which leads to an improvement of the perturbative expansion.
    11 }}}
    125
    13 Finally, we mention that various versions of the Standard Model are actually available for specific studies. For example, in the "minimal SM!'' (sm) the CKM matrix is diagonal while in the smckm model a mixing between the first and second generation is allowed (Cabibbo angle). Another example is the sm_nohiggs model where the Higgs has been eliminated and the EWSB sector behaves as a non-linear sigma-model.
     6The Standard Model of particles and interactions, based on the
     7%$SU(3)''c \times SU(2)''L \times U(1)_Y$ gauge symmetry has been
     8available since the first versions of both MadGraph and more recently
     9of MadEvent.  There is, however, one important differences w.r.t.  the
     10previous version of the package, regarding how the couplings of the
     11models are handled.  As was already mentioned in the previous section,
     12the task of computing from the parameters in the Lagrangian (primary
     13parameters) all the secondary parameters (masses, widths and dependent
     14parameters) needed by MadGraph is left to an external program, the SM
     15Calculator. The output of the SM Calculator is a parameter card,
     16param_card.dat, which contains the numerical values of the main
     17couplings (primary and secondary) of a specific model. The parameter
     18card has a format compliant with the SUSY Les Houches Accord.
    1419
    15 -- !MichelHerquet - 09 Apr 2007
     20A simple example is given by the EW parameters that characterize
     21the gauge $SU(2)''L \times U(1)''Y$ interactions and its breaking:
     22in the Standard Model there are five relevant parameters,
     23%$\alpha_{em}, G_F, \sin \theta ''W, m''Z,m_W$ of which
     24only three are independent at tree level. Various schemes
     25differing by the choice of the parameters considered independent are
     26used in the literature. In the SM Calculator, the default is to take
     27%$G_F,m_Z,m_W$ as inputs and derive $\alpha_{em}, \sin \theta_W$,
     28but other choices are available. As a result a consistent and unique set of
     29values of the couplings appearing in the Feynman rules is derived and
     30used for the computation of the amplitudes.
     31
     32Another sometimes important feature of our SM implementation, is the
     33possibility of distinguishing between the kinematic mass (pole mass)
     34for the quarks and that entering in the Yukawa coupling definition
     35($\overline{MS}$ mass). For the latter, the user can choose to evolve
     36the mass to the scale corresponding to the Higgs mass, which leads to
     37an improvement of the perturbative expansion.
     38
     39Finally, we mention that various versions of the Standard Model are
     40actually available for specific studies. For example, in the ``minimal
     41SM'' (sm) the CKM matrix is diagonal while in the smckm
     42model a mixing between the first and second generation is allowed
     43(Cabibbo angle).  Another example is the sm_nohiggs model where
     44the Higgs has been eliminated and the EWSB sector behaves as a
     45non-linear sigma-model.
     46
     47
     48-- Main.MichelHerquet - 09 Apr 2007
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