CBM Progress Report 2006 - GSI

CBM Progress Report 2006 Simulations
The goal of the CBM experiment is the investigation of
the properties of compressed nuclear matter as produced in
high-energy heavy-ion collisions. The main interest is the
search for in-medium modifications of hadron properties
and for the phase transition boundary between hadronic and
quark-gluon matter at highest baryon densities.
In heavy-ion collisions at relativistic energies, a large
number of particles is produced. In central Au+Au collisions
at 25 GeV/n, about 800 tracks are within the acceptance
of the Silicon Tracking System (STS) of CBM.
The reconstruction of such events, strongly kinematically
focussed due to the fixed-target setup, is very challenging.
Several track finding algorithms are currently being developed
to cope with the extreme environment.
In the Laboratory of High Energy (LHE) at JINR Dubna,
a track finding algorithm based on the approximate solution
of motion equation (ASME) for particles [1] has been developed
with special focusing on the finding of secondary
particles. This algorithm was tested for events of central
Au+Au collisions at 25 GeV/n. The STS setup used consisted
of two MAPS stations (d = 150 µm) at z = 10 cm and
20 cm, two hybrid pixel stations (d = 750 µm) at z = 30 cm
and 40 cm, and four micro-strip stations (d = 400 µm, z =
50, 60, 75 and 100 cm). The single-hit efficieny was 99 %
in the first two stations and 100 % in the others. The fake
hit rate in the MAPS stations was 3 %, while the projective
geometry led to a large number (80 %) of fake hits in the
strip stations. Due to these reasons we have restricted ourselves
to finding tracks with at least four consecutive hits
in stations 2 - 6. With this constraint, the algorithm reconstructs
∼ 600 tracks per event for physics analyses.
efficiency
1
0.8
0.6
0.4
0.2
0
0 1 2 3 4 5
P (GeV/c)
Track reconstruction in the CBM-STS
O. Rogachevsky and A. Jerusalimov
all
primary
secondary
Figure 1: Track reconstruction efficiency as a function of
momentum for all, primary and secondary particles
JINR, Dubna, Russia
5
Figure 1 shows the track finding efficiency as a function
of momentum. We obtain a good efficiency for primary
particles with momenta above 0.5 GeV/c and a reasonable
efficiency for secondary particles, allowing studies of longlived
decaying particles.
The relative momentum resolution obtained from the
track fit is shown in Figure 2 as function of momentum.
Its value is about 1.6 % roughly independent on momentum,
thus demonstrating that it is dominated by multiple
scattering in the STS detector material. A good resolution
for the masses of decayed particle is achieved as well.
P / P (%)
Δ
5
4
3
2
1
0
0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5
P (GeV/c)
Figure 2: Relative momentum resolution for all reconstructed
tracks as function of momentum
The algorithm is very fast (5 − 8 s per event) and the
results obtained with it are promising. Further development
of this algorithm is in progress.
References
[1] A. Jerusalimov: Reconstruction of track parameters in nonuniform
magnetic field, http://www.gsi.de/Documents/DOC-
2005-Jan-58.html