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| Fig.1: d0 corrections results for the modeling of the
Level 2 multijet trigger. Top Left: Scatterplot of d0 versus phi as it appears from the SVT simulation output for data taken with the Jet-20 trigger. Top Right: Same, after our beam position correction. Bottom Left: Number of SVT tracks with corrected impact parameter larger than 100 microns, as a function of the number of tracks used in the beam position fits. Bottom Right: d0 distirbutions after semi-barrel per semi-barrel correction, for half-barrel 3 (yellow) and 5 (blue). |
Until a few runs ago, the beam position was not obtainable by reading any Storable
Bank. It was critical for out studies to keep under control the coordinates of
the beam in order to make corrections on the impact parameters found by the
SVT Simulation.
To make such a correction we need to know the actual position
of the beam in the transverse plane xy and its slope with respect to the z axis. To
determine these parameters we performed a 4-parameter fit using MINUIT, using
tracks that had a chi-squared smaller than 12.6 (the default of the track fitter in
SVT), a Pt larger than 0.7 GeV, and
that begun and ended in the same half-barrel. The chi-squared cut is
useful in order to remove as many fake tracks as possible before performing the fit,
and at the moment it is not
possible to reconstruct tracks that start and end in different half-barrels.
The zero-correlation between d0 and phi after the correction of beam parameters is
shown here in right.
As expected, the performance of the fit improves by incrementing the number
of the tracks used in the fit of the d0-phi plot. We expect that as the determination
of fit parameters increases, the number of tracks with large impact parameter should
decrease until a plateau is reached. The plot on the bottom left shows the number of SVT
tracks with d larger than 100 microns after
beam correction versus the number of tracks used to fit the d-phi plot. Note that we
need to know the minimum number of tracks necessary to have a correct parameter estimate,
since our statistics is low for many of the past stores. Although statistics should be
improved for a definite answer, and the width of the d0 distribution of prompt tracks
should be used for a better determination of the minimum number of tracks to use in the
fits, it tentatively appears that 100 good tracks are enough.
To increase the possibility of obtaining a good correction for all the tracks,
the real impact parameter is calculated half-barrel per
half-barrel. The different result for half-barrel 3 (in full yellow) and 5
(the blue points) is shown in the bottom right plot.
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