Re: [BLAST_ANAWARE] "default" track angle

From: Chi Zhang (zhangchi@MIT.EDU)
Date: Thu Jul 15 2004 - 12:37:05 EDT


Hi Doug,

The current program flow uses 0 theta all the time.

The setup before is:

1. use 0 theta in hit/stub/segment/fast track level.

2. go all the way into Newton, finish first round of tracking with 0
theta.

3. Then the coordinates where the tack passes the first and third wire
plane in each superlayer is used to compute "impact angle",

4. This angle is then passed to t2d functions to compute hit positions
again.

5. A second round of Newton fit.

Aware of the uncertainties in stub and segments, theta correction is based
on the final newton fit track.

The previouse bug of rad to theta conversion suppressed the "impact angle"
passed to t2d by a factor of 57. So for a long time, we have been using 0
theta correction all the time.

After we spot the bug, the problem I stated in the previous email surfaced
again. With any non-trivial theta correction, from default Wire.Cal or
from computed impact angle, it hurts resolution

Chi

On Thu, 15 Jul 2004, Douglas Hasell wrote:

> Hi,
>
> After Chris' s presentation yesterday I looked at how nsed at least
> calculates the track position and it assumes the the track crosses the
> cell at 0 degrees ie perpendicular to the chamber. I don't know where
> to look to say whether or not the reconstruction does this properly or
> not. The so called default angle which is in the wire calibration file
> varies from -53 degree for cells at 20 degrees to 8 degrees for the
> cells at 80 degrees. So nsed at least does not use the default angle
> but rather fixes the track angle to be 0.
>
> Now it has been stated that using the track angle results in worse
> fits. My question here is then at what point is the proper track angle
> used? If the stubs forming the track are selected based on the results
> using 0 degrees and then the same stubs are recalculated using the
> track angle this may indeed result in poorer resolution. The proper
> procedure is to use the default angle as in the wire calibration file,
> determine the stubs from those positions which could result in
> different combinations of hits being chosen, and then refine it with
> the proper track angle once the track is found. Then the change should
> not be so great and possibly improves things. Going from 0 degrees to
> the actual track angle is a big change (eg a change of -50 degrees in
> the forward direction).
>
> The time to distance curve Chris showed yesterday showed the results
> of his improved fit and compared it with the "GARFIELD" result.
> Chris's result rose quickly from zero and then went linear about 2 mm
> above the GARFIELD result which rose less steeply from zero and then
> went linear. This is the behaviour using 0 degree track angle in a
> magnetic field. With higher track angles the rise is steeper. I
> suspect that if the GARFIELD result had been given the correct track
> angle the discrepancy with Chris;s result would not be as great.
>
> Chris: If you tell me which chamber, cell, layer, etc. you used in
> that plot I will calculate the correct "default" time to position
> curve.
>
>
> Cheers,
>
> Douglas
>
> 26-415 M.I.T.
> Tel: +1 (617) 258-7199
> 77 Massachusetts Avenue Fax: +1 (617)
> 258-5440
> Cambridge, MA 02139, USA E-mail:
> hasell@mit.edu
>
>



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