Drift Velocty Variations of TPM

From: J.H. Lee (jhlee@sgs1.hirg.bnl.goV)
Date: Wed Dec 12 2001 - 18:24:57 EST

  • Next message: Ian Bearden: "RE: Drift Velocty Variations of TPM" 

    Hi,

During the data taking, drift velocities for TPC's varied due to some
changes in gas flows, atmospheric pressure, temperature and possibly  
some unknown factors.  As we know, it's important to calibrate the 
variations since the drift velocity is directly proportional to the y positions
of the tracks.  Even though y information is not used for the momentum 
calculations,  it's still important to know it accurately because it's used for  
- Selecting tracks from the vertex  (<y> - y, and BB vertex - y),
- Selecting matched tracks (front track y : back track y),
- Time of flight matching (projected y : tof y) and Path length, 
   and also used for
- Normalization/acceptance corrections (delta y).

Here are some plots illustrating how much the velocities vary in time. 

- Attached 1: y positions of Fibers on the Front of TPM2 
"seen" by TPM2 tracks overlaid by y positions of
TPM1 tracks projected to beam plane as a function of the run number.
- Attached 2: y positions of Fibers on the Back of TPM2 
"seen" by TPM2 tracks overlaid by y positions of
TPM1 tracks projected to beam plane as a function of the run number.
- Attached 3:  "y vs. x (cm)"  of Front and Back Fibers seen by TPM2 tracks.

They are from almost  all "high-pt runs" we took at the end of the run.
(from runs 5853  (Nov. 16) - 5983 (Nov. 25): MRS 90 degree 6KG B)

For some earlier note about the Fibers and the analysis, please 
see http://pii3.brahms.bnl.gov/~jhlee/brahms/analysis/mrs/fiber.html 

These are some of information we've learned from the data about the variations.
-  It confirms that the drift velocity we are using for TPM2, 1.6,  is reasonable.
-  Drift velocities has strong correlations with path of gas flow (see the anti-correlation 
   between back and front in the attached figure 3).
-  Drift velocities change up to 10%.
-  The changes for the drift velocities are due to a common source 
(environmental changes and/or gas flow...) at least for TPM1 and TPM2.  

To reflect the variations on the analysis,
run-by-run y cuts (vertex, track matching, tof matching) need to be used.
And, for more accurate tracking/identification/yield calculations
the dependence of velocities on positions ( x and y) of TPC's need to be
taken into account in the analysis.
 
JH
    fiber_front_run.gif
    fiber_back_run.gif
    fiber_x.gif

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