On effective lumionsity in PbPb and pp runs

Asked by Hesham El Faham on 2021-05-26

Hi,

I am studying some new physics model in pp and in PbPb collisions. For the PbPb runs I modify the mg5 card as follows:

--set pdlabel lhapdf
--set lhaid 901300
--set ebeam1 2750*208
--set ebeam2 2750*208

I realized during the runs that I get effective luminosity for the PbPb run=0.6pb^-1 while for pp=233pb^-1. I don't understand how I should interpret these numbers.

Ultimately, I want to compare the number of events in PbPb and in pp for equal running time at the LHC. For that, and at a later step, I scale the number of events by the integrated luminosity corresponding to a month of running the LHC at pp, and the same for PbPb.

Now that I saw those effective lumionsity numbers produced by mg5, I am not sure how I should scale the number of events, given that mg5 is already considering different luminosity in both cases. May you please elaborate on that?

Best,
Hesham

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Olivier Mattelaer
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Best Olivier Mattelaer (olivier-mattelaer) said :
#1

Hi,

My understanding of
EPPS16nlo_CT14nlo_Pb208 lhapdf implementation
is that you have to set ebeam1/ebeam2 to 2750 and not to 2750*208.
Please double check that point since I'm not an author of that pdf set.

This being said let's go back to your question.
Indeed MG5aMc generates a given number of events (default is 10k) and you can change that number (maximum 1 million per run if think) via "set nevents 100k" (for 100 thousand event)

Madgraph then generates that number of un-weighted events and compute the cross-section at the same-time.
The luminosity can then be calculated using those two numbers.
Please do not use the luminosity quoted by MG5aMC during the run since those are not final but "in progress" value.

> Ultimately, I want to compare the number of events in PbPb and in pp for equal running time at the LHC.

so here you do have
L_pp sigma_pp = N_pp
L_pbpb sigma_pbpb = N_pbpb

so in your case since you want
N_pp/N_pbpb = L_pp sigma_pp / L_pbpb sigma_pbpb

for L_pp and L_pbpb you should use the integrated luminosity corresponding to a month of running the LHC at pp, and the same for PbPb and for sigma_pp and sigma_pbpb use the value given by madgraph.

You should not care at all on the number of events generated by madgraph (but for the precision on the cross-section) neither the associated luminosity

Cheers,

Olivier

> On 26 May 2021, at 13:35, Hesham El Faham <email address hidden> wrote:
>
> lhaid 901300

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Hesham El Faham (helfaham) said :
#2

Thanks Olivier Mattelaer, that solved my question.