Number of photons versus bar number;
θC residual versus bar number and dip angle
The postscript file for this page can be found here.
The plots drawn on this page are the following.
The first two plots have 144 entries, one per quartz bar -- there are 12 bars
per sector. The vertical dashed lines show the sector boundaries. Thus, in
this page, the values printed in red above the graphs are computed using the
144 values (without a pre-average per sector merging 12 bar values together)
taken from the corresponding histogram.
The top plots shows the average number of hits associated to tracks crossing
each of the quartz bars. Some 'cosmics' veto designed years ago is applied to
keep only colliding event tracks.
The triangular-shaped pattern visible in a given sector is due to two effects:
A few bars significant less efficient (sector #0-bar #11; #9-#0; #10-#0 and
#11-#0) are known and always present.
- small holes between bar boxes to which tracks crossing the sector close
to its edges are more sensitive;
- incidence angle of the track with respect to the bar.
This plot shows the per bar mean θC residual, defined
(θC)measured - (θC)expected
The vertical error bars show the RMS of the distribution. Bias and deviation from flatness in this plot show that the DIRC alignment is not perfect. Yet,
misalignments are very small -- signicantly below the mrad level -- and so the
DIRC alignment appears to be quite good.
This plot shows the 'problem' of the DIRC alignment. The profile plot of the
θC residuals versus the dip angle (π/2 - θ, θ
being the polar angle) is not flat. This structure is thought to come
from the way the alignment corrections are setup. A lookup table depending
on the bar number and on the direction of the emitted photon in the bar is
computed using di-muon events. The choice of this particular sample
may bias the corrections. A way to correct this problem would be to use hadron
samples, for instance pion tracks.
Back to the DIRC QA plots page