Optics Meeting Nov 19 2024 0100PM ET

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Agenda

1. Tracking Software (David)
2. Magnetic Field imperfections (David, Kate)
3. C++ Fitting code (Tyler)
4. Kinematic Factor (Vassu)[1]

Attendance

David, Vassu, Sakib, Tyler, Paul S, Kate

Minutes

1. (David) Reported on a meeting with a small working group to discuss event-mode (Tracking) software. The meeting is described in some detail here: [2]. Framework will be Podd, and much of the GEM code (decoding, clustering, track finding/fitting) will be based on SBS code; Chandan Ghosh has done the work so far. Has been tested with cosmic data and MOLLER GEM prototypes. For this group's purpose need to know how it will interface to remoll simulation. Idea is that remoll will produce root trees that will have "ideal" track hits (r,phi,z) at the GEM planes (no resolution smearing, no inefficiency or misalignments). A "post-processor" code will than apply resolution smearing, inefficiency, noise (electronic and low-energy gamma, detector misalignment) and produce a new set of hits (r,phi, z) as input to the tracking code at a place where the subsequent track fitting will be identical for simulated and real data. Then this code will produce output tracks with (r,r',phi,phi') as input to the optics fitting code.
2. (David) Magnetic field imperfections study is going well, but there appears to be an issue that the "no-offset" (symmetric field) simulated data does not fit on the smooth dependences of the shift-coil data (i.e. for things like sieve hole image radial position vs. coil offset). Will ask Juliette and Buddhika to generated an almost symmetric" field (with 0.01 mm shift from nominal) to see if those results fit on the curve - if so, then can try to understand what is different for the shifted fields from the nominal symmetric field.
3. (Tyler) Update on status of C++ Optics fitting code. Is available on github. Probably no work on it will happen over the next few months, as Tyler gets established in his new position at Mainz. Code fits data from one foil at a time, assumes that the HoleID branch already filled (so the original sieve hole for each track has already been established). Applies a simple cut on the number of sigma in r away from the mean GEM r to suppress radiated events. Uses the Root MultiDimFit class to apply a chi-squared fit, which easily adjustable fit function. Splits data into a training set and a testing set. However, problem with fit results at the moment (earlier good results may have been overfitting because a bug meant that had used essentially all the data in training set), does not give stable fit results. Discussion - only using < 10K events in present tests, may need higher statistics to stabilize. Maybe need to add (arbitrary) error bars to data points; maybe tweak some of the knobs in the (ultimately Minuit-based) chi-squared minimization.
4. (Vassu) Will our kinematic factor ("KF") measurement be spoiled by the presence of non-Moller events (radiated ep events) in the tracking data from lH2 target? After "hole" cuts on the sieve hole images there is a background of about 9% of radiated ep events in the hole images in MD ring 5 region under the Moller events. Note: inelastic ep will be a much smaller fraction, so we didn't bother with them. Reminder: the true KF for all MOLLER events has a mean value of 0.9929, but the sieve cuts off some of the acceptance, so the true KF for all lH2 events that pass through the sieve holes (i.e. what we will be able to measure, since we need the sieve data to get best v_z) is different: mean value of KF is 1.060 +- 0.001. We will need to make a correction for this shift to our measured KF. Compare the true KF number to the "reconstructed" KF for the same pure Moller events (using the GEM data, with no v_z correction) which is 1.066 +- 0.001; when we make the first-order v_z correction it shifts the value to 1.062 +- 0.001. However, to get the reconstruction we also need to apply an acceptance cut in order to reconstruct the KF. When we apply this to the same pure Moller events, the KF becomes 1.064 +- 0.001. Note: it doesn't make sense to try to calculate a "true" KF for the eP events, since the E' we use in the KF calculation assumes the energy-angle correlation for Mollers. Now, compare the 1.064 +- 0.001 to the reconstructed KF, with v_z correction, for the proper mix of Moller + ep events: 1.064 +- 0.001. So, within the statistical error, adding the eP events doesn't shift what we extract for KF visibly. This makes sense, since the radiated eP events in a given sieve hole should reconstruct to very similar theta as the Moller events that go through the same sieve hole (and our KF calculation is almost entirely by reconstructed theta).

Meeting link information

See email invitation, or contact David Armstrong, Kate Evans, or Ciprian Gal for Zoom link