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My puzzle with GEANE cov matrix [message #11943] Wed, 01 June 2011 14:22 Go to previous message
Ralf Kliemt is currently offline  Ralf Kliemt
Messages: 507
Registered: May 2007
Location: GSI, Darmstadt
first-grade participant

From: *cb.uni-bonn.de
Hello GEANE experts,

I have a puzzle concerning track parameter propagation and the covriance matrices. I already contacted Lia via mail, but want to share my investigations.

We have the following situation: A track is defined with its first parameter set yielding the following cov matrix:
Start MARS cov (px,py,pz,E,x,y,z): 
6x6 matrix is as follows

     |      0    |      1    |      2    |      3    |      4    |      5    |
------------------------------------------------------------------------------
   0 |  4.059e-05   3.814e-05  -9.897e-05  -2.728e-07   -8.99e-07           0 
   1 |  3.814e-05    3.77e-05  -9.529e-05   1.051e-06  -2.188e-06           0 
   2 | -9.897e-05  -9.529e-05    0.000248  -2.797e-06   2.073e-06           0 
   3 | -2.728e-07   1.051e-06  -2.797e-06   7.092e-06   5.262e-08           0 
   4 |  -8.99e-07  -2.188e-06   2.073e-06   5.262e-08   7.082e-06           0 
   5 |          0           0           0           0           0           0 

After propagating the track parameters to the distance of closest approach of the z axis, I get the following cov:
End MARS cov (px,py,pz,E,x,y,z): 
6x6 matrix is as follows

     |      0    |      1    |      2    |      3    |      4    |      5    |
------------------------------------------------------------------------------
   0 |   5.94e-05  -1.932e-06   2.146e-05   -0.000288   9.157e-06  -0.0001177 
   1 | -1.932e-06   5.631e-05   1.295e-05   2.375e-05   -0.000268  -8.752e-05 
   2 |  2.146e-05   1.295e-05   0.0002708  -0.0005018  -0.0004349  -0.0003691 
   3 |  -0.000288   2.375e-05  -0.0005018    0.002029   0.0004619    0.001021 
   4 |  9.157e-06   -0.000268  -0.0004349   0.0004619    0.001822    0.000857 
   5 | -0.0001177  -8.752e-05  -0.0003691    0.001021    0.000857   0.0007408 

Looking at x,y,z (cols/rows 3,4,5) I get massively increased error values, like from sigma_x=27um to sigma_x=450um.

The residuals look really OK and even the pull distributions for the momenta are well, considering that the beam pipes material blows the cov a bit up.
I can only imagine there is some mismatch between the fortran units and Fairsoft (cm) or a too large contribution of the beam pipe.

I looking forward to some ideas.

Regards. Ralf
 
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