Multi Target operation at the HERAB Experiment

1
OPEN CHARM MEASUREMENT in AuAu COLLISIONS at 25
AGeV in the EXPERIMENT
FAIR CBM
GSI
Iouri Vassiliev and Peter Senger Gesellschaft
für Schwerionenforschung, Darmstadt, Germany
(supported by EU FP6 Hadron Physics and INTAS)
INTRODUCTION One of the major experimental
challenges of the Compressed Baryonic Matter
(CBM) experiment is the measurement of the
D-meson hadronic decay in the environment of a
heavy-ion collision. Due to the extremely low D
multiplicity close to threshold, background
reduction exploiting the D0 displaced vertex
topology is mandatory. The online event
selection, required to reduce the envisaged
reaction rate of 10 MHz down to the archival rate
of 25 kHz, necessitates fast and efficient track
reconstruction algorithms and high resolution
secondary vertex determination. Particular
difficulties in recognizing the displaced vertex
of the rare D meson decays are caused by weak
hyperon decays producing displaced vertices
between the target and first silicon tracking
station and small angle scattering in detectors
and beam pipe limiting the accuracy of track
reconstruction and vertexing. In this poster,
strategies for background suppression in the open
charm measurement are discussed. We concentrate
on the D0 meson which decays into K- and ? with
a branching ratio of 3.8. The D0mean life time
is 123 µm/c. Particles containing heavy quarks
like charm are produced in the early stage of the
collision. At FAIR, open and hidden charm
production will be studied at beam energies close
to the kinematical threshold, and the production
mechanisms of D will be sensitive to the
conditions inside the early fireball. The
anomalous suppression of charmonium due to
screening effects in the Quark Gluon Plasma (QGP)
was predicted to be an experimental signal of the
QGP. Moreover, the effective masses of D-mesons -
a bound state of a heavy charm quark and a light
quark - are expected to be modified in dense
matter similarly to those of kaons. Such a change
would be reflected in the relative abundance of
charmonium and D-mesons. D-mesons can be
identified via their decay into kaons and pions
(D0?K-?,). The experimental challenge is to
measure the displaced vertex of kaon-pion pairs
with an accuracy of better that 50 µm in order to
suppress the large combinatorial background
caused by promptly emitted protons, kaons and
pions.
Physics topics and observables
CBM detector
STS Silicon Tracking Stations.
? In-medium modifications of hadrons ?
onset of chiral symmetry restoration at high ?B
measure ?, ?, ? ? ee- or/and ??-
open charm (D mesons and ?c)
? Indications for deconfinement at high ?B
? anomalous charmonium suppression ?
measure J/?, D ? Strangeness in matter
(strange matter?) ? enhanced strangeness
production ? measure K, ?, ?, ?, ? ?
Critical point ? event-by-event
fluctuations measure K/? ratio
ECAL (12 m)
TOF (10 m)
TRDs (4, 6, 8 m)
RICH
magnet
target
STS (5 - 100 cm)
Figure 2. Silicon tracking stations. Tracking
vertexing challenge up to 10MHz AuAu reactions
at 25 GeV/n, 1000 charged particles/event, up
to 100 tracks/cm2/event, momentum measurement
with resolution lt 1, secondary vertex reconst
ruction (? 40 ?m), high speed data acquisition
and trigger system
Figure 1. CBM detector
Signal and background simulation
Event reconstruction. Tracking performance.

