WHIZARD

How to generate decay chains with non-fixed unstable particle masses

Asked by Xunwu Zuo

Dear experts,

(This may be a dumb question with simple solutions, but I just did not find the correct syntax to make it work. Would be great to hear your suggestions.)
I am trying to generate e+e- -> tt events with exclusive top and W decay chains, in which I want the top and W to have physical width. I tried two ways to write the process, but neither gave me exactly what I wanted.

1. Write a series of decay processes, as mentioned in another question https://answers.launchpad.net/whizard/+question/708455
An example of the syntax I used is
---------------------------
model = SM_CKM
sqrts = 365 GeV

process proc = e1, E1 => t, tbar
process t_dec = t => Wp, b
process tbar_dec = T => Wm, S
process Wp_mn = Wp => E2, n2
process Wm_ud = Wm => U, d

unstable t (t_dec)
unstable T (tbar_dec)
unstable Wp (Wp_mn)
unstable Wm (Wm_ud)
---------------------------
This way I get the processes as I want, but the masses of all the top and W are fixed at their SM value without any spread.

2. Write one process from initial state to final state, and getting intermediate particles with "resonance history".
An example of the syntax I used is
--------------------------
model = SM_CKM
sqrts = 365 GeV

?resonance_history = true
resonance_on_shell_limit = 10
process proc = e1, E1 => (E2, n2, b, U, d, S) {$restrictions = "3+4~W+ && 6+7~W- && !H && ^[W+,W-,A:Z]"}
--------------------------
This way I do get intermediate top and W with mass spread for the majority of the events. But for ~20% of the events, no intermediate particle is found.

Could you let me know if I am using the correct syntax and how I can fix it?

Best,
Xunwu

Question information

Language:
English Edit question
Status:
Solved
For:
WHIZARD Edit question
Assignee:
Juergen Reuter Edit question
Solved by:
Xunwu Zuo
Solved:
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Last reply:
Revision history for this message
Juergen Reuter (j.r.reuter) said : 		#1

Dear Xunwu,
I suppose what you would like is
process top_semilep = e1, E1 => E2, n2, b, U, d, S { $restrictions = "3+4+5 ~ t && 6+7+8 ~ tbar" }
which restricts E2,n2,b to come from a top and U,d,S from an anti-top. This produces only two Feynman diagrams (s-channel photon + Z exchange).

The
?resonance_history = true
is only relevant for parton shower and hadronization in order to tell Pythia to preserve invariant masses of intermediate (hadronic) Ws and tops during the shower evolution and hadronization. There might appear some "non-resonant" background events where no resonant particles show up in the shower history. If you would like to discard these events (which would be non-resonant contamination), you can set resonance_background_factor = 0
Cheers,
    JRR (Juergen)

Revision history for this message
Xunwu Zuo (xunwu-zuo) said : 		#2

Dear Juergen,

Thanks a lot for the answer! It solves my question.
I did not know one can require a parent to 3 particles. This is great!

Best,
Xunwu

Revision history for this message
MR HSNNN (lastreaction) said : 		#3

Your question involves the usage of the WHIZARD event generator to simulate e+e- -> tt events with exclusive top and W decay chains, where the top and W particles have physical width (non-fixed masses).

Let's focus on the second approach using "resonance history," as it seems closer to your goal. The issue you mentioned, where no intermediate particle is found for around 20% of the events, might be related to the on-shell limit setting.

Here's an adjusted version of your second example:

whizard
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model = SM_CKM
sqrts = 365 GeV

?resonance_history = true
resonance_on_shell_limit = 0.1 ! Adjust this value as needed

process proc = e1, E1 => (E2, n2, b, U, d, S) {$restrictions = "3+4~W+ && 6+7~W- && !H && ^[W+,W-,A:Z]"}
In the code above, the resonance_on_shell_limit parameter is set to 0.1. This parameter controls the on-shell condition for intermediate resonances. You may need to adjust this value to find a balance https://infoisthub.com/ between generating events with spread masses and ensuring that a reasonable fraction of events have intermediate particles.

Additionally, you can try increasing the number of events in your simulation to obtain better statistics and see if the issue persists. You can set the number of events using the nevents parameter:

whizard
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nevents = 10000 ! Adjust the number as needed
Experimenting with these parameters and their values can help you find a suitable configuration for your specific simulation requirements. If you encounter further issues or need more detailed guidance, you may want to consult the WHIZARD documentation or community forums for specific assistance related to the version of WHIZARD you are using.