HADES lowmass dilepton, analysis and results

1
HADES low-mass dilepton, analysis and results

• The HADES spectrometer.
• Experimental and simulated data sample.
• Selection of single e/e- tracks.
• Efficiency evaluation.
• Pair Analysis.

2
High Acceptance Di-Electron Spectrometer

• Magnet
• Supercond. Toroid (6 Coils)
• azim. 2p, polar 18o lt J lt 85o
• Track Reconstruction
• MDC Mini-DriftChambers
• Momentum measurement
• Vertex - Reconstruction
• ee- Identification
• RICH C4F10 Photon-Detector
• META TOF Scintillator-Wall
• Pre-Shower Detector
• Event Selection
• 1st LVL Particle Multiplicity
• 2nd LVL e/e- Enhancement

pf-pi 2p sin(Dq/2)
3
Data Sample
Simulation UrQMD 1.2 GEANT 3.21
Digitizer Trigger MMeta gt 3 CC _at_ 1AGeV, B
0,07 T (I400 A) analys. Events 15,6
106 CC _at_ 2AGeV, B 0,5 T (I2497 A) analys.
Events 13,9 106 Data Volume 800 GB
Experiment _at_ SIS - GSI Ibeam 106 / s,
Target C stot 5 Trigger MMeta gt 3
CC _at_ 1AGeV, B 0,07 T (I400 A) analys.
Events 7,4 106 CC _at_ 2AGeV, B 0,5 T
(I2497 A) analys. Events 36,2 106 Data
Volume 350 GB
4
Evaluation of the RICH efficiency
C _at_ 600MeV
q
l
N0Ideal 112 ltN0Expgt 80
5

Sources of ee- pairs
p0 - Production
p0 - Decay
p0 --gt gg
BR 1,2
BR 98,8
p0 -gt e e- g
p0 - Dalitz-Decay in Target
? - Dalitz 0,7 0f the induced p0 ee- -
Paare bei E2AGeV

6
Measured Cherenkov-Rings
C C E 1A GeV

• ring recognition
• Pattern Matrix
• Hough-Transf.

p0-Dalitz decays ?
T. Eberl, TUM E12
7
Selection Criteria for single tracks
q, f matching
e-


S2
S0/S1
min. bias RICH-Signal Tracking RICH Eff. 96
, Suppr. 49, Purity 78
Pre-Shower Condition (Lepton ID)

S3
S4
Calculation of the azimuthal deflection (Reduction
of wrong hit Combinations)
8
Track AnalysisAnalysis Steps S0 ? S4

• Minimum Combinatorial Background for ee- Pairs
• 1 ( Track Purity) 2

Important Identification and Rejection of fake
tracks
9
Different contribution to the fake candidate
10
Momentum Distribution after cut S4
CC _at_ 2AGeV
Tracks/Coll./1MeV/c
Tracks/Coll./1MeV/c
11
ee- Pair Selection
S8 (qgt4) S9 (6ltqlt18)
S6
S5
Not resolved double tracks
Fake tracks
Opening angle cut
META
META
12
ee- Pairs Analysis
CC _at_ 1AGeV B 0,07 T
S/Bexp 3,7 2866 ee- - Pairs, Minv lt 140 MeV/c2
CC _at_ 2AGeV B 0,5 T
S/Bexp 2 10588 ee- - Pairs, Minv lt 140 MeV/c2
13
Combinatorial Background
True combinatorial background
e
?
HADES
CB from evt mixing US pairs Same evt CB LS pairs
e
?0
e-
?
Shape is different !!! Normalization
??? Event characteristics ???
14
ee- Pairs from the target
62 Dielectrons from p0-Dalitz
S9
Real pairs
e-
Simulation
6
18
Opening Angle
15
RICH Pads pro Ring

• good agreement between Experiment and Simulation
• Average number of pads / Ring 22,6
• Dependency of the ring quality on the path
  length in radiator

Polar Angle
Nr. of Pads / Ring
16
Close Pair Rejection
Double single MDC cluster that corresponds to 2
close tracks
Single one MDC cluster, one track
Selection on the base of the cluster parameters
55 Single Efficiency lt-gt 99 double
rejection 70 Single Efficiency lt-gt 80 double
rejection
17
Invariant Mass spectra for unlike-sign pairs
S0-gtS4 single track cuts. S5 not resolved
double tracks S6 fake tracks
18
Signal S- and background B- reconstruction in
experiment and in simulation
experiment
simulation
N-
19
Acknowledgements
Most of the work was done by Thomas Eberl (TUM,
Muenchen) Jacek Otwinowsky (Giessen) Jaroslav
Bielcik (Yale University)
20
Summary and Outlook

• NOV01 sample investigated, p0 channel selected.
• Check of the spectrometer efficiency (must be
  completed via the pp data analysis)
• NOV02 data CC _at_ 2AGeV, 200 Mil. Events, 3 MDC
  planes.
• Combinatorial background checks.

21
Signal S- reconstruction in experiment and in
simulation
simulation
simulation
experiment
Purity 77 (Minv lt 400 MeV/c2) 40
(180 lt Minvlt 400 MeV/c2)
22
ee- Pairs efficiency and fake ratio
Fake Ratio
fake

• Überschrift 1

23
Single lepton distributions
24
Momentum Distribution after cut S4
CC _at_ 1AGeV
Tracks/Coll./1MeV/c
Tracks/Coll./1MeV/c
25
Multiplicityafter cut S4
Multiplicity
26
Multiplizitätnach S4
27
Momentum reconstruction of positrons and
electrons in the HADES spectrometer (simulation)
e
e-
28
Geometrical Acceptance
29
S/B Signal to Background
S/B Nb of recon. ee- / Combinatorial
background
Minvlt140MeV/c2
CB from LS-pairs in same evt technique
S9 Pair - Purity 1AGeV 78 2AGeV 68
30
Minv distributions for like-sign pairs in
experiment and in simulation
experiment
e-e-
ee
simulation
31
Signal S- reconstruction in experiment
(systematic bias corrected) and in simulation
simulation
simulation 0.632
experiment
this factor corresponds to a smaller number of
pions produced by corrected UrQMD code