Madspin weight MUCH larger than the computed max_weight

Hi,

Well this is a tricky question.

The situation is not bad enough to make the code to crash.
But indeed need to you take a decision.

Looks like that the second run under-estimate the maxweight. Which means that you have a small biais.
The biais is kind of under-control since only 49 events have a weight larger than one.
I would prefer 0 but this is still a low number. That’s up to you too see if this is a too larger number for you or not.
If it is, just re-run MadSpin on that sample (and/or increase the time spend for the evaluation of the max_weight)

> ERROR: Found a weight MUCH larger than the computed max_weight (ratio: 15.2179587794).
> This usually means that the Narrow width approximation reaches it's limit on part of the Phase-Space.
> Do not trust too much the tale of the distribution and/or relaunch the code with smaller BW_cut.
> This is for channel (’t_wpb_wp_udx', 'tx_bxwm_wm_mumvmx') with current BW_value at : 10.0827'

This error is more annoying, but this is kind of expected, you have a particle which is quite far from off-shell( at 10 times the width)
and the narrow width approximation starts to break down. (also partially due to a slightly too weak max_weight).
Again one event like this is typically not critical enough to discard the sample (at least not automatically). But again this should be your decision.

Cheers,

Olivier

On 07 Mar 2015, at 12:46, toodles <email address hidden> wrote:

> New question #263341 on MadGraph5_aMC@NLO:
> https://answers.launchpad.net/mg5amcnlo/+question/263341
>
> Hi,
>
> I am using madspin to decay ttbb semileptonically. In my multi_run, the first run is okay, without error or warning, however the second run says there is an error:
>
> ERROR: Found a weight MUCH larger than the computed max_weight (ratio: 15.2179587794).
> This usually means that the Narrow width approximation reaches it's limit on part of the Phase-Space.
> Do not trust too much the tale of the distribution and/or relaunch the code with smaller BW_cut.
> This is for channel ('t_wpb_wp_udx', 'tx_bxwm_wm_mumvmx') with current BW_value at : 10.0827'
>
> WARNING: Found many weight larger than the computed max_weight (49/100000 = 0.049%).
>
> and the madspin_card I am using is :
>
> # set seed 1
> # set Nevents_for_max_weigth 75 # number of events for the estimate of the max. weight
> # set BW_cut 15 # cut on how far the particle can be off-shell
> set max_weight_ps_point 400 # number of PS to estimate the maximum for each event
> #
> # specify the decay for the final state particles
> decay t > w+ b, w+ > j j
> decay t~ > w- b~, w- > l- vl~
> decay w+ > j j
> decay w- > l- vl~
> decay z > all all
> # running the actual code
> launch
>
> do I need to worry about this error? Can I still trust the output of the second run?
>
> I am looking forward to hearing from you!
>
> Cheers,
> Li
>
> --
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Hi, Olivier

Thanks a lot for your detailed explanation! I will close this problem.

Cheers,

Li