[Rgc] [EXTERNAL] RE: Request/proposition of extension of the FTon configuration for RGC

[Sebastian Kuhn]

Dear RGC members,

I want to remind you that the present run plan (30 PAC days with FTon and 90 PAC days with FTout) has been agreed upon by all spokespersons, Hall B leadership and Jefferson Lab leadership (Bob McKeown). It is basically the run plan originally proposed and approved (ERR June 2019) plus 30 extra days for FTOn to take into account the renewed emphasis on DVCS. Based on this agreement (about 1 1/2 years ago), spokespersons and institutional groups have allocated resources and time (in some cases a very significant amount) towards the realization of that run plan. This does of course not mean that we cannot discuss changes to the plan based on NEW and SPECIFIC information - but a general statement that FTCal is of “utmost importance” is not an argument for NOW changing the schedule we have been following up to this point. What I think would be more helpful is a comparison of the most important PHYSICS to be extracted from DVCS (i.e., Compton Form Factors, or tests of existing parametrizations of GPDs) under the 2 scenarios
1) Follow the run plan as it presently stands, with a switchover to FTout by the end of the month (and TAKING INTO ACCOUNT that we will be running with twice the luminosity for the remaining 170 days). 
2) Continue running for 30 more days (15 more PAC days) each on NH3 and ND3 with the present setup (4 nA beam instead of 10 nA for FTOut), smaller raster radius and hence less precise knowledge of the target polarization. 

I want to point out that, according to my accounting, pDVCS will have received about 80% of the integrated charge that was expected for the nominal 15 PAC days allocated in the compromise plan. The same is true for nDVCS, although I will readily concede that the trouble we have been having with target polarization (especially negative target polarization) is more severe in that case. However, it is entirely uncertain (and even unlikely) that things will improve much over the next 2 months if we keep running with the present configuration. For one, we will continue to underraster the target, compromising both the actual polarization and our knowledge of it. We also won’t be able to raise the beam current. And there is absolutely no guarantee that downtime losses (whether due to target problems or accelerator issues) will be any better in September than now. (Hurricane season is supposed to ramp up!) Finally, just to be clear: We won’t need to anneal targets any more often in the FTout configuration than presently, since the larger raster area will compensate for the higher beam current. (But we will KNOW much more clearly when it’s time to anneal…)

Finally, I understand the particular issues that Silvia lists in her point 3 that have made data taking less efficient (and very hard on the RCs). However, keep in mind that we already were given 6 extra days of commissioning, and we added 15 more days to the FTon portion of the run given the early accelerator and target difficulties. As a consequence, the entire time allocated to running in the FTOn configuration is now 79 days (not 60), and there are already only 171 days left for FTout running (not 180). Taking 60 more days away from the latter would lead to roughly 30% larger error bars on ALL channels and reactions that do not require the FT than originally planned. (Note that, after discarding both Torus outbending polarity AND even Solenoid polarity changes, the DVCS spokespeople have made it very hard for all other experiments to make use of the FTOn data as presently collected.) While the inclusive proton DSA will be fine either way, the deuteron DSA was already statistics-starved (from originally 60 days in my PAC proposal to 45 days to now proposed 30 days). This will affect the extraction of quantities like d/u as x->1 that are similarly statistics-poor as DVCS. In any case, I strongly encourage the spokespersons of ALL EIGHT (run group) proposals that have been approved for RG-C to quantify the impact either way. 

Given the likely high interest (and heated discussion) about this issue, I propose the following:
We will have a 2-hour “special” RG-C meeting this coming TUESDAY, 8/23, starting at 8:30 EDT (US). The first hour will be dedicated to presentations of analyses of the data we have already taken (Noemie, Harut,…) and the second hour will be a discussion of the proposal to change our run plan. I hope the spokespersons can come armed with convincing arguments either way. However, keep in mind that we can always revisit RG-C FTOn running later on - either by switching to FTOn over the Christmas break, or by making the argument that that part of the run needs to be given an extension added on at the end because of poor accelerator / target performance.

