# loop-induced negative weight fraction

Hello experts,

I'm trying to produce loop-induced g g > z h h [QCD] process in order to evaluate the cross-section. As far as I understood, this is NNLO correction but the events are all positive. Is this correct or expected? Shouldn't we expect any negative weighted events?

Best,

Chayanit

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Hi,

> As far as I understood, this is NNLO correction but the events are all positive.

This is technically PART of the NNLO correction but this is certainly not the FULL NNLO correction.

This contribution is by construction definite positive (if your PDF are definite positive as well) since this a one loop-amplitude squared against itself so negative contribution are not possible.

Now one can question if this contribution is a real NNLO contribution.

This is actually a "new channel opening at NNLO".

I think it is useful to think about the case of the single higgs production where the LO process is q q > h

and where the NNLO "correction" would be g g > h (loop-induced). In that case you will see that you are dominated by that NNLO "correction".

Cheers,

Olivier

> On 19 Apr 2021, at 09:20, Chayanit <email address hidden> wrote:

>

> New question #696638 on MadGraph5_aMC@NLO:

> https:/

>

> Hello experts,

>

> I'm trying to produce loop-induced g g > z h h [QCD] process in order to evaluate the cross-section. As far as I understood, this is NNLO correction but the events are all positive. Is this correct or expected? Shouldn't we expect any negative weighted events?

>

> Best,

> Chayanit

>

> --

> You received this question notification because you are an answer

> contact for MadGraph5_aMC@NLO.

Chayanit (chayanit) said : | #2 |

Thanks a lot for your prompt reply!

Cheers,

Chayanit