CBM Progress Report 2006 - GSI

CBM Progress Report 2006 Simulations
Muon detector simulations and choice of the RICH mirror shape ∗
V. Baublis, A. Khanzadeev, B. Komkov, V. Nikulin, V. Samsonov, O. Tarasenkova, and E. Vznuzdaev
The PNPI team in 2006 participated in the CBM experiment
in 3 main directions: the studies of the muon system
absorber layout, estimations for the feasibility of modern
tracking detectors for both MuCh designs and development
of the mirror-adjusting mechanics for RICH.
1. The CBM muon identification system (MuCh) is supposed
to be a set of absorbers interlaced with the tracking
chambers. The GSI team works on the design optimized
for the track-following Kalman filter procedure: a
set of absorbers with subsequently increasing thickness.
This design offers an optimal performance, but requires
the expensive high resolution tracking detectors detectors
that can work in very high hit density environment (up to
∼ 1 hit per cm 2 ). Somewhat different design is proposed
by the members of the PNPI team. An ideal absorber
should catch all hadrons, keeping the muons intact. The
absorption power of the media is characterized by the “Interaction
Length” (hereafter λI) - a mean distance between
the hadron interaction, while the influence of the Coulomb
interaction is reflected by the value X0. In “classical” approach
(NA50, NA60, ALICE...) the light-A absorbers (for
example, made of carbon) are chosen because they offer
the smallest value of X0 for given λI: in other words they
offer the smallest multiple scattering for given nuclear absorption.
The proposed MuCh layout consists of the 145 cm thick
carbon absorber (3.86λI) followed by 60 cm thick iron
(3.58λI). The total absorption length is the same as for
the GSI option; at this thickness the ratio of the absorption
powers for muons (due to the Coulomb scattering) and
hadrons (due to hadron showers) reaches a plateau. Placing
a thick carbon layer before iron one gives a possibility
to reduce the hit density of secondary tracks in coordinate
detectors. Thus the influence of the neighboring tracks is
minimized.
The total thickness of carbon and iron layers has been
fixed, but the number of iron layers and number of detectors
was varied. The GEANT3-based CBMroot framework
v.OKT04 was used for track transport through the detector.
The background and the signal (decay muons from J/Psi
and LMVM) tracks for 25 GeV Au+Au central and minimum
bias events were simulated with UrQMD and Pluto
generators respectively. The so-called cone method has
been used for the track-finding: each STS track was considered
as a muon one provided there is as minimum one
hit within the multiple scattering cone around its extension
in each tracking station.
The analysis demonstrated that the proposed layout of
absorbers (with thick first carbon layer) allows to reach a
∗ Work supported in part by INTAS grant 06-1000012-8781
PNPI, Gatchina, Russia
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pretty high signal to background ratio. For the minimum
bias events the achieved values of the efficiency and signalto-background
ratio are quite close to the results of the GSI
team (see Figure 1). For the case of central events the performance
of the muon system is obviously degrades due
to high hit density in tracking stations. Additional inefficiency
is inherent to the cone method;the resulting efficiency
drops to the level of S/B 4 × 10 −3 .
Signal/Background
Graph
1.2
1
0.8
0.6
0.4
0.2
χ 2 / ndf
9.512e-06 / 1
p0 1.534 ± 2.784
p1 -0.02487 ± 0.1693
p2 0.0001686 ± 0.002222
Efficiency
2
χ χ 2
2 / / ndf
ndf 9.512e-06 0.00383 / / -2 -3
p0 1.534 ± 2.784
p0 -0.23 ± 1.225
p1 -0.02487 ± 0.1693
p1 p2 0.0001686 0.02247 ±
± 0.002222 0.02981
Omega Meson
Signal/Background
20 30 40 50 60
0
Iron Thickness, cm
Figure 1: The signal-to-background ratio (blue line) and
the selection efficiency (red curve) as a function of the
number of 15 cm thick iron absorbers for the minimum bias
events.
2. It has been demonstrated that the existing modern
position-sensitive detectors (micromegas, GEM, even the
cathode pad chambers in peripheral zone) could be used in
case of MuCh. Even for the hottest zones with hit density
of about 1 cm −2 the gas detectors could be built. However,
the significant angular distribution implies the limitation on
the spatial resolution of the detectors: according to our estimations
one will not be able to achieve the resolution better
than 0.25 mm, most likely the best resolution will be of order
∼ 0.3 mm.
3. The designer team presented a conception of the design
for RICH mirrors adjustment and fixation mechanics.
Two prototypes were aso built: the first uses the mirror fixation
at its center, in the second the mirror is fixed at 3
points in its peripheral zone. The analysis of high precision
measurements prove the design.
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Reconstruction Efficiency