MadGraph5_aMC@NLO

Top decay (t > b e+ ve mu+ mu-) not working, width too big

Asked by Jiashu Huang

Hello MADGraphers,

I am fairly new to MADGraph and please help. I am studying t > b w z using the following card: t > b e+ ve mu+ mu-. Whenever I generate it, it gives me a width of 0.4144 +- 0.005706 GeV. This makes no sense, since the t > b w z decay is a rare one. I expect the width to be on the level of 1e-7 or 1e-8.

I have read #698251 (https://answers.launchpad.net/mg5amcnlo/+question/698251) and set the electron and muon masses to be non-zero. Still doesn't change the width.

I am using version 3.5.0.

Attached is the log:

MG5_aMC>generate t > b e+ ve mu+ mu-
INFO: Trying process: t > b e+ ve mu+ mu- @1
INFO: Process has 10 diagrams
1 processes with 10 diagrams generated in 0.014 s
Total: 1 processes with 10 diagrams
MG5_aMC>launch
output command missing, run it automatically (with default argument)
INFO: initialize a new directory: PROC_sm_136
INFO: remove old information in PROC_sm_136
INFO: Organizing processes into subprocess groups
INFO: Generating Helas calls for process: t > b e+ ve mu+ mu- @1
INFO: Processing color information for process: t > b e+ ve mu+ mu- @1
INFO: Creating files in directory P1_t_blvlll
INFO: Generating Feynman diagrams for Process: t > b e+ ve mu+ mu- @1
INFO: Finding symmetric diagrams for subprocess group t_blvlll
Generated helas calls for 1 subprocesses (10 diagrams) in 0.030 s
Wrote files for 32 helas calls in 0.331 s
ALOHA: aloha starts to compute helicity amplitudes
ALOHA: aloha creates 6 routines in 1.946 s
save configuration file to /isilon/data/users/jhuan166/MG5_aMC_v3_5_0/PROC_sm_136/Cards/me5_configuration.txt
INFO: Use Fortran compiler gfortran
INFO: Use c++ compiler g++
INFO: Generate jpeg diagrams
gs: /cvmfs/sft.cern.ch/lcg/views/LCG_99/x86_64-centos7-gcc10-opt/lib/libtiff.so.5: no version information available (required by /lib64/libgs.so.9)
gs: /cvmfs/sft.cern.ch/lcg/views/LCG_99/x86_64-centos7-gcc10-opt/lib/libtiff.so.5: no version information available (required by /lib64/libgs.so.9)
INFO: Generate web pages
Output to directory /isilon/data/users/jhuan166/MG5_aMC_v3_5_0/PROC_sm_136 done.
Type "launch" to generate events from this process, or see
/isilon/data/users/jhuan166/MG5_aMC_v3_5_0/PROC_sm_136/README
Run "open index.html" to see more information about this process.
output done: running launch
************************************************************
* *
* W E L C O M E to *
* M A D G R A P H 5 _ a M C @ N L O *
* M A D E V E N T *
* *
* * * *
* * * * * *
* * * * * 5 * * * * *
* * * * * *
* * * *
* *
* VERSION 3.5.0 2023-05-12 *
* *
* The MadGraph5_aMC@NLO Development Team - Find us at *
* https://server06.fynu.ucl.ac.be/projects/madgraph *
* *
* Type 'help' for in-line help. *
* *
************************************************************
INFO: load configuration from /isilon/data/users/jhuan166/MG5_aMC_v3_5_0/PROC_sm_136/Cards/me5_configuration.txt
INFO: load configuration from /isilon/data/users/jhuan166/MG5_aMC_v3_5_0/input/mg5_configuration.txt
INFO: load configuration from /isilon/data/users/jhuan166/MG5_aMC_v3_5_0/PROC_sm_136/Cards/me5_configuration.txt
Using default text editor "vi". Set another one in ./input/mg5_configuration.txt
No valid eps viewer found. Please set in ./input/mg5_configuration.txt
No valid web browser found. Please set in ./input/mg5_configuration.txt
