In order to quantify the impact of the SSD on the STAR tracking performances,
various simulations have been performed for the
Tracking efficiency and ghost track probability have been calculated for two
different vertex detector geometries (SVT only and SVT+SSD) and two
tracking chains (tfs+stk+svm and tfs+est) :
figure 1 :
Number of findable, correct and ghost tracks using EST and
the four layer geometry found in 5 Hijing events :
A) all tracks versus the transverse momentum,
B) primary tracks vs the transverse momentum
C) secondary tracks vs the transverse momentum
D) all tracks vs the pseudo-rapidity
figure 2 : Number of hits in
the track segments found by EST for the SVT+SSD geometry :
left : findable segment and reconstructed segments.
middle : correct and ghost segments
right : position of the first wrong hit in the ghost segments
(from the outermost to the innermost layers, ie mostly 1=SSD)
The reconstructed segment size distribution peaks at 4 hits demonstrating
the EST is performing relatively well. The first wrong hit in the ghost
segment is usually 1 indicating that a correct hit association in the SSD
is critical in order to reconstruct correctly the full segment.
figure 3 :
Average number of findable, correct and ghost tracks per Hijing event for
primary and secondary tracks :
top : for the STK+SVM tracking method
bottom : for the EST tracking method
Of the order of 20% more primary tracks are reconstructed with the SVT+SSD
compared to the SVT only and independently of the tracking method used.
With the SSD and the EST tracking method an increase by a factor of 3
is observed for secondary tracks compared to the three layer geometry
and the STK+SVM chain.