[Halld-offline] Kinematic fitting and error matrices

[Will Levine]

Hi,

I've been looking into kinematic fitting and just thought I should share a
couple issues I've been running into, in case anyone has any ideas or
thoughts.

Using the error matrices from the Kalman fitter, if I run a kinematic fit,
and cut out events with confidence level<.01, it seems to do its basic job
of cutting out most "bad" events and keeping some good events.

However, there are a few problems, which lead to too few events passing the
cut.

Pulls indicate that the error matrices are somewhat off, in particular, that
the errors are too small. Simply scaling the error matrix (as given in
px,py,pz) does not solve the problem, so it will be necessary to look at the
errors of the tracking variables.

A few of the error matrices are totally bogus. i.e.

7x7 matrix is as follows

    |      0    |      1    |      2    |      3    |      4    |
------------------------------------------------------------
  0 | 9.425e+141 -4.773e+142  2.402e+142 -6.569e+140  1.786e+143
 1 |-4.773e+142  2.379e+143 -1.197e+143  3.273e+141 -9.047e+143
  2 | 2.402e+142 -1.197e+143  6.031e+142 -1.649e+141  4.555e+143
 3 |-6.569e+140  3.273e+141 -1.649e+141  4.507e+139 -1.245e+142
  4 | 1.786e+143 -9.047e+143  4.555e+143 -1.245e+142  3.473e+144
 5 | -8.99e+143  4.554e+144 -2.293e+144  6.266e+142 -1.748e+145
  6 |-4.053e+141  2.053e+142 -1.033e+142  2.826e+140 -7.805e+142

I'll look into whether these tracks are identifiable or bogus in any other
way.

If any one has any insight into these issues, I'd be happy to hear it.

Will
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