mg272 and mg273 give different xs for longitudinally polarized processes

Asked by xiao jie on 2020-07-17

Dear authors,

I found mg273 and mg272 give different cross sections for longitudinally polarized VBS processes.

For WLZL:
    set process:
        generate p p > w+{0} z{0} j j QED=4 QCD=0, w+ > l+ vl, z > l+ l- @ 1
        add process p p > w-{0} z{0} j j QED=4 QCD=0, w- > l- vl~, z > l+ l- @ 2
    and set reference frame:
        3,4,5,6 = me_frame
    The cross section under mg273 is 0.001368 pb
    The cross section under mg272 is 0.00158 pb
    mg273 is 13.4% less than mg272

For WLWL:
    set process:
        generate p p > w+{0} w+{0} j j QED=4 QCD=0, w+ > l+ vl @ 1
        add process p p > w-{0} w-{0} j j QED=4 QCD=0, w- > l- vl~ @ 2
    and set reference frame:
        3,4,5,6 = me_frame
    The cross section under mg273 is 0.003051 pb
    The cross section under mg272 is 0.003286 pb
    mg273 is 7.2% less than mg272

For WLWL no decay:
    set process:
        generate p p > w+{0} w+{0} j j QED=4 QCD=0 @ 1
        add process p p > w-{0} w-{0} j j QED=4 QCD=0 @ 2
    and set reference frame:
        3,4 = me_frame
    The cross section under mg273 is 0.02858 pb
    The cross section under mg272 is 0.02858 pb
    mg273 is same as mg272

Do you know reason?

Best,
Jie

Question information

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MadGraph5_aMC@NLO Edit question
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Solved by:
Olivier Mattelaer
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Revision history for this message
Best Olivier Mattelaer (olivier-mattelaer) said :
#1

Hi,

This should be related to this message in the UpdatesNotes:

OM: Fix a bug in the longitudinal polarization for off-shell effect, leading to deviation at large invariant mass.

Cheers,

Olivier

Revision history for this message
xiao jie (talalal) said :
#2

Thanks Olivier Mattelaer, that solved my question.

Revision history for this message
Prachi Arvind Atmasiddha (patmasid) said :
#3

Hello Olivier,

Does this bug-fix affect the kinematic distributions as well or just the cross-sections? What all does this bug-fix affect?

Regards,
Prachi

Revision history for this message
Olivier Mattelaer (olivier-mattelaer) said :
#4

Yes the main impact is on the Breit-Wigner shape that differs since the change was a modification of the propagator used for the longitudinal propagator. Both propagator are the same in the onshell limit but are different when going off-shell limit.

Revision history for this message
Prachi Arvind Atmasiddha (patmasid) said :
#5

Hi Olivier,

Thank you very much.

Regards,
Prachi