TWIST Technote 97

Tests of the Energy Loss Correction in the Helix Fitter

Robert MacDonald, 20 May 2005

The helix fitter includes code to correct the reconstruction to account for energy loss. I tested this code using Geant-generated mono-energetic positrons. I have found that the energy loss correction did not work correctly in its original state, but removing the momentum-dependent component and the fudge factor yields something very close to predictions. Several tests show that this change is reasonable; ``harder'' Bremsstrahlung is effectively unseen by our detector. Therefore, it is recommended that the energy loss correction be turned on by default in the helix fitter, as a momentum-independent correction as predicted by just the ``ionization'' energy loss calculations.

Addendum: 22 June 2005

TN 97.1: Helix Fitter Energy Loss Correction on Through-Going Tracks

The helix fitter energy loss correction was recently validated for ``standard'' positron tracks beginning at the target. This short study shows that the correction also works properly for through-going tracks which start at the upstream end of the detector and pass through the full stack. The tracks were fit separately in the upstream and downstream halves of the detector, and when the energy loss correction was applied the effect on the upstream tracks was the same in magnitude and opposite in sign from the downstream tracks---as expected. (The mean $\chi^2/dof$ is not affected in the same way for the upstream and downstream tracks, which seems strange. It can at least be said that the energy loss correction does not make the $\chi^2$ \emph{worse} on the upstream side.) I believe the energy loss correction can be safely used to analyze ``upstream stops'' data.

Addendum: 29 June 2005

TN 97.1.1: On the Intercept of $p_{rec}$ vs $1/\cos\theta$

This is just a short note explaining why I believe the intercepts in the table of $p_{rec}$ vs $1/\cos\theta$ (Technote 97.1, Table 1) make sense, and why that's not where energy loss due to the target comes into the equation---since someone asked the question, and it took me a moment to figure it out.


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