sPHENIX is a new detector at RHIC.

[John Lajoie <lajoie AT iastate.edu>]

Dear John, Gunther, Dave, Rosi, Craig:

     I definitely agree with an approach that seeks to clarify that the descoping presents its own set of challenges and its not an easy set of decisions that have little or no impact.

    I would like to reword what John H. said a bit - I think it is not right to keep bringing up the bugaboo of "leakage" since, as we have seen, it is really not really the main issue we are dealing with (although I'm guilty of this). The average jet is made of many lower-energy particles and therefore is typically well-contained in the detector.  This is really the potential for making the fragmentation dependence of jet reconstruction worse, i.e. when a jet fragments into a high-z particle, so I would rather describe it as a fragmentation dependence than "leakage". I think this is more specific about the physics compromise we are making.

    So I would suggest:

The changes largely preserve the unique sPHENIX capabilities for hard probe physics at RHIC.  However, the truncated acceptance of the EMCAL reduces the statistical reach by as much as 25%, and the reduction in thickness of 1-2 interaction lengths and empty space within the calorimeter volume creates the risk of a jet fragmentation bias that may have consequences which are not fully understood.  It will not be possible to restore the EMCAL acceptance once the sector construction begins.

Regards,
John

On 10/24/2017 12:50 PM, John Haggerty wrote:

Gunther and Dave,

On 10/24/17 11:56 AM, Gunther M Roland wrote:

Dear Craig, Rosi, John et al,

Many thanks for the valuable comments! As time is somewhat limited, it might be most efficient if we could discuss a few concrete suggestion for replacing the key statements about the impact of the proposed descoping solution on the expected physics performance. It is probably most useful to focus on the sentence that is repeated in executive overview and summary (with a typo already fixed):
"The changes, which result in significant but manageable reduction in expected statistical reach and jet reconstruction performance, preserve the unique sPHENIX capabilities for hard-probe physics at RHIC."

Here are some words which are certainly not be the final, but I know this is urgent so I thought I'd try and put some words out there to try and push this conversation along and perhaps we can communally come up with something better wordsmithed:

The changes largely preserve the unique sPHENIX capabilities for hard probe physics at RHIC, although the truncated acceptance of the EMCAL reduces the statistical reach by as much as 25% and the reduction in thickness of 1-2 interaction lengths and empty space within the calorimeter volume creates the risk of leakage of energy out of the detector which may have consequences which are not fully understood.  It will not be possible to restore the EMCAL acceptance once the sector construction begins.

One we have found an improved _expression_ of the collaboration's sentiment, we'll update the other relevant paragraphs in the document accordingly.

We should probably be mindful that the exact choice of phrasing may have real consequences with respect to Berndt's planned visit to DOE - i.e., if the changes are considered "catastrophic" one would presume that we will only go ahead once funding for the full scope is identified or if we say that "The changes,...., threaten the unique sPHENIX capabilities..." one might want to reconsider going for CD-1 in early 2018. We need to make sure this describes the impact accurately and supports the path forward the collaboration is pursuing.

I would also avoid "catastrophic," I had a problem with that word in a previous incarnation, and I don't think what we're talking about is really catastrophic, but the closer we get to building something that's not that different from what is already done at RHIC, the less reason our handlers would have for paying for it.

Best,

Gunther and Dave

On Oct 24, 2017, at 11:22 AM, woody <woody AT bnl.gov <mailto:woody AT bnl.gov>> wrote:

Hi All,
  I just want to support what Rosi and John have said. I don't think we want to imply in the Descoping Document that "we're fine" with the proposed cuts in either the EMCAL or the HCAL, and I would not use words like "these losses are not catastrophic", since I believe that is exactly how this would be interpreted. For the EMCAL, I looked more carefully at how we would save the $1.1M before contingency from the EMCAL alone (not counting the electronics), and, because of the fixed costs involved in building the sectors, the cost savings does not scale exactly with reducing the acceptance, although it does to a fairly good approximation (|eta| < 0.82 rather than |eta| < 0.85). However, we should remember that the acceptance for the baseline design is |eta| < 1.1 so that we can have a fiducial regon of |eta| < 1.0 for physics, and we would still need to impose a similar fudicial cut on the reduced acceptance detector. This would imply our physics acceptance would be more like |eta| < 0.72, which would imply a loss of statistics of ~ 50% for the upsilon. Also, the boundary between the reduced acceptance EMCAL and the full acceptance HCAL (which may only be the outer HCAL) will certainly cause significant non-uniformities in jet reconstruction that we will presumably only really find out about after we've started actually analyzing the data. All of this certainly goes against the original design motivation for sPHENIX for having a larger acceptance jet spectrometer with *uniform* coverage, and I personally don't this this should be stated as "not catastrophic".

