Dilepton Trigger Performance of the HADES Spectrometer GSI

1
Dilepton Trigger Performance ofthe HADES
Spectrometer _at_ GSI
Uncorrelated thoughts about correlated events
Alberica Toia II Physikalisches
Institut Justus-Liebig-Universität Gießen,
Germany for the HADES Collaboration

• Motivation and Requirements of the 2nd Level
  Trigger
• Functionality
• Components
• Algorithms
• Performance
• Suppression factor
• Efficiency
• Comparison Online-Offline Analysis

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HADES Physics Program

• Modification of vector meson properties due to
  interaction with the medium
• Access
• normal nuclear matter p, p beam
• dense nuclear matter heavy ion beam

Electromagnetic form factors of hadrons in time
like region w transition form factorfew
existing data not in agreement with VMD models
SPECTRAL FUNCTION OF r MESON
w TRANSITION FORM FACTOR
ee- -gt p0 w
Model M.Post et al., Nucl.Phys.A 689 (2001)
753 University of Giessen European Graduate School
w -gt p0mm-
need for electromagnetic probes(no strong final
state interaction) r,w -gt ee-(branching
ratio 10-5 10-6 )
Model F.Klingl, et al., Nucl.Phys.A 624 (1997)
527 Data R.Dzhelyadin et al., Phys.Lett.B 102
(1981) 296 V.Druzhinin et al., INP84-93
Novosibirsk
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HADES Detector

• Features
• 106 events / s
• 45 geometrical acceptance
• up to 200 charged particles
• 1 mass resolution

• Components
• MOMENTUM RECONSTRUCTION
• Mini Drift Chambers (MDC) in front and behind a
  supreconducting toroid magnet
• LEPTON IDENTIFICATION
• Hadron-blind Ring Imaging Cherenkov (RICH)
  detector
• Electromagnetic Shower detector
• Time of Flight (TOF) wall

TOF
SHOWER
MDC
RICH
MAGNET
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Trigger System
First Level Trigger multiplicity -gt reaction
centrality
1100

• Second Level Trigger I (Image Processing Units -
  IPU)Identification of electron candidates
• Cherenkov rings
• Electromagnetic showers
• Time-of-flight
• Second Level Trigger II
• (Matching Unit - MU)
• Selection of lepton pairs
• Correlation of electron candidates in front of
  (RICH) and behind magnetic field (Shower, TOF)
• Invariant mass determination

• 60 VME boards
• FPGA, CPLD, DSP implemented
• decision 10 ms
• process 3 GByte/s raw data

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The Matching Unit

• Combines lepton signatures from various IPUs
• Deflection in magnetic field
• polar angle q
• correction dependent on azimuthal angle f
• Matching window variable in f
• Determines momentum from (q, Dq, f)
• Combines unlike sign leptons
• Calculates invariant mass of pairs

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Performance 1- Suppression Factor
Time of Flight Cut

• fakes at low pad multiplicity
• efficiency loss for higher thresholds

Shower Condition

• local maximum
• increasing charge

Matching Unit Selectivity
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Performance 2- Efficiency
Single Lepton Recognition Efficiency

• Efficiency studied with simulations of
• pure e-
• e- embedded in CC

p (MeV)

• No momentum dependence
• Increases with polar angle (due to longer
  radiator path length)

q (degrees)

• SIMULATIONS
• Uncertainty of event generator
• Uncertainty of hit digitization
• Need for an absolute reference system pp
  -gt pph

Efficiency
Fakes/evt pure e- (10 lt ? lt 90
100 lt p lt 1000 MeV 88.7 0.17Noise
1) CC(1.9AGeV) e- 87.6 0.4
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Performance 3 Comparison Online-Offline
Analysis
electrons
p 1/DqMETA - MDC
positrons
Low magnetic field better understanding of
background sources (p0 Dalitz, conversion)
MDC opening angle
Compatible results at large opening angle
satisfactory reconstruction of open
pairs Discrepancies at small opening angle due to
different screening mechanism close pairs ?
efficiency loss ?
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Trigger in Production BeamtimeLepton Enhancement
DEC 2002

• Enhancement of correlated rings in triggered
  events
• Ring quality distribution enhanced but not
  distorted

Normalized to the same number of events
Background suppression by a factor 10
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Summary and Outlook
Alberica Toia I.Fröhlich, A.Gabriel, D.Kirschner,
W.Koenig, W.Kühn, J.Lehnert, E.Lins, T.Pérez,
M. Petri, J.Ritman, D.Schäfer, M.Traxler II.
Physikalisches Institut Justus-Liebig-Universität
Gießen, Germany GSI Darmstadt

• Characterization of second level trigger and its
  components
• Estimation and methods for
• Suppression factor
• Single lepton efficiency
• Pair ratios online/offline and vice versa
• Low magnetic field
• possibility of resolving close pairs
• better understanding of the ring properties
• Production run background suppressed by a
  factor gt 10

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Matching Unit window
1 LEPTON
2 LEPTONS

• Azimuthal angle f
• in 1st order no deflection
• matching between RICH and
  TOF/Shower
• Polar angle q
• momentum dependent deflection Dq
• Correction for small f dependence of deflection
• Invariant mass calculation of pairs of unlike
  sign leptons