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<p>Hello all,</p>
<p>I will briefly comment on the 12 degree systematic determination,
although I'll once again point you towards our theses for complete
details on various aspects of the analyses. In particular,
Section 5.2 of my thesis covers the 12 degree analysis including a
rather long discussion of systematic uncertainties in Section
5.2.8. The dominant contributions to the 12 degree
species-relative measurement systematic uncertainty were as
follows:</p>
<ol>
<li>ToF trigger efficiency: 0.19%</li>
<li>Magnetic field: 0.15%</li>
<li>Knowledge of the elastic form factors: 0.14%</li>
<li>Fiducial cuts: 0.12%</li>
<li>Lepton tracking efficiency: 0.18%<br>
</li>
<li>Elastic selection: 0.27%</li>
</ol>
<p>These effects account for ~97% of the total uncertainty quoted
for the 12 degree point. The first three are related to the fact
that we ran in only one field configuration. For #1, the
electrons and positrons tracked in the 12 degree arm sampled
different distributions of ToF bars for the associated proton
trigger (shown in Figure 5-19 of my thesis), including
substantially different sampling of the rearmost ToF bars that had
leading-edge discriminators and needed to be treated differently
in the simulation than the rest of the bars (see Section 4.3.4 of
Becky's thesis. The magnetic field uncertainty arises from the
fact that the 12 degree arms were mounted in the region of the
field with the strongest field gradients (near the coil pinch)
where our uncertainty in the field measurements and model were
largest. Due to the small acceptance of the telescopes and the
strong slope in the cross section in this region, these field
uncertainties can create a clearly visible effect (Figures 5-23
and 5-24 of my thesis). The form factor systematic could, in
principle, be reduced by future measurements, but is fundamentally
limited by the fact that the telescopes sampled different average
Q^2 for each species in the same field configuration. Attempts
were made to cross-check these systematics by using the limited
amount of negative field data, however, there were insufficient
negative field data in the fall run in which running conditions
were at all similar to main production running (i.e., most
negative field runs had material on the target windows, rolled-out
detections, etc.) and the February running conditions were
sufficiently different from the Fall run (in particular with
regard to tracking the protons) to make any clear analysis
effectively impossible. Section 5.2.1 of my thesis discusses the
limitations of a single-arm measurement (i.e., requiring no
information from a proton track (merely the trigger), which
results in ~1%-level uncertainties).<br>
</p>
<p>The latter three effects are a result of the fact that the MWPCs
were not initially designed to be the main (and, in fact, only)
tracking elements of the 12 degree telescopes. Although they
performed extremely admirably and "saved-the-day" for the 12
degree measurements, ultimately the limitation to three tracking
planes and ~1-mm hit position resolution fundamentally limited the
reconstruction. As noted, Section 5.2.8 covers how these various
effects contributed to the systematics and how they were tested by
varying various elements of the analysis. Section 5.2.2 of my
thesis explains why the GEMs needed to be excluded from the 12
degree measurement.<br>
</p>
<p>Many of these effects are estimated very conservatively, and it
is likely true that they are not completely orthogonal. In
particular, I suspect that the fiducial cuts and magnetic field
uncertainties are highly-correlated since the field is related to
the widths of the vertex distributions that go into the fiducial
cuts. Some of this is symptomatic of the fact that typically the
more systematic uncertainties you investigate, the larger your
uncertainty estimate becomes. If Axel or Jan would like to
comment on some of the forward main spectrometer point
uncertainties, they might be able to illuminate a bit more, but in
general the wider acceptance of the drift chambers washes-out some
of these effects.</p>
<p>Let me know if you have any questions.</p>
<p>Brian<br>
</p>
<div class="moz-cite-prefix">On 03/24/2017 08:22 AM, Belostotski,
Stanislav wrote:<br>
</div>
<blockquote cite="mid:58D50F9D.7010909@desy.de" type="cite">
<pre wrap="">Dear Douglas,
Let me first to congratulate you with so successful presentation of the
OLYMPUS results at DESY PRC.
Given the TPE is certainly may affect the precise measurements at
small Q**2 may I ask you which question on r_p (proton radius) is in
the comment section, as mentioned in your mail?
My second issue is related to the published data.I mean the 12 deg.
telescope point.
Why is the systematic error bar so large in this point? Seems to be
larger than those in other (BLAST spectrometer) points.
With best regards StanB
On 23.03.2017 19:28, Douglas K Hasell wrote:
</pre>
<blockquote type="cite">
<pre wrap="">Dear Colleagues,
This past Tuesday the DESY PRC meeting was held. I gave the talk in the open session that I think was well received. My slides are available on the OLYMPUS Wiki.
In the closed session Uwe and I were present. Elke Aschenauer was also present and gave a brief summary of the OLYMPUS experiment and results. She started from the proposal and listed the progress as well as the problems along the way. I think her assessment was fair and very supportive. She congratulated us on several innovative solutions to the problems that arose and also for the quality of the final result that she considered as a definitive measurement in this momentum transfer region in spite of the small effect. She pointed out that theory predictions at the time of the proposal lead us to expect a larger effect that unfortunately was not seen. She also pointed to the tight time constraints that we had to deal with. She made recommendations to DESY about future projects like this to encourage broader collaboration and readiness reviews.
In response Uwe and I thanked her and the PRC for their support and agreed with most of her statements. I then showed our plans for the future papers and presentations.
Offline many people from the PRC and others from DESY congratulated us on the result and commiserated with us about the small effect.
Leslie Rosenberg pointed out that the OLYMPUS web site is woefully out of date. Typically we have been using the Wiki to post results. However, the Wiki is private so the public face of OLYMPUS is hasn’t been updated since around 2012. This is my fault since it is hosted at MIT and I am the main person responsible for these web pages.
Therefore, if there are no objections, I will update several things on the web site, copy our public presentations and publications, including theses, from the Wiki to the web page and remove or make private anything that needs to remain OLYMPUS only.
FYI the Physics Today web site has a brief article on OLYMPUS on their web site:
<a class="moz-txt-link-freetext" href="http://physicstoday.scitation.org/do/10.1063/PT.5.7358/full/">http://physicstoday.scitation.org/do/10.1063/PT.5.7358/full/</a>
Perhaps Jan would like to respond to the proton radius question in the comment section?
I spoke to Dr. Steven Blau, one of the Physics Today editors, last week and will have a telephone conversation with him this afternoon about a longer article scheduled to appear in Physics Today in May. I have put together a few pictures that might be used in the May article. If anyone has good photos of their particular detector that might be included please send them to me.
Finally, the next collaboration meeting will be on Monday, April 10, at 10:00 EST (16:00 CET).
Cheers,
Douglas
26-415 M.I.T. Tel: +1 (617) 258-7199
77 Massachusetts Avenue Fax: +1 (617) 258-5440
Cambridge, MA 02139, USA E-mail: <a class="moz-txt-link-abbreviated" href="mailto:hasell@mit.edu">hasell@mit.edu</a>
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