Setting me_frame for polarized same when the production includes a decay chain

Asked by Siyuan Sun on 2020-04-28

I read the manual for how to set me_frame but it only includes instructions without specifying a decay chain.

When we try

generate pp > w+{0} z{0} j, w+ > l+ vl, z > l+ l-

[1,2] = me_frame

This seems to work as expected where the polarization is defined in the initial parton CM frame.
When we plot the angle between the W's lepton in the W boson CM frame against the W Boost from the parton CM frame to the W frame this seems to follow the sin^2 distribution. Essentially this is the W lepton decay angle when the boost is evaluated from the parton CM frame and not the WZ CM frame. Plotting the W lepton decay where the W boost is from the WZ CM frame gives a slightly broader distribution than sin^2. I assume this is due to the additional j making the WZ CM frame somewhat different from the parton CM frame where the polarization is defined.

So far so good.

Next we tried

generate pp > w+{0} z{0} j, w+ > l+ vl, z > l+ l-

[3,4] = me_frame

We thought this should correspond to the CM frame of the WZ since particle 3,4 in the LHE list corresponds to the W and Z. However when we plot the W lepton decay angle where the W Boost is from the WZ CM frame, the distribution is completely wrong being much more broad when compared to the [1,2] = me_frame case. At the same time, the W lepton decay angle where the W Boost is evaluated from the parton CM frame is also very broad.

We are not sure what is wrong with the syntax since we thought this should match. However the manual only show the case where there is only generation "pp > W+{0} z{0}" without the subsequent decay so we weren't sure if this does not work so well for intermediate particles.

Next we tried
[5,6,7,8] as this corresponds to the decay product of the W and Z in the LHE file. Still this did not work. The distributions for the W lepton decay angle was completely identical to the case where [1,2]=me_frame. It seems like this was ignored and the default partonic frame was used by the program.

We tried this all and got the same results for
generate pp > w+{0} z{0}, w+ > l+ vl, z > l+ l-
where the additional jet was removed.

We got the same result where [3,4] was much broader than sin^2.

How does me_frame work when an additional decay chain is set?

In the end, we would like to generate the 0 jet and the 1 jet at the same time

generate pp > w+{0} z{0}, w+ > l+ vl, z > l+ l- @0
generate pp > w+{0} z{0} j, w+ > l+ vl, z > l+ l- @1

and we would like to set the me_frame to the WZ CM frame.

-Sun

Question information

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Solved by:
Olivier Mattelaer
Solved:
2020-04-28
Last query:
2020-04-28
2020-04-28
 Olivier Mattelaer (olivier-mattelaer) said on 2020-04-28: #1

Hi,

You need to look at the diagram plotted by madgraph in order to know the ordering of each particles.
Intermediate particle does not have a number in that scheme, only final state particles.

for this syntax (but please check on the diagram plotted
> generate pp > w+{0} z{0} j, w+ > l+ vl, z > l+ l-

l+ has index 3
vl has index 4
l+ has index 5
l- has index 6
j has index 7
so if you want the w+ z frame you need to use
[3,4,5,6]

they are no index 8

> generate pp > w+{0} z{0}, w+ > l+ vl, z > l+ l- @0
> generate pp > w+{0} z{0} j, w+ > l+ vl, z > l+ l- @1
>
> and we would like to set the me_frame to the WZ CM frame.

The above syntax works for both.

Cheers,

Olivier
> On 28 Apr 2020, at 09:53, Siyuan Sun <email address hidden> wrote:
>
> New question #690270 on MadGraph5_aMC@NLO:
>
>
> I read the manual for how to set me_frame but it only includes instructions without specifying a decay chain.
>
> When we try
>
> generate pp > w+{0} z{0} j, w+ > l+ vl, z > l+ l-
>
> [1,2] = me_frame
>
> This seems to work as expected where the polarization is defined in the initial parton CM frame.
> When we plot the angle between the W's lepton in the W boson CM frame against the W Boost from the parton CM frame to the W frame this seems to follow the sin^2 distribution. Essentially this is the W lepton decay angle when the boost is evaluated from the parton CM frame and not the WZ CM frame. Plotting the W lepton decay where the W boost is from the WZ CM frame gives a slightly broader distribution than sin^2. I assume this is due to the additional j making the WZ CM frame somewhat different from the parton CM frame where the polarization is defined.
>
> So far so good.
>
> Next we tried
>
> generate pp > w+{0} z{0} j, w+ > l+ vl, z > l+ l-
>
> [3,4] = me_frame
>
> We thought this should correspond to the CM frame of the WZ since particle 3,4 in the LHE list corresponds to the W and Z. However when we plot the W lepton decay angle where the W Boost is from the WZ CM frame, the distribution is completely wrong being much more broad when compared to the [1,2] = me_frame case. At the same time, the W lepton decay angle where the W Boost is evaluated from the parton CM frame is also very broad.
>
> We are not sure what is wrong with the syntax since we thought this should match. However the manual only show the case where there is only generation "pp > W+{0} z{0}" without the subsequent decay so we weren't sure if this does not work so well for intermediate particles.
>
> Next we tried
> [5,6,7,8] as this corresponds to the decay product of the W and Z in the LHE file. Still this did not work. The distributions for the W lepton decay angle was completely identical to the case where [1,2]=me_frame. It seems like this was ignored and the default partonic frame was used by the program.
>
> We tried this all and got the same results for
> generate pp > w+{0} z{0}, w+ > l+ vl, z > l+ l-
> where the additional jet was removed.
>
> We got the same result where [3,4] was much broader than sin^2.
>
> How does me_frame work when an additional decay chain is set?
>
> In the end, we would like to generate the 0 jet and the 1 jet at the same time
>
> generate pp > w+{0} z{0}, w+ > l+ vl, z > l+ l- @0
> generate pp > w+{0} z{0} j, w+ > l+ vl, z > l+ l- @1
>
> and we would like to set the me_frame to the WZ CM frame.
>
> Thank you for your help,
>
> -Sun
>
> --

 Siyuan Sun (ssun) said on 2020-04-28: #2

Thank you Olivier.