sPHENIX EMCal discussion

[Craig Woody <woody AT bnl.gov>]

Hi Sasha,
   Thanks very much for your comments. Yes, you are right, there is a price to be paid for tilting the towers both in eta and phi, and we have discussed this at great length within the EMCAL group. As Jin showed in his simulations, having perfect projectivity produces the narrowest shower size, which is good for not only measuring electrons and photons, but also for our e/h rejection. However, because of the vertex distribution, the pointing will never be perfect, and the effect of the non-uniformities in the blocks is quite large. As we've seen from the test beam data, the tilting does improve the uniformity considerably, and while it is indeed a tradeoff, the benefit of tilting is very substantial. It may degrade the position resolution slightly, but our position resolution for photons will already be degraded significantly by the underlying event in central collisions.
   Also, the tilt angles we are talking about are actually quite small. In the current sPHENIX configuration, the eta tilt is only about 8 degrees for the projective part of the calorimeter, and the phi tilt decreases with eta from less than 5 degrees at central rapidity to less than 3 degrees at large eta (see the attached plot that Jin generated showing this). We believe that we need a tilt greater than about 4 degrees to really see an improvement in the uniformity, which is why we've just tested the V2.1 prototype at larger phi tilt angle at Fermilab this week. We haven't seen the results of that test yet, but hopefully we'll see some improvement.

Cheers,
Craig

On 5/3/2018 4:13 PM, Alexander Bazilevsky wrote:

Dear Craig and All,

I have one major concern on your slides. Probably it is not an issue at all, but just the way you present it.

On slide 4 you justify the necessary of (increasing of) the tilt angle by the uniformity of the EMCal response. I'm afraid this logic is not completely right. Obviously, the larger the tilt angle (or the larger shower size on the projection on EMCal plane) the better uniformity. If you checked 20 or even 30 degrees tilt in test beam (or even in simulation), you would get even better uniformity.  What matters here is EMCal efficiency (1 minus fraction of photons/electron tunneled without inducing a shower) and resolutions. Introducing the (large) tilt angle you deteriorate other things, e.g. position resolution, the ability of shower profile for photon/electron ID etc. So, those should also be judged when moving to larger angle. ... Probably 10 degrees is still ok: e.g. it will add (only?) ~1.2mm to position resolution.

Position dependence of the measured energy would not be a problem at all if it is correctable. And I believe it is (as shown by Joe).  My personal feeling is that what important here is not the (average) response non-uniformity but a fraction of tunneled photons/electrons (or the size of the tail to very low energy in EMCal response to a fixed energy photon/electron), and the average EMCal response vs position doesn't directly reflect it.

... my 2 cents.

Sasha.

On 5/3/18 12:32 PM, Craig Woody wrote:

Sorry, but in my haste to get this out, I forgot one important item on the last slide. Please have a look at these slides instead of the first ones I sent out.

Thanks,
Craig

On 5/3/2018 12:22 PM, Craig Woody wrote:

Dear All,
  There has been a lot of discussion lately about what we say in the CDR about the energy resolution for the EMCAL and how we present this to the CD-1 Review Committee. During the practice for the plenary talks yesterday, Gunter showed our "Ultimate Performance Parameters", which are physics deliverables that we claim that we will be able to achieve in the final detector. They are important quantities since we are essentially promising the DOE that we will be able to deliver these levels of performance, and we better be sure we can achieve them.
  I've prepared a few slides that I plan to show at the General Meeting tomorrow (or at least some subset of them) which I wanted to distribute before then to get feedback from people about how they think we should present this. Please have a look at the attached slides and let me know your comments.

Many thanks,
Craig

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Alexander Bazilevsky
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Attachment: Average Tilt For Surfaces Jin 3-29-18.pdf
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