MadGraph5_aMC@NLO

MadGraph Cross-section

Asked by ouseph cj

Dear Madgraph Experts,

I am computing the Deep inelastic neutrino nucleon scattering cross-section mediated by Z and Zprime, According to the analytical formula, the total cross-section (with z and Zprime) should overlap with the SM cross-section. But My madgraph results are not overlapping with the SM cross-section and in the Higher mass regime, the total cross-section differs by 2 to 3% from the SM one.

Is this a computation error associated with the Madgraph? How can I solve this problem? Or is it a problem with my model file

I am attaching my model file with this email, I am looking forward to hearing from you.

M$ModelName = "Tutorial";
M$Information = {
  Authors -> {"****"},
  Version -> "1.0.0",
  Date -> "15. 07. 2020",
  Institutions -> {"*****"}

};

M$InteractionOrderHierarchy = { {QCD,1}, {NP,1}, {QED,2} };
M$ClassesDescription = {
     V[6] == {
     ClassName -> Zp,
     SelfConjugate -> True,
     Indices -> {},
     Mass -> {MZp, 3000},
     Width -> {WZp, 600},
     ParticleName -> "Zp",
     PropagatorLabel -> "Zp",
     PropagatorType -> Sine,
     PropagatorArrow -> None,
     FullName -> "Zprime"
   }
};

M$Parameters = {

(* External Parameters *)

              (**** Overall Coupling****)

         gZp == {
         ParameterType -> External,
         Value -> 3.0,
         InteractionOrder -> {NP,1},
         TeX -> Subscript[g,Zp],
         BlockName -> NPZPCOUP,

         Description -> "Overall Zp coupling strength"
   },

              (***** neutrino coupling ******)

          gee == {
         ParameterType -> External,
         Value -> 1.0,
         InteractionOrder -> {NP,0},
         TeX -> Subscript[g,ee],
         BlockName -> NPZPCOUP

   },

          gem == {
         ParameterType -> External,
         Value -> 1.0,
         InteractionOrder -> {NP,0},
         TeX -> Subscript[g,em],
         BlockName -> NPZPCOUP

   },

          get == {
         ParameterType -> External,
         Value -> 1.0,
         InteractionOrder -> {NP,0},
         TeX -> Subscript[g,et],
         BlockName -> NPZPCOUP

   },

          gmm == {
         ParameterType -> External,
         Value -> 1.0,
         InteractionOrder -> {NP,0},
         TeX -> Subscript[g,mm],
         BlockName -> NPZPCOUP

   },

          gme == {
         ParameterType -> External,
         Value -> 1.0,
         InteractionOrder -> {NP,0},
         TeX -> Subscript[g,me],
         BlockName -> NPZPCOUP

   },

           gmt == {
         ParameterType -> External,
         Value -> 1.0,
         InteractionOrder -> {NP,0},
         TeX -> Subscript[g,mt],
         BlockName -> NPZPCOUP

   },

          gtt == {
         ParameterType -> External,
         Value -> 1.0,
         InteractionOrder -> {NP,0},
         TeX -> Subscript[g,tt],
         BlockName -> NPZPCOUP

   },

          gte == {
         ParameterType -> External,
         Value -> 1.0,
         InteractionOrder -> {NP,0},
         TeX -> Subscript[g,te],
         BlockName -> NPZPCOUP

   },

         gtm == {
         ParameterType -> External,
         Value -> 1.0,
         InteractionOrder -> {NP,0},
         TeX -> Subscript[g,tm],
         BlockName -> NPZPCOUP

   },

        (***** quark coupling******)

          gur == {
         ParameterType -> External,
         Value -> 1.0,
         InteractionOrder -> {NP,0},
         TeX -> Subscript[g,ur],
         BlockName -> NPZPCOUP

   },

          gul == {
         ParameterType -> External,
         Value -> 1.0,
         InteractionOrder -> {NP,0},
         TeX -> Subscript[g,ul],
         BlockName -> NPZPCOUP

   },

          gdr == {
         ParameterType -> External,
         Value -> 1.0,
         InteractionOrder -> {NP,0},
         TeX -> Subscript[g,dr],
         BlockName -> NPZPCOUP

         },

         gdl == {
         ParameterType -> External,
         Value -> 1.0,
         InteractionOrder -> {NP,0},
         TeX -> Subscript[g,dr],
         BlockName -> NPZPCOUP

         },

    (***** leptons coupling******)

       glR == {
         ParameterType -> External,
         Value -> 1.0,
         InteractionOrder -> {NP,0},
         TeX -> Subscript[g,lR],
         BlockName -> NPZPCOUP

         },

       glL == {
         ParameterType -> External,
         Value -> 1.0,
         InteractionOrder -> {NP,0},
         TeX -> Subscript[g,lR],
         BlockName -> NPZPCOUP

         }

};

Ln = gZp* (gee* (vebar.Ga[\[Mu]].left[ve]) +
    gem *(vebar.Ga[\[Mu]].left[vm]) +
    get *(vebar.Ga[\[Mu]].left[vt]) +
    gmm *(vmbar.Ga[\[Mu]].left[vm]) +
    gme *(vmbar.Ga[\[Mu]].left[ve]) + gmt *(vmbar.Ga[\[Mu]].left[vt]) +
    gte*(vtbar.Ga[\[Mu]].left[ve]) + gtt *(vtbar.Ga[\[Mu]].left[vt]) +
     gtm *(vtbar.Ga[\[Mu]].left[vm]) + gul*(ubar.Ga[\[Mu]].left[u]) +
    gur*(ubar.Ga[\[Mu]].right[u]) + gdl*(dbar.Ga[\[Mu]].left[d]) +
    gdr*(dbar.Ga[\[Mu]].right[d]) + glL*(lbar.Ga[\[Mu]].left[l]) +
    glR*(lbar.Ga[\[Mu]].right[l]))*Zp[\[Mu]]

L =Ln+LSM

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Olivier Mattelaer (olivier-mattelaer) said : 		#1

Hi,

What is the theoretical uncertainty quoted? and what is the statistical uncertainty quoted?
Typically statistical uncertainty depend of the number of events requested (default is at the order of the % level)
theoretical uncertainty depends of the power of alpha_S present in your process (for the renormalization scale uncertainty which is the dominating one in general) but at LO you typically expect error of 40%. So a disagreement of 2-3% can be easyily explained with a difference in the renormalization scale. (at that level of precision, theoretical error in PDF and factorization scale can also explained such small difference.)

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