I ran 1000 FAKE_EVENT 20 GeV Pt negative
muons through GEANT. Of these, 74 failed to hit
the chambers correctly. 84 times there was no stub found. I am investigating:
this seems an unacceptably high failure rate.
The resulting stubs have distributions which can be seen in
this PostScript file. The first two plots
are the x/y position and the Z position of the muon as it exits the
last cell in the Bmu system. This is provided by the monte-carlo.
The first shows that the stack angles are not quite correct after the most
recent revisions, and I need to revisit these. We see here hits in two
adjacent stacks. The second plot shows the z-distibution of these hits.
The lower left plot is a chi-squared distribution, modulo some scale
factor. The resolution found on the bench does not seem to agree with
that found in the lab.
The lower right plot is the deviation between the position of the last
hit as found by the monte-carlo and the position of the stub. There is
a slight offset. The offset is larger for
positive muons. I suspect the difference in the offsets (.00020 vs
.00017) has to do with there being
a residual non-radial component to the direction of the muon after
passing through the magnetic fields. The rest of the offset is probably
the alignment issue mentioned above in the orientation of the Bmu stacks.
With positive muons 66 missed the last cell and 66 were not found. The
rate of muons missing the outer stack is consistent with the rate expected
(75) due to cell wall and inter-cell gap inefficiencies.
1-March-2001
Update
There is a difference between the direction of positive and negative
muons in the Bmu system, as tracked by GEANT. The first page of
this postscript file shows the difference between
its direction angle and its position angle, which reflects the deviation from
radial direction. The second page shows the X/Y position of the missing
tracks. These line up very nicely on the whole with the position of
the gaps. I conclude that the inefficiency is largely geometric. I
require 3 hits or more for a track, and a radial track which passes through
one inter-cell gap is likely to pass through a second one, leaving only
2 hits behind.