sPHENIX HCal discussion

[Jamie Nagle <jamie.nagle AT colorado.edu>]

Hello All,

At the sPHENIX Simulations meeting yesterday (https://indico.bnl.gov/conferenceDisplay.py?confId=2710), there was a good discussion about the different energy response in the calorimeters for hadronic and electromagnetic showers.   Christof asked about jets that have a leading pizero and thus a larger electromagnetic component, which is the opposite to those jets selected for fragmentation function measurements (i.e. with a leading charged hadron and thus no leading pizero).    In the TTrees generated by Dennis, the pizero are already decayed into two photons before compiling the particle listing (particle_pid[i]).    Thus, at Dennis' suggestion, I have plotted the jet reconstructed energy for truth jets > 50 GeV as a function of the fraction of the truth particle energy in electromagnetic form (in this case in photons and electrons incident on the front of the calorimeter).    I have plotted this for the Default sPHENIX Calorimeters, though it looks similar (maybe a bit worse) for the Aluminum with and without readout.

The figure attached shows a ~20% variation in the reconstructed energy response as a function of truth electromagnetic fraction.    It aligns with Jin's comment that the default in the reconstruction code is the EMCal energy scale set for pure EM showers.    If the jet consists almost entirely of a very high z pizero, one gets a reconstructed energy of 50 GeV, i.e. very close to the truth.    As the electromagnetic fraction is smaller, the reconstructed energy total decreases.  This is part of my concern expressed today that when we investigate changes in the inner HCal and as a function of leading z hadron -  we are likely walking up and down this curve.

After I am back from the Initial Stages meeting, I would be interested to see how this energy response varies with reconstructed energy fraction in the EMCal (which is something we will measure).  Also, as a small-step start in the direction Christof mentioned for particle flow jets, I would like to implement something that finds EMCal showers that look like pure EM showers (with a probability variable cut like we use in PHENIX).   For such showers above some energy (1-2 GeV), one could use the current energy scale, and then for all the remaining energy in the EMCal use a different hadronic response energy scale.   One can see if that already will improve the resolution by removing the effect shown in the plot.    All of this should lead to some eventual documented scheme for calibrating the jet energy scale in reality.

Sincerely,

Jamie

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|| James L. Nagle   
|| Professor of Physics, University of Colorado, Boulder
|| EMAIL:   jamie.nagle AT colorado.edu
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