Craig

On 10/24/2017 8:35 AM, Rosi Reed wrote:

Hello Everyone,

First, attached are two plots for single jet and dijet containment.
For the single jet containment, this is the number of jets a given jet
pT within the EMCal acceptance over all jets of that pT.  For the
dijet containment plot, this is versus the leading jet transverse
momentum so they are on the same scale.  This is the ratio of the
number of leading jets of a given pT where both the leading and
subleading jet are within the EMCal acceptance (and the subleading jet
has pt > 5) over the total number of leading jets with that momentum.
The sub-leading and lead jets are |delta phi| > 2.35, there is no
requirement that the sub leading jet be the highest pt sub-leading
jet.

I think this shows the statistical hit that we take with the reduced
emcal, as we are unlikely to analyze Calo jets that are not fully
contained within both the HCal and the EMcal.  The reason we would
want to do this is illustrated in Figure 1.5.

I can adjust these plots as needed up until about 4 pm today.

Line 91: I highly doubt we can improve upon the nominal configuration
using the reduced configuration with corrections ... the JES and JER
are both affected for the worse, and this means the correction and
unfolding will be more difficult.  In lines with what John has stated,
I don't think we want to make it too optimistic.

Regards,

Rosi



On Tue, Oct 24, 2017 at 12:26 AM, John Haggerty<haggerty AT bnl.gov>  wrote:

Gunther and Dave,

Speaking as a high official of the project, it is good to see that the
collaboration can agree on a plan to bring the costs within the boundaries
the lab has asked.  As you know, I have looked into more radical changes
without expending engineering effort on them, but I cannot see any better
alternative to this proposal that could come anywhere near being ready in
202{1,2,3}

Speaking as a member of the collaboration, I think we should try to convey
more of a sense of loss and sacrifice.  Cutting the emcal acceptance in
particular seems like a double whammy, because it not only cuts the
acceptance, it makes the whole calorimeter system thinner, and I have even
wondered whether the un-descoped detector is thick enough, and with the
Inner HCAL probably gone, the fiducial region is pretty small.  Experiments
often recover that region with endcap or end plug calorimeters, but I don't
think that's a viable option for us, because it introduces a boundary which
is inherently non-uniform.

Another concern is the unknown unknowns we have introduced with the odd
longitudinal segmentation in the calorimeters...  sure, there is now a lot
of evidence that the signals look not so bad with this arrangement, but
violating the conventional wisdom is often punished harshly by nature.  I
don't know that we can say with certainty that the vast background ocean of
low pt particles, jets, and slow secondaries which are now spinning around
inside the magnet but outside the emcal do not have some very unfortunate
effects.  I have not been able to put my finger on exactly what we should
look for, but I think we have to keep looking for trouble.

I know these are not very well formulated objections, and so may be
impossible to take account, but I thought it might be useful to put them out
there in case it inspires someone smarter than me to see more clearly where
we might be going off the rails.


On 10/21/17 12:02 AM, Gunther M Roland wrote:

Friends,

As discussed at the General Meeting today, we are forwarding draft 1 of
our document outlining the detector scope for a $32M cost cap. The pdf file
can be found at
https://www.dropbox.com/s/5wyutbndogozm5q/sPH-GEN-2017-002_v1.pdf?dl=0
(we'll provide another link tomorrow for those that can't access dropbox)

Please send your comments in reply to this mail, keeping the
[sPH-GEN-2017-002] tag in the subject line***. Comments received by
close-of-business on Monday, 10/23, will be most useful.

Cheers,

Gunther and Dave

***we will move future reviews to ansphenix-notes-l AT bnl.gov  list, but the
list couldn't be generated in time for this note.
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John Lajoie

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Iowa State University

 

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