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

[silvia at jlab.org]

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