900 tracks per event
Fifure 5. Track reconstruction efficiency versus
momentum of particle in the STS detector.
Figure 6. Track momentum resolution versus track
momentum.
Figure 7. Residuals between reconstructed and
MC z-positions of the primary verteses
Figure 3. UrQMD event AuAu at 25 AGeV Track
trajectories inside the STS detector calculated
by GEANT3
Figure 4. The same event. STS Hits by hitsprodu-
sers. Blue hits are from MAPS detectors, green
Hybrid pixels, red strip detectors.
The total reconstruction efficiency for all
tracks is about 97. The reconstruction
efficiency clearly depends on the particle
momentum. High energetic particles have
efficiencies larger than 99. Secondary tracks
from D0 decay have momentum larger than 1 GeV/c
and, in addition, come from the target region,
which can be used during the reconstruction, and,
therefore they have an even higher efficiency of
99.90. Most of the other secondary tracks are
low energetic tracks and suffer significant
multiple scattering in the detector material. The
efficiency of low energetic tracks is about 90.
Particles with momentum lower than 200 MeV/c are
mostly outside of the geometrical acceptance of
the STS detector. There is no splitting of
reconstructed tracks into short parts as it can
bee seen from the negligible clone rate. The
level of wrongly reconstructed tracks (ghost
tracks) is less than 1 and these tracks are
similar to tracks of short low energetic
particles.
The primary vertex was determined from all tracks
reconstructed in the STS excluding those which
formed well detached vertices like K0S and ?
decays. The Kalman filter based algorithm
reconstructs the primary vertex with high
accuracy.
In the simulations we used the STS detector of 7
stations positioned at 5, 10, 20, 40, 60, 80 and
100 cm from the target which is made of a 20 µm
thick gold plate. The first 3 stations are placed
in vacuum in order to decrease the effect of
multiple scattering in the carbon beam tube on
track parameters at the target. Station thikness
depends of the detector type. Monte Carlo (MC)
track impact points have been smeared with
a Gaussian assuming typical values of s 3 µm
for the first two Monolithic Active Pixel Sensors
(MAPS) stations close to the target. For hybrid
pixel s 10 µm and for the other four silicon
strip stations 50 µm pitch was used by the strip
hit producer. All detectors have realistic
response ( fake hits, efficiency losses, pile-up
etc.). The asymmetric magnetic field has been
used to trace particles through the detector.
Open charm detection. Strategy and results.

• Strategy
• The prime goal of the D0 detection strategy is a
  suppression of the background by many orders of
  magnitude with, at the same time, achieving a
  maximum for the efficiency of the signal. For the
  background suppression several main cuts have
  been defined and applied in the following order
• Single track parameters based cuts -
• ?2 cut on the track impact parameter including a
  cut on upper and lower values
• track momentum p-cut
• track transverse momentum pt -cut
• Multiple track parameters based cuts -
• ?2 cut on the geometrical fit of the two-track
  secondary vertex
• KS and ? suppression
• cut on z-position of the secondary vertex
• cut on reconstructed D0 momentum pointing to the
  primary vertex (safety)
• ?2 cut on the topology of the primary and
  secondary vertex fit.

Background study with "super-event" technique.
S/B ratio.
The background origin study shows that 76 of
back-ground events were produced with proton
tracks. Such type of backgrounds was elimina-ted
by proton identification technique using TOF
sub-detector.
NOT proton

protons
not protons
Figure 11. Z-vertex resolution (? 38?m) after
the geometric and topologic fit for D0 ? K- ?
recon-struction is the key point to achieve
maximum of efficiency (? 8.5).
Figure 10. Left part reconstructed secondary
vertices for the background (black line) and for
the signal (red line). Right part the ratio
S/?(SB) with a maximum at z 250 µm.
KS and ? suppression
Clusters of the events shaped according to the
kinematics of K0S?? ?- and ? ?p ?- decays are
clearly visible. The cut ptct gt 0.23 GeV/ccm
was applied to reduce the background from K0S and
? decays, where pt is the transverse momentum
relative the K0S or ? line-of-flight and t is
their proper lifetime. Excluding K0S and ? decay
products from the final invariant mass spectrum
decreases the hyperons decay depending background
by the factor of two.
Figure 13 Invariant mass simulated signal plus
background spectrum. The D0 peak is clearly
visible. Estimated data taking time is about 4
months at 0.1 MHz interaction rate. Signal to
background ratio is 3.4
Conclusions The MC simulations of D mesons
decaying into the K? channel shows the
feasibility of a D-mesons collection rate of
about 1000 per day using a combination of MAPS,
Hybrid(s) and strip STS stations in the CBM
detector. The developed track finding and track
fitting procedures in the inhomogeneous magnetic
field allow to obtain an invariant mass
resolution of 10 MeV. The resolution of the D0
vertex is about 40 µm in the longitudinal
direction. The efficiency of D0 detection is
8.5. The achieved signal to background ratio is
about 3.4 in the 2? invariant mass region.
Figure 12. The Armenteros-Podolanski plot
transverse momentum pt of the oppositely charged
decay products versus their asymmetry in
longitudinal momentum .