Event weighting strategy -3 and -4

Asked by Hannes on 2019-09-25

Hi,

we recently had problems when the event reweighting strategy -3 was set in LHE events that were generated with MadGraph and processed with Pythia.
To avoid this, I am trying to make sure we always use strategy -4.
When I set event_norm = sum, does this make sure strategy -4 will always be used, for LO and NLO mode, or are there cases when this wouldn't be the case?

Cheers,
Hannes

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2019-09-25
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2019-09-25

Hi,

I'm not an expert on that variable.
In principle they should not have any differences between event_norm=sum and event_norm=average in both case I would expect strategy 4 (or -4) to be used.

Now if you put event_norm=unit then in that case, I would expect the strategy to be change to 3 (or -3 if you have at least one negative events).

If that matches what you observed? How did you get a strategy -3 used?

Cheers,

Olivier

> On 25 Sep 2019, at 22:13, Hannes <email address hidden> wrote:
>
> New question #684257 on MadGraph5_aMC@NLO:
> https://eur03.safelinks.protection.outlook.com/?url=https%3A%2F%2Fanswers.launchpad.net%2Fmg5amcnlo%2F%2Bquestion%2F684257&amp;data=02%7C01%7Colivier.mattelaer%40uclouvain.be%7Cb06f740d15a844dead4908d741f4ce90%7C7ab090d4fa2e4ecfbc7c4127b4d582ec%7C0%7C0%7C637050392597388614&amp;sdata=ooYjoPxHiWJIk3weM219alV5bifS1o3Pe1v3HlLwPnk%3D&amp;reserved=0
>
> Hi,
>
> we recently had problems when the event reweighting strategy -3 was set in LHE events that were generated with MadGraph and processed with Pythia.
> To avoid this, I am trying to make sure we always use strategy -4.
> When I set event_norm = sum, does this make sure strategy -4 will always be used, for LO and NLO mode, or are there cases when this wouldn't be the case?
>
> Cheers,
> Hannes
>
> --
> You received this question notification because you are an answer
> contact for MadGraph5_aMC@NLO.

Hannes (hannes3) said : #2

Hi Olivier,

thanks for the quick reply.
First, sorry, I meant event_norm=average seems to leads to strategy -4, while event_norm=sum -> strategy=-3.
I just tried it again (in 2.6.5).

To be clear, I am talking about the "-3" in the LHE header:
<init>
2212 2212 6.500000e+03 6.500000e+03 0 0 247000 247000 -3 1
8.360400e+04 7.893500e+01 8.360400e+00 1
<generator name='MadGraph5_aMC@NLO' version='2.6.5'>please cite 1405.0301 </generator>
</init>

I am not entirely sure whether this is a general problem with Pythia or just in our setup, right now I am just trying to be sure we always use -4. The "average" normalization is anyhow better for us and we have been told from the Pythia side to use strategy -4.

The strategies are also documented here
http://home.thep.lu.se/~torbjorn/pythia82html/LesHouchesAccord.html

Cheers,
Hannes

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