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

[Angela Biselli]

Dear all
As a representative of pDVCS and based on my experience with the TSA and DSA analysis of eg1-dvcs I am also in absolute favor of extending the FTon configuration. 
Best regards 
Angela

Sent from my iPhone

> On Aug 19, 2022, at 12:07 PM, silvia at jlab.org wrote:
> 
> 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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