MadGraph5_aMC@NLO

particle-dependent cut

Asked by Inês Ochoa

Hi,

I would like to know if it is possible to define a generation cut that would select only events with at least one b-quark in the final state. I want to generate inclusive 5FNS Z+jets events, and I understand it isn't possible to enhance a particular flavour component - therefore it would be useful to filter out at generation level those flavours which I am not interested in.

Thanks!
Ines

Question information

Language:
English Edit question
Status:
Solved
For:
MadGraph5_aMC@NLO Edit question
Assignee:
Paolo Torrielli Edit question
Solved by:
Inês Ochoa
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Revision history for this message
Olivier Mattelaer (olivier-mattelaer) said : 		#1

Hi,

I guess that you are speaking about NLO generation right?

In that case, I'm not 100% sure that this is a physical things to do due to the shower-counterterm.
Paolo should be able to say if it make sense or not. (or say for which shower it can be done)

Otherwise for the technical aspect of your question. We do not have any automated way to include such cut.
You can however use our lhe parser
(madgraph/various/lhe_parser.py) and look at the example at the end of the file.
It should then be straightforward to define such filter and write a new lhe event with only the events that you want.

If Paolo says that you can not do that at the level of the lhef file, then you need to read the output of the shower (hepmc/stdhep) and do the same but in that case, I'm not able to help.

Cheers,

Olivier

Revision history for this message
Paolo Torrielli (paolo-torrielli) said : 		#2

Hi Inês,
I don't see problems form the MC-counterterm point of view.
But in general, setting a cut on a single flavour in a NLO
computation in general is IR unsafe, so you should be careful
when doing that.
Cheers,
Paolo

Revision history for this message
Inês Ochoa (inesochoa) said : 		#3

Hi Paolo,

Apologies for the slow reply and thank you for your answer.
Is it possible to instead run an inclusive generation + showering and apply a filter on the events with a b-hadron matched to a jet only at the output stage (such that the output file is smaller)?

Best,
Ines

Revision history for this message
Paolo Torrielli (paolo-torrielli) said : 		#4

Hi Inês,

I'm not sure I understand correctly. If your aim is simply to have an event file
smaller than the original Z+jets one, with all events containing at least a b quark,
then you could generate pp>Z+jets, select from that file the events with at least
a b quark (to be possibly stored to another file), and shower only them.

Note that the shower could convert gluons in bb pairs, hence in principle also
events that do not contain b quarks at the Les-Houches level may pass the b-jet
cuts in an analysis, after showering. These should be a small fraction, though,
but you may be interested in them.

Cheers.
Paolo

Revision history for this message
Inês Ochoa (inesochoa) said : 		#5

Hi Paolo,

Yes, that would be my aim at this point, since hepmc files get quite large. I guess you mean a simple grep would be applicable? In which case, this would also work after showering.

Thanks!
Ines