sPHENIX HCal discussion

[John Lajoie <lajoie AT iastate.edu>]

Dear HCAL Folks,

    My apologies for being a little behind on this, but this email is to lay out what *I* think we need for the pCDR, after consulting with many of you. As you are all aware the time that we have to prepare the pCDR is rather short so we need to be very targeted and focused.

     First a quick comment about the "p" in the "pCDR". This is NOT the complete CDR.  The full CDR will have to demonstrate that (a) the design of the sPHENIX subsystems is reasonable and well thought-out and (b) the combined detector is capable of achieving the physics goals of sPHENIX. We need (a) before (b), and I think the focus of the pCDR should be (a), (b) will have to wait for the full CDR.

    So what does it mean to demonstrate that "the design of the HCAL is reasonable", at least as far as simulations are concerned. I think it means simulations focused how key parameters of the design affect the energy resolution of the device.

(NOTE: In what follows for the simulations when I say "energy resolution" I mean the energy resolution of the the combined EMCal+HCAL device. It makes *no sense* to talk about the HCAL by itself as we will never use it that way, it will always have an EMCal in front of it. When I talk about variations of the HCAL design I am assuming all simulations will be done with a baseline EMCal in front of it.)

(1)  Single particle energy resolution for pions and protons, from let's say 2-60 GeV for the baseline HCAL design.  These simulations should quantify the energy resolution and the non-Gaussian nature of the response function (high and low-side tails), energy leakage, etc.   We have already seen, for example, that the low-energy resolution seems to be much better than anticipated - Abhisek and I think this has to do with the fact that you need to take into account the light collection efficiency and fluctuations at low energies, and he is working on this.

Wherever possible, (1) should incorporate our best knowledge about light collection efficiency, calibration (MeV per SiPM pixels, etc.) from the test benches at BNL and Boulder to achieve a realistic response from the MC.

With (1) in hand, there are several variations that should be a part of the pCDR:

(2) Sensitivity of the HCAL single particle resolution to the inner and outer HCAL tilt angle. I know that Liang did some work on this originally, but I think there have been some fixes to the simulations since that time and likely this needs to be redone in a coordinated fashion.  Characterizing the response should include *not only the resolution, but the non-Gaussian aspects (tails) as well*.

(3) Sensitivity of the HCAL single particle resolution to the uniformity of the sampling fraction as a function of depth. In other words, we should try to quantify the effect on the resolution of grading the coating on the tiles to even out the sampling fraction. Note that this means that we are throwing away a lot of light (!) and should include any possible degradation of the photostatistics that may be important at lower energies.

Doing (1), (2) and (3) correctly is *a lot of work*, and I think that this is reasonably what we can achieve in the next month or so.  If we can do this, I think it will serve as a solid foundation for the CDR, and with this foundation in hand we can redo some of the jet unfolding simulations from the proposal to establish that the sPHENIX can do the physics.

So - what did I leave out? If you would propose doing more, think about what you might leave out in order to accomplish it all in a month. I'd like to start marshalling people to actually do (1)-(3) so if you are interested in participating please let me know.


Regards,
John Lajoie

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John Lajoie

PHENIX Deputy Spokesperson

Professor of Physics

Iowa State University

 

(515) 294-6952

lajoie AT iastate.edu

Contact me: john.lajoie