On what grounds the LHCO output has ID=0 or 4?

[rfrances@…]

The manual says ... "Electrons and photons may create a candidate in the jet collection."

So, fine, electrons and photons do interact with the calorimeter and, as jets are made from calorimetric information, hence the energy deposits of photons and electron might affect jet making.

Despite this "undistinguishablity" of gammas and jets the output of Delphes in LHCO assign ID =0 or 4, as to let understand that the given object is believed to be a photon or a jet.

I cannot find anywhere on what basis the ID=0 or 4 is assigned in the LHCO output. My naive hypotesis was that large had/em should be a jet, and small had/em should be a photon, but I see that ID=0 is given to objects with any had/em, and the same is true for ID=4. So had/em seems to be not the relevant quantity.

On what grounds the LHCO has ID=0 or 4?

Thanks for your messages
Roberto

[favereau]

Hi,

In Delphes, towers hit by a photon (MC information) are placed in a specific collection (which may contain towers hit by other objects). This list is later used to tag photons with the right ID.

[rfrances@…]

Ok. So, the MC truth is used.

I am rather puzzled then by the output in this event (the rest of the sample has the same wierd feature)

# typ eta phi pt jmas ntrk btag had/em dum1 dum2
0 0 0
1 0 -1.987 1.658 49.464 0.000 0.000 0.000 0.000 0.000 0.000
2 0 0.479 1.353 33.038 0.000 0.000 0.000 0.000 0.000 0.000
3 0 -1.785 -1.833 16.520 0.000 0.000 0.000 0.000 0.000 0.000
4 0 0.479 0.480 11.499 0.000 0.000 0.000 4.840 0.000 0.000
5 0 -1.174 -2.138 11.419 0.000 0.000 0.000 49.395 0.000 0.000
6 3 0.462 0.535 34.580 2.293 1.000 0.000 3.490 0.000 0.000
7 4 -1.190 -2.009 87.380 25.129 9.000 0.000 1.514 0.000 0.000
8 4 -1.987 1.658 49.464 -0.042 0.000 0.000 0.000 0.000 0.000
9 4 0.473 1.343 35.380 3.024 1.000 0.000 0.048 0.000 0.000

10 4 0.462 0.535 34.580 2.293 1.000 0.000 3.490 0.000 0.000
11 4 -1.831 -1.851 31.572 5.579 4.000 0.000 0.777 0.000 0.000
12 4 -3.015 1.133 22.740 4.105 0.000 0.000 2.290 0.000 0.000
13 6 0.000 -0.035 8.297 0.000 0.000 0.000 0.000 0.000 0.000

As you can see the particle 1 and 8 are basically identical in the four vector, but one is tagged as photon the other as jet.
As the had/em is zero I would have said this to be a photon, but who knows what is going on here!

How can this happen? (all the events in the sample have one or more jet matching a photon!)

Thanks for your replies
Roberto

[favereau]

That's normal. The photon collection and jet reconstructions are separate, delphes first reconstructs photons based on MC truth then when the jet algorithm runs, it reconstructs a jet based on the calo tower from the photon. You can exclude jets in a close cone to photons to avoid this.