Changes between Version 2 and Version 3 of StandardModel
 Timestamp:
 04/04/12 14:28:20 (8 years ago)
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StandardModel
v2 v3 1 = Standard Model interactions = 1 2 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. 2 4 3 == Standard Model interactions == 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 4 7 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="textdecoration: 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 }}} 5 12 6 The 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 8 available since the first versions of both MadGraph and more recently 9 of MadEvent. There is, however, one important differences w.r.t. the 10 previous version of the package, regarding how the couplings of the 11 models are handled. As was already mentioned in the previous section, 12 the task of computing from the parameters in the Lagrangian (primary 13 parameters) all the secondary parameters (masses, widths and dependent 14 parameters) needed by MadGraph is left to an external program, the SM 15 Calculator. The output of the SM Calculator is a parameter card, 16 param_card.dat, which contains the numerical values of the main 17 couplings (primary and secondary) of a specific model. The parameter 18 card has a format compliant with the SUSY Les Houches Accord. 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 nonlinear sigmamodel. 19 14 20 A simple example is given by the EW parameters that characterize 21 the gauge $SU(2)''L \times U(1)''Y$ interactions and its breaking: 22 in the Standard Model there are five relevant parameters, 23 %$\alpha_{em}, G_F, \sin \theta ''W, m''Z,m_W$ of which 24 only three are independent at tree level. Various schemes 25 differing by the choice of the parameters considered independent are 26 used in the literature. In the SM Calculator, the default is to take 27 %$G_F,m_Z,m_W$ as inputs and derive $\alpha_{em}, \sin \theta_W$, 28 but other choices are available. As a result a consistent and unique set of 29 values of the couplings appearing in the Feynman rules is derived and 30 used for the computation of the amplitudes. 31 32 Another sometimes important feature of our SM implementation, is the 33 possibility of distinguishing between the kinematic mass (pole mass) 34 for the quarks and that entering in the Yukawa coupling definition 35 ($\overline{MS}$ mass). For the latter, the user can choose to evolve 36 the mass to the scale corresponding to the Higgs mass, which leads to 37 an improvement of the perturbative expansion. 38 39 Finally, we mention that various versions of the Standard Model are 40 actually available for specific studies. For example, in the ``minimal 41 SM'' (sm) the CKM matrix is diagonal while in the smckm 42 model a mixing between the first and second generation is allowed 43 (Cabibbo angle). Another example is the sm_nohiggs model where 44 the Higgs has been eliminated and the EWSB sector behaves as a 45 nonlinear sigmamodel. 46 47 48  Main.MichelHerquet  09 Apr 2007 49 50 51 52 53 54 55 56 15  !MichelHerquet  09 Apr 2007