"Reconstructing the jets by hand" shows different results with Delphes

[Ke-Pan Xie]

Hi,

I tried to reconstruct the jets by hand, and compared the results with those in Delphes. I use the fully hadronic final state of p p > t t~ as a sample.

By "reconstructing jets by hand", I read the 4-momentums from branchEFlowTrack, branchEFlowNeutralHadron and branchEFlowPhoton, and used the JetDefinition function to reconstruct the jets. The algorithm is antikt, same as the default one in Delphes; the DeltaR is also tuned to be the same, so as the JetPTMin.

By "use the Delphes jets", I just read the 4-momentums from branchJet.

And I compared resultant jets from above two methods. The result is confusing. The eta's and phi's from the two different methods are the same, but the jets read from Delphes typically have a bigger PT than the jets reconstructed by hand. The differences are ~2 GeV, for all jets. Below is a example:

1) The jets reconstructed by hand:
No. 0 jet: E=386.351 GeV, pt=177.164 GeV, eta=1.41365, phi=2.19596;
No. 1 jet: E=1216.73 GeV, pt=162.375 GeV, eta=2.70263, phi=5.22539;
No. 2 jet: E=326.076 GeV, pt=71.9835 GeV, eta=2.19107, phi=1.03263;
No. 3 jet: E=49.1064 GeV, pt=42.6502 GeV, eta=-0.521058, phi=4.01831;
No. 4 jet: E=133.434 GeV, pt=38.5038 GeV, eta=1.91403, phi=2.1752;

2) The jets read from Delphes directly:
No. 0 jet: E=392.017 GeV, pt=179.762 GeV, eta=1.41365, phi=2.19596;
No. 1 jet: E=1233.06 GeV, pt=164.554 GeV, eta=2.70263, phi=5.22539;
No. 2 jet: E=336.585 GeV, pt=74.3035 GeV, eta=2.19107, phi=1.03263;
No. 3 jet: E=52.3742 GeV, pt=45.4884 GeV, eta=-0.521058, phi=4.01831;
No. 4 jet: E=141.665 GeV, pt=40.879 GeV, eta=1.91403, phi=2.1752;

I cannot find the answer. Maybe I missed something? Could you help me? Thanks!

Best,
Ke-Pan

[Pavel Demin]

What card are you using?

For example, in delphes_card_CMS.tcl, the momentum of the jets is corrected by the jet energy scale:

https://cp3.irmp.ucl.ac.be/projects/delphes/browser/git/cards/delphes_card_CMS.tcl#L688

It could explain the difference that you observe.

[Ke-Pan Xie]

Replying to pavel:

Hi pavel,

Thanks a lot! The JetEnergyScale setting solved my problem. I used the CMS configuration.

Best,
Ke-Pan

What card are you using?

For example, in delphes_card_CMS.tcl, the momentum of the jets is corrected by the jet energy scale:

https://cp3.irmp.ucl.ac.be/projects/delphes/browser/git/cards/delphes_card_CMS.tcl#L688

It could explain the difference that you observe.