MadGraph5_aMC@NLO

VBF Zh process with wrong sign interference

Asked by Carlos Henrique de Lima

Hi,

I was studying the VBF process e+ e- > ve ve~ h Z in the SM and noticed that the interference between the Z and W diagrams is occurring with the wrong sign. In order to pick only the VBF topology, I used the following syntax: e+ e- > ve ve~ h Z / ve e- , which should be fine since we are excluding diagrams that have Zee and Z veve interactions, that we can set to zero without messing with gauge invariance.

The interference effect between the W and Z diagrams should be negative, as it is in the hard process W+ W- > h Z (which in the hard process madgraph gives the correct interference).

I have tried to change the setting for the Phase-Space Optimization strategy and all of them give the same results. For example, for 3TeV CM without cuts, the madgraph results for the W-only cross-section is around 2pb, the Z-only cross-section is around 4 pb, and the interference between W and Z cross-section gives 6pb. The W-only and Z-only cross-sections look consistent with the estimations that we can get from equivalent W approximation, but the interference is not.

Question information

Language:
English Edit question
Status:
Answered
For:
MadGraph5_aMC@NLO Edit question
Assignee:
No assignee Edit question
Last query:
Last reply:
Revision history for this message
Olivier Mattelaer (olivier-mattelaer) said : 		#1

Are you sure that gauge is ok?
If I run:
check e+ e- > ve ve~ h Z / ve e-

I do have the following output... which is failing the gauge check by trying to compare the result between the two gauge.

Lorentz invariance results:
Process Min element Max element Relative diff. Result
e+ e- > ve ve~ h 5.1128032766e-12 5.1128032766e-12 9.0846699132e-14 Passed
Summary: 1/1 passed, 0/1 failed
Gauge results (switching between Unitary/Feynman/axial gauge):
Process Unitary Feynman Relative diff. Result
e+ e- > ve ve~ h z 3.2153073793e-10 5.8064009289e-11 8.1941381510e-01 Failed
   JAMP 0 1.2861229517e-09 2.3225603715e-10 8.1941381510e-01 Failed
Summary: 0/1 passed, 1/1 failed
Failed processes: e+ e- > ve ve~ h z
Process permutation results:
Process Min element Max element Relative diff. Result
e+ e- > ve ve~ h z 1.3349335423e-15 1.3349335423e-15 2.8069481971e-15 Passed
Summary: 1/1 passed, 0/1 failed

Cheers,

Olivier

> On 23 Jan 2024, at 20:20, Carlos Henrique de Lima <email address hidden> wrote:
>
> e+ e- > ve ve~ h Z / ve e-

Revision history for this message
Carlos Henrique de Lima (carloscdlima) said : 		#2

It should be gauge invariant since it is consistent from the point of view of the Gauge symmetry to set the Z couplings to fermions to zero. I noticed that in the Feynman gauge, the VBF process where the W is exchanged by the goldstone boson is not present, and also the diagram with s-channel G0 instead of Z, which should be why the check is failing.

Revision history for this message
Olivier Mattelaer (olivier-mattelaer) said : 		#3

Are you using massive lepton?

I do not see how you can have a g0 in S-channel if it only couple to h/Z and not to electron/neutrino.
the default "SM" does not have mass/yukawa for the electron/neutrino and therefore I do not see how such diagram exits.
The same is True for the VBF process with the g+.

Interessingly the process:
check e+ e- > ve ve~ h Z / Z
does pass the gauge check (and also does not have those diagram, but the same as yours plus three Z diagram where the Z diagram couple to electron/neutrino. But I do not fully understand why this process pass the gauge invariance.

Can you help with this problem?

Provide an answer of your own, or ask Carlos Henrique de Lima for more information if necessary.

To post a message you must log in.