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

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:
> https://answers.launchpad.net/mg5amcnlo/+question/690270
>
> Hi Madgraph experts,
>
> 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
>
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