Process scan, to try and see what each of process is generating, and which should B and S really be.

SM processes (background)


g g > t t~ t t~ and g g > t t~ all, all > t t~ [QED,QCD default]

72 diagrams in the first case, with only gluons as intermediate particles. σ = 8.33 fb

17 diagrams in the second case. "all" is only gluons, so we're generating g g > t t~ g, (g > t t~) σ = 25.14 fb

process g g > t t~ t t~ g g > t t~ all, all > t t~
gg_tttt_diagram1.png yes yes
gg_tttt_diagram3.png yes yes
gg_tttt_diagram11.png yes no
gg_tttt_diagram12.png yes yes
gg_tttt_diagram17.png yes yes
gg_tttt_diagram18.png yes yes
gg_tttt_diagram67.png yes yes
gg_tttt_diagram68.png yes yes
It seems there is only one type of diagram (11) that is not present in g g > t t~ all, (all > t t~). However, in this case

there are way less "permutations" (why?)

g g > t t~ t t~ and g g > t t~ all, all > t t~ [QED = 99 ,QCD=99]

216 diagrams in the first case. Gluons, Z, photon and Higgs as intermidiates. Number of diagrams ~x4 with respect to default orders

16+8+8+8 diagrams in the second case. Now "all" is either g, z, a or h. So, g g > t t~ {a,z,h,g}, ({a,z,h,g} > t t~)

The diagram distribution is going to be similar to that in the table above, with Z, photons and higgs entering the processes now.

From this it seems that all the processes in g g > t t~ all, all > t t~ are contained in g g > t t~ t t~. Generating the latter should contain the former, so there is no need of adding both.

From LaunchPad:

The syntax g g > t t~ g, (g > t t~)

means generate a gluon on shell and then decay the on shell particles to t t~…. not very physical

Actually the cross-section of g g > t t~ ALL, ALL > t t~ is not expected to be equal to g g > t t t~ t~ exactly because the first syntax force the particle “ALL” to be on shell which is not the case of the second syntax. The first syntax should only be use when Narrow width approximation is valid. Therefore I would encourage to NOT use multi-particle label with decay chain syntax.

Since the syntax “g g > t t~ g, (g > t t~) “ is meaningless, I have no idea (and do not care) how madgraph succeeds to returned a number to it. The actual number that I would expect madgraph to returns for the above syntax is 0 due to the restriction of the gluon to be on shell. What probably happens is that MG discard the condition of the gluon to be on shell. But even if that assumptions is correct and even if MG does it in a fully sensible way. You result is still meaningless since you break gauge invariance, meaning that your cross-section has no physical meaning (your cross-section is likely to be boost dependent, i.e. not lorentz invariant). The reason of this breaking of gauge invariance and lorentz invariant is that you are discarding important set of Feynman diagram, diagram which interferes strongly with the one that you kept. You can see that even from your number. Your cross-section is likely to be bigger simply because you discard Feynman diagram which interferes negatively with the Feynman diagram that you kept.

That means that g g > t t~ all, (all > t t~) is in general incorrect, unless we want "all" to be on-shell for some reason.

-- AlbertoGasconBravo - 2015-03-24

Topic attachments
I Attachment History Action Size Date Who Comment
Unknown file formateps diagrams_0_gg_ttxall_all_ttx.eps r1 manage 28.5 K 2015-03-24 - 10:18 AlbertoGasconBravo g g > t t~ all, (all > t t~)
Unknown file formateps diagrams_0_gg_ttxall_all_ttx_A99.eps r1 manage 20.0 K 2015-03-24 - 10:31 AlbertoGasconBravo g g > t t~ A, (A > t t~) QED=99 QCD=99
Unknown file formateps diagrams_0_gg_ttxall_all_ttx_G99.eps r1 manage 28.5 K 2015-03-24 - 10:32 AlbertoGasconBravo g g > t t~ G, (G > t t~) QED=99 QCD=99
Unknown file formateps diagrams_0_gg_ttxall_all_ttx_H99.eps r1 manage 20.0 K 2015-03-24 - 10:32 AlbertoGasconBravo g g > t t~ H, (H > t t~) QED=99 QCD=99
Unknown file formateps diagrams_0_gg_ttxall_all_ttx_Z99.eps r1 manage 20.0 K 2015-03-24 - 10:31 AlbertoGasconBravo g g > t t~ Z, (Z > t t~) QED=99 QCD=99
Unknown file formateps diagrams_0_gg_ttxttx.eps r1 manage 84.8 K 2015-03-24 - 09:57 AlbertoGasconBravo g g > t t~ t t~
Unknown file formateps diagrams_0_gg_ttxttx_99.eps r1 manage 229.1 K 2015-03-24 - 10:13 AlbertoGasconBravo g g > t t~ t t~ QED=99 QCD=99
PNGpng gg_tttt_diagram1.png r1 manage 14.7 K 2015-03-24 - 10:40 AlbertoGasconBravo  
PNGpng gg_tttt_diagram11.png r1 manage 6.6 K 2015-03-24 - 10:49 AlbertoGasconBravo  
PNGpng gg_tttt_diagram12.png r1 manage 6.5 K 2015-03-24 - 10:49 AlbertoGasconBravo  
PNGpng gg_tttt_diagram17.png r1 manage 6.5 K 2015-03-24 - 10:49 AlbertoGasconBravo  
PNGpng gg_tttt_diagram18.png r1 manage 6.8 K 2015-03-24 - 10:49 AlbertoGasconBravo  
PNGpng gg_tttt_diagram2.png r1 manage 7.2 K 2015-03-24 - 10:49 AlbertoGasconBravo  
PNGpng gg_tttt_diagram3.png r1 manage 7.8 K 2015-03-24 - 10:49 AlbertoGasconBravo  
PNGpng gg_tttt_diagram67.png r1 manage 10.3 K 2015-03-24 - 10:49 AlbertoGasconBravo  
PNGpng gg_tttt_diagram68.png r1 manage 10.0 K 2015-03-24 - 10:49 AlbertoGasconBravo  
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Topic revision: r2 - 2015-03-24 - AlbertoGasconBravo
 
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