03/31/2020

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Present: M.D., A.D., S.Š, J.S.

  • General:
  • SŠ: Coded the unpolarized and polarized Bethe-Heitler formulae, see his report
    • AD's comments on Simon's document:
      • BH asymmetries seem very small. For GDH, we are looking at asymmetries at the 3% level (see e.g. Helbing review's Fig. 38: Δσ 10 μb, with A=Δσ/(2σ₀). It seems the BH asymmetry is lower by significantly more an order of magnitude. Further, Mark simulation indicate a 10% trigger efficiency for BH, so if it is true, Δσ is further suppressed by an order of magnitude. This points toward the possibility that we can ignore the BH for the proposal.
      • Regarding the structure functions for the asymmetry, the atomic form factors should not be needed since the atoms are not polarized.
      • For g1 and g2, probably only their values at very small-x that are truly relevant. If so, one can use a simple Regge parameterization of g1, see e.g. arXiv:1808.03202. For g2, we could just assume g2ww(x,Q²)=-g1(x,Q²)+ ∫x¹g₁(y,Q²)/y dy.
      • For question #2, the angle coverage, we are planing to use the Compton Calorimeter, which cover down to 0.2°. (Note: we do not have the ComCal in the simulation yet).
  • AD:
    • Found out how to write Simon's initial properly: Š
    • Discussed with Richard Jones and Sasha Somov how easy it would be running in Hall D at 0.5 pass. The bottomline is: very difficult because of the smaller Lorentz boost and the long distance between the radiator and the collimator. Here are the details:
      • Richard Jones: It is going to be difficult to tag that beam with our tagger, for two reasons.
        1. only a very small fraction of the photon beam (rough estimate of 0.03%) will make it through the collimator, while the tagger has to count the entire beam. This means that accidentals will be high even at very modest photon beam intensity at the target.
        2. the height of the "stripe" of post-bremsstrahlung electrons at the tagger focal plane will be an order of magnitude larger. This will lead to substantial scraping on the poles of the tagger magnet, and only a fraction of them actually being detected by the tagger. This needs to be simulated, but it might be that the quadrupole can be used to help mitigate this. Those electrons that are detected in the tagging counters without scattering from the poles will have an enhanced probability of having their photons pass the collimator, which may partially offset issue #1.
        • Bottom line is that it may work but not easily. The biggest irreducible issue will be the substantial scattering from the poles of the tagger by post-bremsstrahlung electrons. If the showers from these interactions dominate the tagger rate, it will be difficult to use the tagger at all for this measurement. If that is the case, can you make a meaningful measurement without tagging?
        • Why not consider an alternative: run at the full energy, and tag the low-energy part of the spectrum. You need to build a custom low-energy tagger, which might be a diamond strip detector like they used in the Compton in hall c. You could place it in the vacuum near the exit edge of the tagger magnet on a movable ladder, and sweep it around to cover different parts of the low-energy spectrum. These photons would be fully collimated, and very clean because the electron beam current would be turned way down to keep the low-energy flux at a level compatible with tagging. This sounds like something I would be interested in working on, provided that the physics of the low-energy part of the measurement justifies the effort.
      • Sacha Somov:
        • You can easily simulate this run condition using gxtwist.
        • The problem at small beam energies is the bremsstrahlung angle (m/E), as you have already discussed (I believe that the quad would not help). The rate of accidentals in the tagger has to be considered.
        • The magnet acts as a collimator (the gap is 3 cm); we see this effect at the endpoint energy. A photon, which bremsstrahlung electron interacts wit the magnet pole, will likely not make it through the collimator, but you'll get some background.
        • You may consider to slightly increase the collimator diameter (you don't need linear polarization (?)). If there is a problem with the AC hole size, you can run with the beam profiler.
        • All in all, I believe that this would not be easy to do, and will require some studies/optimizations.