Difference between revisions of "Track Finding/Fitting 2006"

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(Non-uniform B-field)
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momentum. This is because the fit is a pure helix and the momentum is derived from the radius of the helix (and  
 
momentum. This is because the fit is a pure helix and the momentum is derived from the radius of the helix (and  
 
the theta angle). In other words, using a helix assumes a uniform, constant magnetic field. When an inhomogeneous
 
the theta angle). In other words, using a helix assumes a uniform, constant magnetic field. When an inhomogeneous
field map is used, the angles of the particle at the vertex (phi and theta) are reconstructed reasonably well as shown
+
field map is used, the reconstructed parameters aren't so accurate as shown in the following plots:
in the next plot.  
+
 
 +
Here are the reconstructed angles (phi, theta) of single pi+ events with energies ranging from 200MeV to 9GeV.
 +
The top plots shows the difference between thrown and reconstructed phi angle in radians. The bottom plot shows
 +
the ratio of sin(theta_thrown) to sin(theta). The ratio of sin(theta)s are plotted since the total momentum is calculated
 +
by dividing the transverse momentum by sin(theta).
  
 
[[Image:delta_angle.gif]]
 
[[Image:delta_angle.gif]]
  
The momentum magnitude, however, is not as shown here:
+
The following plot shows the ratio of total momentum, thrown to reconstructed.
  
 
[[Image:momentum_non-const.gif]]
 
[[Image:momentum_non-const.gif]]

Revision as of 13:02, 23 May 2006

May 22, 2006

In preparation for the GlueX presentation at PAC 30, I have started looking at what kind of tracking resolutions can be obtained from the current GlueX track finding code. This is built on the earlier work documented in [GlueX Note 528] .

It is important to note that the goal of the code used here is to find tracks and give initial parameters for input to the final (Kalman) fitter. The final fitter is not yet written. Resolutions obtained from the simple helical fits of the track finder are useful for:

  • Upper limits on tracking resolutions
  • Resolutions seen by Level-3 trigger

Non-uniform B-field

One of the problems with the current tracking code is that it is not terribly accurate at determining the particle's momentum. This is because the fit is a pure helix and the momentum is derived from the radius of the helix (and the theta angle). In other words, using a helix assumes a uniform, constant magnetic field. When an inhomogeneous field map is used, the reconstructed parameters aren't so accurate as shown in the following plots:

Here are the reconstructed angles (phi, theta) of single pi+ events with energies ranging from 200MeV to 9GeV. The top plots shows the difference between thrown and reconstructed phi angle in radians. The bottom plot shows the ratio of sin(theta_thrown) to sin(theta). The ratio of sin(theta)s are plotted since the total momentum is calculated by dividing the transverse momentum by sin(theta).

Delta angle.gif

The following plot shows the ratio of total momentum, thrown to reconstructed.

Momentum non-const.gif

This next plot shows the z-component of the magnetic field as a function of z (along the beamline) in cm. The values of Bz are plotted for distances from the beamline(R=0) out to the BCAL (R=65cm). The locations of the CDC and FDC packages are shown. From this you can see that the field drops dramatically between the first and last FDC package.

Bfield cdc fdc.gif

Helical Parameters

Ro vs theta no sin theta.gif

Uniform vs. Non-uniform Magnetic Fields

Ro vs theta 1GeVpi-.gif

Parameter Resolution

Ro vs theta ConstField.gif Ro vs theta FieldMap.gif Ro vs theta FieldMap z-cut.gif