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.
Back to the TWIST Technotes...