Is P_T of electrons smeared by calorimeter, or is it all in tracker?

[Michael Albergo]

I'm trying to isolate the source of P_{T} mismeasurement in the delphesPythia8 electron gun example.

The Delphes academic paper from 2014 (Delphes 3) says: "The full electron reconstruction usually involves combining information from
the tracking system together with the electromagnetic calorimeter. In Delphes, we circumvent
these reconstruction complexities by parameterizing the combined reconstruction
efficiency as a function of the energy and pseudorapidity. As for muons, the electron reconstruction
efficiency vanishes outside the tracker acceptance and below some energy threshold.
For the electron energy resolution, we use a combination of the ECAL and tracker
resolution. At low energy, the tracker resolution dominates, while at high energy, the
ECAL energy resolution dominates."

This discussion is for energy, however. Is the PT just from gaussian smearing in the tracker depending on the pseudorapidity? I'm just trying to exactly understand what is the force behind the mismeasurement in the PT under the "Electron" section of the output ROOT file.

Thanks!

Michael

[Michele Selvaggi]

Hi,

what is written in the paper is not true anymore.
Electrons are smeared as tracks at low energy and as calorimeter towers at higher energy.
The energy at which simga_trk=sigma_ecal depends on the card you are using.

To answer your question typically yes, the resolutions are pseudo-rapidity dependent, as in here:

​https://github.com/delphes/delphes/blob/master/cards/delphes_card_CMS.tcl#L160-L175
​https://github.com/delphes/delphes/blob/master/cards/delphes_card_CMS.tcl#L301-L304

[Michael Albergo]

My Delphes Card

[Michael Albergo]

Hi there,

Thanks kindly for this information! I am using Delphes 3.3.3 and I have attached my Delphes card to the description.

Given this balance of ECAL and tracker efficiency, I was wondering if there was an equation you could provide me with that describes how they are balanced given a certain energy or PT or such? Specifically in context of the "Electron" branch in the output root file's measurement of PT. I understand that both are used, but is the PT calculated in that branch just the sum of the ECAL and tracker PT mismeasurements?

Best,
Michael

[Michele Selvaggi]

It is simply a weighted average using the trk and ecal uncertainties, as done in here:

​https://github.com/delphes/delphes/blob/master/modules/Calorimeter.cc#L589-L592

The individual formulae for trk and ecal are those in the card that I have linked previously.

[Michael Albergo]

Thanks for pointing me to this -- just one last clarification. This seems to be for the energy measurement. Is the P_T derived this way as well? How does Delphes calculate the PT from this?

Thanks a bunch.

Michael

[Michael Albergo]

​https://github.com/delphes/delphes/blob/master/modules/Calorimeter.cc#L603

Is it calculated using the rescale facetor here?

[Michael Albergo]

I seem to be a bit confused about something, perhaps the ordering.

The output ROOT file has both an "Electron" branch and "Tower" branch. The "Electron" branch has no "E" measurement, only PT. Is this PT measurement the one you were talking about? From the code you referenced, that seems to only have to do with the calorimeter. Is the PT smearing for the tracker calculated in tracker.cc

I see in ​https://github.com/delphes/delphes/blob/46894810e595b68c79417873df683f93a750fbac/modules/TreeWriter.cc#L499 where the Electron data is written, but it is still unclear to me the order of operations that occurs to make those values. Are the values in the "Electron" branch those that come from the smearing of the PT in the inner tracker and calorimeter? There is no energy in the "Electron" branch, so I'm a confused how the energy might be used to smear the PT even though the energy itself is not saved.

Very sorry for all the confusion, I'm just trying to make sense of the order of operations that is occurring to make the measurements I see in the ROOT file. I really appreciate your help :)