Silvia, if you like, I can organize the entire meeting for this Tuesday. I hope you can organize future analysis meetings on (every other) Tuesday(s) going forward.

- Sebastian

> On Aug 19, 2022, at 5:06 PM, DEFURNE Maxime <Maxime.Defurne at cea.fr> wrote:
> 
> Dear all,
> 
> I am currently on vacation with very poor internet Access. So please apologize for the short e-mail written on my cellphone.
> 
> Based on my RG-A analysis, FTCal is of outmost importance for the dvcs acceptance. It covers small phi (0-90 dégrées/ 270-360) in a wide area in q2/xb/ small t where WE have most statistics (highest cross section) from which the phi-harmonics are the most constrained. Any additional data Taken with FTCal-on would greatly benefit to the accuracy of the pDVCS measurement. 
> 
> Therefore I wholeheartedly support silvia's request. 
> 
> Best regards,
> 
> Maxime
> ________________________________
> De : Rgc <rgc-bounces at jlab.org> de la part de silvia at jlab.org <silvia at jlab.org>
> Envoyé : vendredi 19 août 2022 18:07:33
> À : rgc at jlab.org
> Objet : [Rgc] Request/proposition of extension of the FTon configuration for RGC
> 
> Dear Sebastian, and dear all,
> I’d like to propose to postpone the configuration change to FToff by about
> two months, moving it to the end of October.
> This request is a consequence of :
> 1)      The assessment that I did a few days ago, following Volker’s
> suggestion, of the statistics currently collected for nDVCS (and the one
> of pDVCS will be only a factor of ~20 more, which is still pretty poor and
> would not allow 4D binning for the TSA and DSA of pDVCS)
> 2)      The PbPt analysis carried out by Noémie on ND3, which, albeit still
> with poor statistics, hints to a quite low negative polarization, which
> worsens my already appalling estimate for the perspectives for the nDVCS
> target-spin asymmetry.
> 3)      The fact that this first part of RGC has been, first, devoted to
> commissioning, and has then been plagued by various problems: the twice
> broken cold tank circuit for the ND3 NMR, which has not allowed us to
> monitor and optimize appropriately the ND3 polarization, forcing us to use
> a bigger target sample and under-rastering it ; the poor performances of
> the accelerator, due both to the weather (almost a storm per day during
> the month of July, with strong impacts on the beam delivery and a few
> major downtimes) and to the high-current operation of Hall C ; the beam
> time devoted to the ND3 irradiation, for which also the FToff part of the
> run will benefit.
> The target group is planning to fix the cold tank circuit doing the
> upcoming downtime at the end of August. This would allow us to finally
> collect ND3 data with a better knowledge and control over the
> polarization. It would be highly desirable to get these data in the
> optimal configuration for DVCS, which is FTon.
> Extending RGC-FTon until the end of October would still leave ~5 months of
> running to the FToff configuration. For the DIS and SIDIS experiments,
> statistics is not a major issue, while it is crucial for the
> statistics-starved, fully exclusive, low cross section DVCS experiments.
> The factor of 2 higher current that is, on paper, obtainable in the FToff
> configuration (and this still needs to be proven in practice, as the
> target performances at 8 nA have yet to be tested, and likewise for the DC
> occupancies with ELMO), doesn’t compensate the acceptance loss by more
> than a factor of 3, induced by running without the FT.
> I am fully aware that this proposition I am making is arriving at the very
> last minute, and it goes against the agreements we had come to as a Run
> Group. I am sorry for this, all I can say is that I had imagined a
> smoother running than the one we have been dealing with so far, and I
> needed to evaluate all the data collected to fully realize the situation.
> I am also aware that I personally represent a Run-Group proposal, which
> should by definition follow the decisions taken for the PAC-approved
> proposals. I am, on the other hand, pretty confident that this proposition
> would be agreed upon by the pDVCS representatives, although I prefer that
> they speak for themselves.
> I hope this proposition will be at least taken in consideration and
> discussed upon.
> Thanks a lot in advance.
> Best regards,
> Silvia
> 
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