semileptonic decays in madspin

Asked by Piero Viscone

Dear all,
I am trying to generate semileptonic decays from ttbar.
So I want that if t > b(W+ > qq~) then t~ > b~ (W- > ell- vl~) and if t > b(W+ > ell+ vl) then t~ > b~ (W- > q q~)

My cards are:

import model sm-ckm_no_b_mass
generate p p > t t~ @0
add process p p > t t~ j @1
add process p p > t t~ j j @2
add process p p > t t~ j j j @3


set ms_dir ./madspingrid

set Nevents_for_max_weight 250 # number of events for the estimate of the max. weight
set max_weight_ps_point 400 # number of PS to estimate the maximum for each event
set max_running_process 1

define ell+ = e+ mu+ ta+
define ell- = e- mu- ta-
define q = u u~ d d~ c c~ s s~ b b~

decay t > w+ b, w+ > c b~, t~ > w- b~, w- > ell- vl~
decay t > w+ b, w+ > ell+ vl, t~ > w- b~, w- > b c~



But the final results are completely wrong (W and t~ will not decay)
Which is the correct syntax?

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Olivier Mattelaer (olivier-mattelaer) said :


This is indeed not a valid syntax.

Each decay line should be associated to a dedicated particle so in your case, you need (at least) one line for the top and (at least one line for the anti-top.
Your syntax is interpreted for the full matrix- element as
p p > t t~, (t > w+ b, w+ > c b~, t~ > w- b~, w- > ell- vl~), (t > w+ b, w+ > c b~, t~ > w- b~, w- > ell- vl~)
note the paranthesis that specify that "w+", "t~" and "w-" should only apply to the decay of the top given by "t > w+ b"
But since you do not have "t~" and "w-" generate those are ignored and your syntax is just:
p p > t t~, (t > w+ b, w+ > c b~), (t > w+ b, w+ > c b~)
(madspin does not raise warning for that because you would have a lot of false alarm if MS was doing it).

So far we do not have a way to do semi-leptonic automatically. The official solution is to split the sample in two (with some poisson distribution) and then run each mode.

But for the fun, I have tried the following patch:

which allow the following MG5aMC script (with the command to edit the madspin_card automatically (hence the --add which allows to not overwrite the decay line for the t since one is already existing):
generate p p > t t~
set fixed_scale 400
decay t > w+ b, w+ > e+ ve @1
decay t > w+ b, w+ > j j @2 --add
decay t~ > w- b~, w- > j j @1
decay t~ > w- b~, w- > e- ve~ @2 --add

those command does generate the madspin_card as
decay w+ > all all
decay w- > all all
decay z > all all
# running the actual code

decay t > w+ b, w+ > e+ ve @1
decay t > w+ b, w+ > j j @2
decay t~ > w- b~, w- > j j @1
decay t~ > w- b~, w- > e- ve~ @2

As you can guess the @1/@2 in the decay syntax allows to correlate those decay together to reach the effect that you want.

Now, this seems to work a bit too easily to my point of view, cross-section seems correct (with no modification which is surprising).
and I have not found one event with two lepton (which is expected with the above patch).

So in other word, do not use this blindly. It is certainly not a feature that I would consider as validated even for tt~ production at LO.
Adding MLM (which is your example case) in top of it might break some symmetry that make it work in the above example/...
When I have time, I will start a proper validation procedure but this will certainly not be part of 3.5.0 (since first alpha release is schedule for this or next week).

If you try it and have success (or issue) do not hesitate to comment here or on github.



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Piero Viscone (pviscone) said :

Thank You for your quick response.
Unfortunately, I will not be able to test it right now (I am forced to use the madgraph/madspin version embedded in CMSSW), but I will try it as soon as I can.

Can you help with this problem?

Provide an answer of your own, or ask Piero Viscone for more information if necessary.

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