[Pwg] Reader 1 PR12-21-006 review

[Xiaochao Zheng]

Hi Yves:

Thanks.  One more related question: If we have e+ beam in the hall and it's energy turns out to be 1E-3 (or larger) different from the e- beam that we took data with (for whatever reason), is it possible to tune the arcs so the energy of the e+ matches that of the e- beam to within the 5E-4 level we can measure?

Or do you think that is a very unlikely scenario given the ability of selecting the energy at the injector and setting the arc?

BTW Not sure why everyone is using 1E-4, we always assumed 5E-4 on the beam energy measurement and in the proposal.

Thanks,

Xiaochao

________________________________
From: Yves Roblin <roblin at jlab.org>
Sent: Friday, July 9, 2021 2:13 PM
To: Xiaochao Zheng <xiaochao at jlab.org>; pwg <pwg at jlab.org>; solid at jlab.org <solid at jlab.org>
Subject: Re: Reader 1 PR12-21-006 review

I will let Joe describe in more details the process of generating the e+ but in a nutshell, the resulting positrons are momentum selected and then accelerated via the injector cryomodules and subsequently, the linac.

For the injector, we have a spectrometer we can use to set/check the momentum of the outgoing beam. I expect that this should work as well.

For the linac, we use the next arcs (arc1 for NL and arc2 for SL) to set the acceleration such that a beam with the desired momentum exactly results in going in the center of each dipole.  In other words we set the arc dipole currents to exactly correspond to the momentum we want. A beam at that momentum will stay on this design orbit. We adjust the linac gains accordingly to achieve that condition. Those dipoles are stable at  better than 1e-4 in relative over a long time so once we set it up, it stays that way during the experiment. All we then have to do is monitor and control the pathlength to insure that we end up at the same overall momentum when we reach the halls.

The final step is to measure the momentum in Hall A using the energy measurement there and do any further accelerating gain adjustments as needed.

Note that what I just described is relevant for the beam momentum centroid.
We can pretty much guarantee this will be the same for e- and e+ within our ability to measure it in Hall A (1e-4 or so).

The intrinsic energy spread around that momentum will be mostly dominated by the synchrotron radiation at 11 GeV so by the time you get to Hall A, any difference in energy spread between e+/e- at the start of the machine will be erased.











________________________________
From: Pwg <pwg-bounces at jlab.org> on behalf of Xiaochao Zheng <xiaochao at jlab.org>
Sent: Friday, July 9, 2021 11:40 AM
To: pwg <pwg at jlab.org>; solid at jlab.org <solid at jlab.org>
Subject: [Pwg] Reader 1 PR12-21-006 review

Dear All:

We received some questions and comments from Steven Dytman on the proposal, please see his email below.  (Shufang Su is the 2nd reader and we may receive more questions from her later).

To the positron team (Joe, Yves, Jay, et al): could you please provide some insights to 1b and 1c?

To my theory colleagues: any suggestion on 2? (I can probably make a draft sketch on its answer.  And I may also expand the last theory update in my other 4-page PDF answer to theory/tech comments).

I think I can answer 5, but suggestions are welcome.

I am not sure I understand 3 and 4.. Any ideas?   I think since both e- and e+ are secondary beams, we can't use polarized e-, so we can only do "unpolarized DIS measurement" but I can't figure out how to use it as a "cross check".

I hope to draft a full answer over the weekend and send it back to our readers by Tuesday next week.

Best regards,

Xiaochao

________________________________
From: Steven Dytman <dytman at pitt.edu>
Sent: Friday, July 9, 2021 11:26 AM
Subject: [EXTERNAL] PR12-21-006 review

Xiaochao,
I hope I have your name correct.  Here are my questions on your proposal.

1.  The main issue is the positron beam.
     a.  What parameters are most important properties to control, how well?
     b.  Proposal emphasizes importance of *equality* of beam energies.
Is this a strong limitation on the lab?  TAC report also mentions this.
What if the consistency was 50% worse than you request?  What is your
estimate of how much the systematic errors would increase?
     c.  What is earliest time you plan to have a suitable e+ beam for
your experiment?
     d.  What is the maximum time separation between e+ and e- beam
running you seek?

2.  Since DIS has no sharp signal, backgrounds must be calculated (high
twist, radiative corrections seem most difficult).  How will those be
controlled?

3.  Will you do unpolarized or single polarized DIS measurements as a
cross-check?

4.  How does this relate to other positron experiments? Why was this
chosen to be first?

5. What are the differences between your proposal and the existing CERN
measurement?  I think that comes from the fact that their beam was
polarized and yours will be unpolarized.

regards,
Steve



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