[Sbs_gems] [EXTERNAL] SBS GEM optics

[Andrew Puckett]

Hello all,

I’m reproducing here my response to Sean from the Slack channel:

The basic thing we need to do is figure out what we will be able to put together quickly for implementation of SBS arm tracking constraints. Basically the only information we have to go on, assuming we don’t attempt to use the BigBite tracking results in any way, is the clustering results from HCAL. From SBSGEMSpectrometerTracker and SBSBigBite you can see how this works for the BigBite GEMs. Basically, if the “useconstraint” flag is set, all of the GEM reconstruction proceeds in SBSGEMSpectrometerTracker::FineTrack, otherwise this all happens in CoarseTrack.
1:14<https://superbigbite.slack.com/archives/D01J8JLLB7G/p1663089269422669>
In SBSBigBite::CoarseReconstruct, the generic THaSpectrometer::CoarseReconstruct is invoked, which triggers calling the CoarseProcess methods for all the “non-tracking” detectors in BigBite, and then there are some lines of code where the constraint points are calculated for the GEMs and propagated down to the GEMs.
1:15<https://superbigbite.slack.com/archives/D01J8JLLB7G/p1663089300300809>
For SBS, we have to implement a similar logic in the (unfortunately named) SBSEArm class
1:15<https://superbigbite.slack.com/archives/D01J8JLLB7G/p1663089350475149>
In first approximation, we would put the back constraint point at the surface of HCAL, and tracks would be required to point to the highest-energy cluster in HCAL
1:16<https://superbigbite.slack.com/archives/D01J8JLLB7G/p1663089370365049>
with HCAL at a large distance from the target, this constraint should be fairly restrictive.
1:17<https://superbigbite.slack.com/archives/D01J8JLLB7G/p1663089463123029>
Then, depending on the SBS field setting, we can define a range of trajectories that a charged particle can have consistent with originating from the target, undergoing deflection by SBS, and striking HCAL at the detected location.
1:19<https://superbigbite.slack.com/archives/D01J8JLLB7G/p1663089540528729>
Clearly, this useful range depends on whether we assume positive or negative charged particles (in most cases in GEN I assume we’ll run with positive particles upbending and therefore mainly focus on searching positively charged tracks like protons or pions)
1:20<https://superbigbite.slack.com/archives/D01J8JLLB7G/p1663089649831869>
If we could somehow hack the analyzer algorithm to force BigBite reconstruction to happen first, we could also use the BigBite tracking results (vertex in particular) to set tighter constraints at the front of the SBS GEMs. But I also suspect that this won’t necessarily be needed. For a given HCAL and SBS magnet distance, there is a limited range of angles and track momentum originating from the target that can pass through the magnet acceptance and strike HCAL at any given location. We need to use the simulation to look at this, and then proceed from there to write the necessary code and set up the necessary database information. I believe I should be able to carve out some time for this over the next week.

Andrew

From: Sean Jeffas <sj9ry at virginia.edu>
Date: Tuesday, September 13, 2022 at 11:04 AM
To: Andrew Puckett <puckett at jlab.org>
Cc: Sbs_gems at jlab.org <sbs_gems at jlab.org>
Subject: [EXTERNAL] SBS GEM optics
Hi Andrew,

You mentioned at the SBS Monday meeting that we have not developed the optics yet for the SBS side and that this is a significant concern because we cannot reasonably run the tracking algorithm without some reasonable calorimeter constraints. It seems like this is critical to work on if we want to use the SBS GEMs at all. Do you have any plans or ideas for working on this or the timeline? Also could you point us students to what needs to be done?

Best,
Sean
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