[Halla_molpol] Hall A Moller polarimetry meeting at 1PM

[Asar Ahmed]

Hi Don,

Thank you for clarifying the experimental setup. Earlier, I was thinking in the wrong direction and assuming that the full measurement would be primarily track-based. I agree that the MOLLER coincidence trigger changes the situation significantly.

My concern about using only three GEM planes was mainly related to standalone tracking, alignment, and especially unbiased plane-by-plane efficiency measurements in a more general or high-background environment. If the primary goal is to study the spatial and angular distributions of MOLLER-triggered events, rather than to make a precision efficiency measurement for each GEM plane, then three XY GEM layers may be sufficient.

The clean left-right coincidence trigger, low accidental rate, and small phase-space region defined by the limiting collimator should strongly reduce the pattern-recognition ambiguity. In this case, the GEMs are not searching blindly over the full detector area; they are mostly reconstructing the distribution of already-selected MOLLER electron pairs.

The remaining concern with three planes is not so much whether tracks can be found, but whether the reconstructed distributions could be biased by alignment, residual backgrounds, or possible fringe-field curvature. A fourth plane would still provide useful redundancy for residual checks, rejection of wrong hit combinations, and validation of alignment or curvature effects. However, I would not say that three planes are impossible for the MOLLER-triggered distribution measurement.

The efficiency scan is a separate study, mainly needed to determine the optimal operating voltage for the chamber, which we are doing in the test lab. That type of test does require four layers, and this was the point I was referring to in yesterday’s meeting.

Best,
Asar

On May 5, 2026, at 8:26 AM, Donald Jones <jonesdc at jlab.org> wrote:

Hi Asar,
We were talked yesterday about the difficulty of alignment, finding tracks and measuring efficiency with only 3 GEMs. It seemed like you were saying it was nearly impossible and we discussed the possibility of installing the 4th GEM plane as well to allow for redundancy for track finding. One thing we didn't discuss is the Moller trigger. I'm not sure if the Moller coincidence trigger would obviate the necessity of a 4th plane. We have a very clean left-right coincidence trigger for the Moller electron pairs. The accidental rate is typically <1%. The electrons contributing to the trigger in MOLLER will cover an area of only a few square cm (<20cm^2 per side) of the GEMs due to a limiting collimator we will install. I don't have a good feeling for how many X-rays or low energy single-arm electrons will hit the GEMs, but if we have a 2-4kHz source of electron  Moller pairs as our GEM trigger and a phase space of only a few square cm, I am wondering what you think about finding tracks then with only 3 planes. One complicating factor is that there might be a gentle curvature of tracks due to fringe field of the dipole spectrometer magnet.
-Don

Donald Jones
Hall A/C Staff Scientist
Jefferson Lab
Newport News, VA
________________________________
From: Asar Ahmed <asar at jlab.org>
Sent: Monday, May 4, 2026 1:46 PM
To: Donald Jones <jonesdc at jlab.org>
Cc: halla_molpol at jlab.org <halla_molpol at jlab.org>; thejimnap at gmail.com <thejimnap at gmail.com>
Subject: Re: Hall A Moller polarimetry meeting at 1PM

Hi Don

Please find the slides.

Best regards
Asar

On May 4, 2026, at 8:28 AM, Donald Jones <jonesdc at jlab.org> wrote:

Hi Folks,
We will have our usual Hall A Moller polarimetry meeting today at 1PM.
https://jlab-org.zoomgov.com/j/1606426957
Passcode: 314159
-Don

Donald Jones
Hall A/C Staff Scientist
Jefferson Lab
Newport News, VA

-------------- next part --------------
An HTML attachment was scrubbed...
URL: <https://mailman.jlab.org/pipermail/halla_molpol/attachments/20260505/bea7d9b4/attachment.htm>