sPHENIX HCal discussion

[John Lajoie <lajoie AT iastate.edu>]

Hi Edward,

    I certainly understand your discontent, we are in a position here where we think we have a pretty good design and are pushing forward, but we need to back up a bit and dot our "i"'s and cross our "t"'s to justify what we are doing to an external review committee.  There is real value in going back and examining key assumptions in an attempt to put together a cogent argument - and often there are important things to be learned. 

    That's the way I am approaching the pCDR exercise, not that we are ripping everything up and starting with a blank page, but rather that we are trying to be diligent to backfill the justification for our design choices.

Regards,
John

On 9/3/2015 7:18 AM, Edward Kistenev wrote:

John, 

I wander if we could avoid the impression that every time when we write new document related to sPHENIX we begin everything from scratch meaning that we know nothing of the detector we are planning to build.  

If we have a problem it is with presenting what we already learned, and to a large extent it is probably my fault - never had time enough to stop and describe what is already done, there is always something more interesting to do. 

Reality is starkly different - we are already building the prototype which we feel we will not need to change. Our expectation is that what will be left to do after this prototype is built and tested is to build large “wood bench” to complete a learning process of how to work with detector. Please do not think I see no need for new simulation - yes we need a lot of it but the outcome must be something directing us towards goal of running detector for physics on day one not so much to prove the chosen value of the tilt angle. sPHENIX is all about notion that we will not even need an engineering run and are building our strategy around delivering physics in two years of physics running. There should be an urgency in proving that  detector conforms to this  promise - measure high pt jets and help to identify electromagnetic probes in the pt range where they are produced - remember we’ll  be (not me!!!)  looking for the differences between those two not for similarities. 

Finally - I see nothing bad in low pT resolution being better then “expected”. In the detector we designed this is natural - particles with different energies are measured in different calorimeters even if geometrically this is the same detector.

Edward 

 

Edward Kistenev, PhD

PHENIX Physicist

On Sep 2, 2015, at 8:04 PM, John Lajoie <lajoie AT iastate.edu> wrote:

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

--

John Lajoie

PHENIX Deputy Spokesperson

Professor of Physics

Iowa State University

 

(515) 294-6952

lajoie AT iastate.edu

Contact me: 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