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<p>Hello,</p>
<p>Apologies for the delayed reply. I have addressed the status of
this previously and I wouldn't say anything has substantially
changed since then, so I have copied what I wrote then below. In
addition to what I wrote regarding the 12 degree determination,
there is the additional uncertainty contribution from the
multi-interaction luminosity determination to the extraction of
the ratio, which is of the same order as the 12 degree uncertainty
and would thus need to be similarly reduced to make any
significant headway against the total uncertainty on the
epsilon=0.98 point. On that latter point, I would have to defer
to Axel and the recent paper on that measurement:
<a class="moz-txt-link-freetext" href="http://www.sciencedirect.com/science/article/pii/S0168900217310288?via%3Dihub">http://www.sciencedirect.com/science/article/pii/S0168900217310288?via%3Dihub</a><br>
</p>
<p>For reference, my thesis is available at the following link and
the relevant section is 5.2: <a class="moz-txt-link-freetext" href="https://arxiv.org/pdf/1705.04740.pdf">https://arxiv.org/pdf/1705.04740.pdf</a>.
In general, I don't see any clear way to reduce the uncertainty
significantly, certainly not by anything approaching an order of
magnitude. If there is sufficient interest and manpower for an
analysis with the goal of reducing the uncertainty by ~20%, this
might be feasible, but is not something I would be able to do
beyond providing assistance in getting started.</p>
<p>For the sake of completeness, it is possible that a careful
analysis of the 4-fold ratio using negative field data from the
February run could significantly reduce the uncertainty. This,
however, would be a serious undertaking, likely requiring at least
one grad student working full time on it as a thesis project,
given that many of the calibrations and analyses of the ToFs,
magnetic field, tracking, 12 degree telescopes, etc. would need to
be redone (or at least thoroughly checked for consistency with the
fall run). Even with that sort of manpower, there are
uncertainties in the 4-fold analysis that may not cancel, however,
and we have no means of measuring now. In particular, I am not
sure if we have sufficient data from the magnetic field
measurements in negative current operation to constrain the
systematic due to imperfect field reversal in the 12 degree region
(where field gradients are the highest and are extremely sensitive
to movements of the coils, asymmetries, etc.).<br>
</p>
<p>Brian<br>
</p>
<blockquote type="cite">
<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>
</blockquote>
<br>
<div class="moz-cite-prefix">On 01/14/2018 06:21 AM, Belostotski,
Stanislav wrote:<br>
</div>
<blockquote type="cite" cite="mid:5A5B3D57.4020000@desy.de">
<pre wrap="">Dear Douglas, Michael and all,
This is a good news.
The OLYMPUS results presented by me at the Annual session of the PNPI
NICKI Council were actively discussed.It is important of course to
measure the TPE contribution in a wide range of Q^2 and epsilon.
One more motivation is to carefully measure the TPE at a very small
Q^2. The PNPI experiment to measure proton radius with a highest
possible precision (using Recoil technique: TPC filled with hydrogen) is
in preparation phase now at the Mainz accelerator. The TPE
correction, though expected to be small at small Q^2, will be the only
one unknown value which might affect the derivative at Q^2->0. Most
of other RCs are small in the case of the Recoil technique.
In this conjunction, let me ask why the systematic error of the charge
asymmetry measured with the two-arm telescope is so large? Might it be
possible to revisit this analysis? What are the dominating factor, and
isn.t it possible to reduce these systematic uncertainties.If yes, this
would be a big help in solving the proton radius problem, at least by
normalization of the theory at small Q^2.
With best regards stanB
On 13.01.2018 20:13, Douglas K Hasell wrote:
</pre>
<blockquote type="cite">
<pre wrap="">Dear Colleagues,
This is just to let you know that the Physics Today article on OLYMPUS written by Steve Blau is being republished in Japan in the March, 2018 issue of Parity.
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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