Final state charged leptons appearing with pt below cut values

Hi,

You need to distinguish generation cut and analysis cut.

The generation cut are those specify within run_card.dat and they are applied on the event/particle at parton level BEFORE showering. The parton-shower, done via another program (typically pythia8 or HW7), will change the kinematic of the events (in order to generate the soft and collinear radiation) and therofore some variable will be impacted and some events will now be removed if one applied the same cut AFTER parton-shower. Hadronization, detector simulation and jet reconstruction might also slightly change the kinematic of some observables (but this is typically less relevant for lepton observables).

So this is normal to not have a 100% efficiency if you apply the same cut at generation level and at analysis level.
However, setting both cut as equal is WRONG. Indeed, you also have the opposite migration (events that would not pass the generation cut that would pass the analysis cut). Therefore, the generation cut needs to be quite loose version of the analysis cut.
Loose enough such that the analysis cut does not depend of the cut at generation cut but hard enough to remove the singular region of the phase-space (if any) and hard enough such that you have a decent efficiency of the analysis cut.

Cheers,

Olivier

Hi Olivier,

Thank you for your response. I am not applying an analysis cut, only a generation cut, so I guess it makes sense that some subleading leptons have pt below the generation cut. I am wondering why I don't see any leading leptons with pt below the cut, however? Also, why do the events with subleading lepton pt below 10 GeV occur in such a specific phase space region, in terms of cos(Theta*)?

Thanks,
James

Hi,

The exact spectrum that you would get might depend of the parton-shower code/algorithm that you use and in particular in the recoil algorithm that is used by the program. I'm not an expert here and therefore can not really comment here.

However, I would risk a "simple" explanation (which is certainly not 100% correct but I hope that the intuition is at least correct).
you can think that at the level of the generation cuts, the Z has exactly a 0 pt and therefore both lepton have the same PT.
Which is due to your cut bigger than 10 GeV.
The parton-shower should be QCD (or mainly QCD) and therefore the effect is only initial state radiation, which will be recoiled by Z which will now have a non zero PT. I would naively think that your lepton PT would then be |PT_z + PT_l| and |PT_z-PT_l| (this is likely too naive since some angle might enter the expression). So I would expect that one is "increased" and one would be "decreased".
making difficult for both to be below the minimal value.

I would not try to explain the cos(Theta*) with such type argument but I guess that such situation can occur only if the PT_l is close to 10 and that you do not have hard ISR. The first requirement means some constraint on the direction of the lepton (but since your angle is AFTER parton-shower it is difficult for a non parton-shower expert to comment on this).

Cheers,

Olivier