CBM Progress Report 2006 - GSI

CBM Progress Report 2006 Simulations
Fast simulation of low-mass electron pair measurements with CBM
Investigation of the invariant mass spectrum of short
lived neutral vector mesons (ρ, ω, φ) via detection of their
electron-positron pairs is one of the major issue of the CBM
physics. Besides of the electron-positron pairs that originate
from the meson decays, there is a large number of
electrons and positrons from π 0 -Dalitz decays and the γ
conversions in the target and the detector material budged.
In addition, the charged pions misidentified as electrons
contribute to the background.
Our study focus on Au+Au central reactions at 25 AGeV.
The physics input was generated by the UrQMD event generator
that accounts for the general event characteristics in
terms of hadron production, and by PLUTO event generator
that provides leptonic (e + e − pairs) and semi-leptonic
decays of light vector mesons. The assumed multiplicities
of mesonic “cocktail” together with the considered decay
channels and their branching ratios can be found in [1].
The CBM analysis framework with embedded GEANT3
package [2] was used to transport primary particles through
the CBM experimental set up geometry. The assumed STS
geometry consists of enlarged first 6 STS stations to maximize
the geometrical acceptance for low momentum particles.
The magnetic field was set to 70% of its maximum
value and the target thickness was set to 25µm. The reconstructed
electrons are defined as these that passed through
each of the 8 STS stations (tracking) and hit both RICH
and TRD detector (high efficiency and purity of particle
identification). The unlike sign pair invariant mass spectrum
for the measured electrons is shown in Fig. 1 for uncorrelated
(background, 10 5 events) and correlated (signal,
16 × 10 6 events) components by the solid line and the solid
dots, respectively. It is seen that except of low mass region
(gamma conversion and π o -Dalitz) the combinatorial
background exceeds signal by large factor that depends on
invariant mass. In the range between 0.2 GeV and 0.6 GeV,
specially interesting regarding the CBM physics, the signal
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background pairs
signal pairs
0 0.2 0.4 0.6 0.8 1 1.2 1.4
M + - [GeV]
Figure 1: Combinatorial invariant mass spectrum for the
measured electrons for uncorrelated pairs (background)
and for correlated pairs (signal).
P. Staszel, R. Karabowicz, Z. Majka
e e
Jagiellonian University, Poland
17
to background ratio is on the level of 3 × 10 −3 . In order
to suppress the combinatorial background, the following
cuts have been applied: (a) cut on the invariant mass
(Minv < 16MeV ) of the partially reconstructed unlike
sign pairs (partially means that one track forming the pair is
not identified), (b) cut on the invariant mass of fully reconstructed
unlike sign pairs (Minv < 55MeV ) and (c) cut on
the particle transverse momentum (pT < 0.3GeV ).
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combinatorial invariant mass
background pairs
signal pairs
o ρ
ω - Dalitz
η - Dalitz
0 0.2 0.4 0.6 0.8 1 1.2
M + -
e e [GeV]
Figure 2: The combinatorial invariant mass spectrum of
accepted tracks that survived all cuts. Realistic momentum
resolution, track reconstruction efficiency and the pion
suppression factor have been assumed.
The analysis is based on the Monte Carlo tracks and does
not take into account the realistic performance of the detection
system as well as the reconstruction ability of the existing
reconstruction software. To obtain the more realistic
results we took the momentum resolution, track reconstruction
efficiency and the pion suppression factor determined
from data digitization and reconstruction CBM procedures
[3, 4]. The combinatorial invariant mass spectrum of accepted
tracks that survived all cuts, together with the signal
and background components are plotted in Fig. 2 as black
symbols, red symbols and by the solid line, respectively.
The obtained result is very promising showing that the
concept of the CBM detector geometry is well suited for
the di-electron measurement in the low mass regime. This
analysis, however, does not include realistic particle identification
efficiency. This effect will be considered in the
forthcoming studies.
References
[1] T. Galatyuk and J. Stroth, CBM-PHYS-note-2006-001.
[2] M. Al-Turany et al., FairRoot/CbmRoot Simulation and
Analysis Framework, this report.
[3] C. Höhne et al., Electron identification with RICH and TRD
in CBM, this report.
[4] T. Galatyuk et al., Full reconstruction of low-mass electron
pairs in CBM, this report.