                Note that since 2.3. The launch for 1>N pass in event generation
                For efficient width computation see arXiv:1402.1178.
generate_events run_01
The following switches determine which programs are run:
/=================== Description ===================|============= values ==============|======== other options ========\
| 1. Choose the shower/hadronization program | shower = OFF | Pythia8 |
| 2. Choose the detector simulation program | detector = OFF | Delphes |
| 3. Choose an analysis package (plot/convert) | analysis = Not Avail. | Please install module |
| 4. Decay onshell particles | madspin = OFF | ON|onshell|full |
| 5. Add weights to events for new hypp. | reweight = OFF | ON |
\=======================================================================================================================/
Either type the switch number (1 to 5) to change its setting,
Set any switch explicitly (e.g. type 'shower=Pythia8' at the prompt)
Type 'help' for the list of all valid option
Type '0', 'auto', 'done' or just press enter when you are done.[60s to answer]
>
Do you want to edit a card (press enter to bypass editing)?
/------------------------------------------------------------\
| 1. param : param_card.dat |
| 2. run : run_card.dat |
\------------------------------------------------------------/
 you can also
   - enter the path to a valid card or banner.
   - use the 'set' command to modify a parameter directly.
     The set option works only for param_card and run_card.
     Type 'help set' for more information on this command.
   - call an external program (ASperGE/MadWidth/...).
     Type 'help' for the list of available command
 [0, done, 1, param, 2, run, enter path][90s to answer]
>1
open /isilon/data/users/jhuan166/MG5_aMC_v3_5_0/PROC_sm_136/Cards/param_card.dat
Do you want to edit a card (press enter to bypass editing)?
/------------------------------------------------------------\
| 1. param : param_card.dat |
| 2. run : run_card.dat |
\------------------------------------------------------------/
 you can also
   - enter the path to a valid card or banner.
   - use the 'set' command to modify a parameter directly.
     The set option works only for param_card and run_card.
     Type 'help set' for more information on this command.
   - call an external program (ASperGE/MadWidth/...).
     Type 'help' for the list of available command
 [0, done, 1, param, 2, run, enter path]
>
INFO: Update the dependent parameter of the param_card.dat
WARNING: update the strong coupling value (alpha_s) to the value from the pdf selected: 0.13
WARNING: For model consistency, update mass with id [11] to value 0.0
WARNING: For model consistency, update mass with id [13] to value 0.0
Generating 10000 events with run name run_01
survey run_01
INFO: compile directory
Not able to open file /isilon/data/users/jhuan166/MG5_aMC_v3_5_0/PROC_sm_136/crossx.html since no program configured.Please set one in ./input/mg5_configuration.txt
compile Source Directory
Using random number seed offset = 21
INFO: Running Survey
Creating Jobs
Working on SubProcesses
INFO: Compiling for process 1/1.
INFO: P1_t_blvlll
INFO: P1_t_blvlll
INFO: Idle: 4, Running: 16, Completed: 0 [ current time: 04h57 ]
INFO: Idle: 3, Running: 16, Completed: 1 [ 0.88s ]
INFO: Idle: 0, Running: 0, Completed: 20 [ 3.2s ]
sum of cpu time of last step: 30 seconds
  === Results Summary for run: run_01 tag: tag_1 ===

