User name riordan
Log entry time 13:25:03 on April 29, 2006
Entry number 172375
keyword=Updated Vertex ReconstructionI've come up with a set of corrections for the vertex reconstruction as
well as an updated geometry for the placement of the spectrometer
relative to the beam.
The correction introduce a factor of 2 improvement in resolution along
the beamline, going from 1cm sigma to about 5 or 6mm sigma. The proper
placement of the spectrometer has the chambers pointing directly to the
center of the target as well as producing the proper spacing between
foils. (Figure 1) The fourth foil seems to be exceptionally narrow and
I'm open to suggestions why that is.
The geometry was obtained strictly from survey information and appears to
work very, very well, so I think we should stick with that.
The corrections are first order corrections in x and y and appear to be
of the same order of magnitude. Until these corrections are done there
appears to be a dependence on momentum, suggesting that these are due to
magnetic effects. Plotting the vertex against X, we can see the that for
extreme X there appears to be a distortion in the same regions that we
have had difficulty fitting momentum to (Figure 2). Since these are
momentum dependent, these corrections in theory can be different for all
of the different configurations we have had. I will investigate this and
come up with a set of numbers that we can put in all of the databases.
Also, since this introduces deflection in the y (horizonatal) direction,
I've defined the new deflection angle to be the arccos of the dot product
between the electron vector before the magnet and after the magnet. This
preliminarily seems to make the yp dependence in the momentum
coefficients go away, so my first guess is it seems like the right thing
to do.
A copy of this log entry has been emailed to: bogdanw@jlab.org, nl8@virginia.edu, feuerbac@jlab.org
Figure 1
Figure 2
