[Olympus] OLYMPUS article from Physics Today being republished

[Axel Schmidt]

Dear colleagues,

IMHO, as far as the MIE luminosity analysis is concerned, we’re really up against a hard limit there. I could imagine improving our confidence in the systematics if the dynamic range of the SyMBs had included more pile-up points. But as things stand, I don’t think the luminosity uncertainty can be reduced further, even with manpower. The dominant systematic comes from the accuracy of the optical survey. Unfortunately, it’s too late to do anything about that.

Sorry that the news isn’t better,
Axel


On Jan 18, 2018, at 10:51 , Brian S. Henderson <bhender1 at mit.edu> wrote:

> Hello,
> 
> 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: 
> http://www.sciencedirect.com/science/article/pii/S0168900217310288?via%3Dihub
> 
> For reference, my thesis is available at the following link and the 
> relevant section is 5.2: https://arxiv.org/pdf/1705.04740.pdf. 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.
> 
> 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.).
> 
> Brian
> 
>> 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:
>> 
>> 1. ToF trigger efficiency: 0.19%
>> 2. Magnetic field: 0.15%
>> 3. Knowledge of the elastic form factors: 0.14%
>> 4. Fiducial cuts: 0.12%
>> 5. Lepton tracking efficiency: 0.18%
>> 6. Elastic selection: 0.27%
>> 
>> 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).
>> 
>> 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.
>> 
>> 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.
>> 
>> Let me know if you have any questions.
>> 
> 
> On 01/14/2018 06:21 AM, Belostotski, Stanislav wrote:
>> 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:
>>> 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: hasell at mit.edu
>>> 
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