     Width : 0.05274 +- 104 GeV
     Nb of events : 0

INFO: End survey
refine 10000
Creating Jobs
INFO: Refine results to 10000
INFO: Generating 10000.0 unweighted events.
sum of cpu time of last step: 30 seconds
INFO: Effective Luminosity 227541.30591036423 pb^-1
INFO: need to improve 9 channels
- Current estimate of cross-section: 0.05273767745430651 +- 103.99122928674376
    P1_t_blvlll
INFO: Idle: 3, Running: 16, Completed: 0 [ current time: 04h58 ]
INFO: Idle: 2, Running: 16, Completed: 1 [ 0.89s ]
INFO: Idle: 0, Running: 16, Completed: 3 [ 13.7s ]
INFO: Idle: 0, Running: 11, Completed: 8 [ 34.4s ]
INFO: Idle: 0, Running: 10, Completed: 9 [ 39.9s ]
INFO: Idle: 0, Running: 6, Completed: 13 [ 46.2s ]
INFO: Idle: 0, Running: 4, Completed: 15 [ 1m 42s ]
INFO: Idle: 0, Running: 3, Completed: 16 [ 2m 20s ]
INFO: Idle: 0, Running: 2, Completed: 17 [ 2m 27s ]
INFO: Idle: 0, Running: 0, Completed: 19 [ 4m 14s ]
INFO: Idle: 0, Running: 0, Completed: 19 [ 4m 14s ]
INFO: Combining runs
sum of cpu time of last step: 3m15s
INFO: finish refine
refine 10000 --treshold=0.9
Creating Jobs
INFO: Refine results to 10000
INFO: Generating 10000.0 unweighted events.
sum of cpu time of last step: 3m15s
INFO: Effective Luminosity 37513.44176248967 pb^-1
INFO: need to improve 3 channels
- Current estimate of cross-section: 0.31988533805430647 +- 0.013166393166405586
    P1_t_blvlll
INFO: Idle: 1, Running: 5, Completed: 0 [ current time: 05h02 ]
INFO: Idle: 0, Running: 4, Completed: 2 [ 37.2s ]
INFO: Idle: 0, Running: 3, Completed: 3 [ 1m 20s ]
INFO: Idle: 0, Running: 2, Completed: 4 [ 1m 34s ]
INFO: Idle: 0, Running: 1, Completed: 5 [ 1m 42s ]
INFO: Idle: 0, Running: 0, Completed: 6 [ 3m 6s ]
INFO: Idle: 0, Running: 0, Completed: 6 [ 3m 6s ]
INFO: Combining runs
sum of cpu time of last step: 4m51s
INFO: finish refine
INFO: Combining Events
WARNING: Order of particle in the event did not agree with parent/child order. This might be problematic for some code.
INFO: fail to reach target 10000
failed to generate enough events. Please follow one of the following suggestions to fix the issue:
  - set in the run_card.dat 'sde_strategy' to 1
  - set in the run_card.dat 'hard_survey' to 1 or 2.
  - reduce the number of requested events (if set too high)
  - check that you do not have -integrable- singularity in your amplitude.
  === Results Summary for run: run_01 tag: tag_1 ===

     Width : 0.4144 +- 0.005706 GeV
     Nb of events : 3949

store_events
INFO: Storing parton level results
INFO: End Parton
reweight -from_cards
decay_events -from_cards
INFO: storing files of previous run
INFO: Done

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Olivier Mattelaer (olivier-mattelaer) said : 		#1

Hi,

Note that your syntax is NOT "t > b w z " it also contains "t > b w a".
This is "problematic" since "t > b w a" is one sub-part of the NLO correction of "t>wb", the issue being obviously that you do not include the associated loop contribution with that syntax and that separately the two contribution are infinite (with opposite sign such that their sum is finite.

So in your example, you do have two singularities
1) when the photon (or virtual Z) is soft
2) when the photon (or virtual Z) is collinear with the b quark
(and looking at the Feynman Diagram, I'm clearly not exhaustive in the list of singularity)

Now, if you looked at the colored part of your log, you will see those two lines:
WARNING: For model consistency, update mass with id [11] to value 0.0
WARNING: For model consistency, update mass with id [13] to value 0.0

So you do not have massive muon, that can effectively act as a regulator for the soft component.
Independently, I'm not sure about your setup for the collinear singularity.

I'm not an expert in such computation which are non trivial to me since you can not use Narrow-width approximation.
So a syntax asking for onshell "W" and onshell "Z" will not make any sense.

 So my advise here is to check literature to see how they do compute such contribution (do they do cuts --likely--, use other approximation,...) and then use MG5aMC --if it is compatible with the approximation used in the litterature-- do the same then.

Cheers,

Olivier

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