Probing the inmedium properties of hadrons with dielectrons

1
Probing the in-medium properties of hadrons with
di-electrons
Thomas EberlPhysik-Department E12 Technische
Universität München
High Acceptance Di-Electron Spectrometer
2
High Acceptance Di-Electron Spectrometer

• Introduction
• Physics goals
• The HADES experiment
• setup
• data analysis
• Results comparison with theory
• pp 2.2 GeV
• CC 2 AGeV
• CC 1 AGeV

3
Goal and Concept

• USE
• (p, p, A) A collisions at SIS GSI,
  0 ? r ? 3 r0 , 0 ? T ? 80 MeV
• Dielectron two-body decays of light Vector
  Mesons r, w, f
• High resolution spectroscopy of ee- pairs, no
  final state interaction !

Rare probes at bombarding energies of 1 2 AGeV
!!!
4
Heavy ion collisions at moderate beam energies
Enhancement of baryon density Dt(r/r0 gt 2) 10
fm/c
comparatively long life-time ... ... at moderate
densities
5
Prediction Vector meson modification
e.g. T. Renk et al., PRC 66 (2002) 014902

• melting of the ?-meson
• mass shift and broadening of the ?-meson
• small effects on the f-meson

Vector mesons are interesting probes to
investigate in-medium effects
6
Spectrometer concept

• Geometry
• Full azimuth , polar angles 18o - 85o (y 0
  - 2)
• Pair acceptance ? 0.35
• 80.000 ch., seg. solid or LH2 targets
• Fast particle identification
• RICH CsI solid photo cathode, C4F10 radiator
  N0 ? 80-90, p - suppression 104
• TOF 384 scintillator rods , s ? 150 ps
• TOFino 24 scintillator paddles, s ? 450 ps
  temporary solution, RPC in future
• Pre-Shower 18 pad chambers lead converters
• Online ee- identification
• RICH, TOF, Pre-Shower IPUs Matching Unit
• Momentum measurement
• Magnet super-conducting toroid Br 0.36 Tm
• MDC 24 multi-wire drift chambers, sy ? 100
  mm single cell resolution

1 m
7
The HADES experiment _at_ GSI
BEAM
8
Data analysis

• Single electron analysis
• classical 2-dim cuts
• on RICH rings, Shower, p vs b, hit matching
  ...
• or Bayes theorem cut on pid prob.
• track fitting quality
• ee- pair analysis
• close pair cuts
• opening angle q gt 9 (tracks removed)
• corrections for detector and reconstruction
  (in-)efficiencies

e-
e
p1
q
RICH
p2
Target
9
Di-electrons from pp 2.2 GeV
10
Exclusive h production in pp reactions at 2.2GeV
dN / dMee
VDM
QED
Mee MeV/c2
... to be continued with pp _at_ 3.5 GeV
11
Di-electrons from CC collisions at E 2AGeV
12
CC 2AGeV ee- invariant mass spectrum
No Efficiency / Acceptance corrections
signal lt 140 MeV/c2 20971 counts signal gt 140
MeV/c2 1937 counts
Statistical errors only !

• Combinatorial background (CB)
• from like-sign pairs
• CB
• Signal S- Nee- - CB

13
CC 2AGeV ee- invariant mass spectrum

• efficiency corrected spectra
• - detector efficiency
• - reconstruction efficiency
• normalized to the pion yield

Invariant mass of e and e-
12C12C 2AGeV
in HADES acceptance
Statistical errors only ! Systematic errors still
under investigation, but lt 20
14
Comparison to models
15
PLUTO a fast event generator for HADES

• Uses ....
• known prod. cross sections or extrapolations
• known branching ratios
• angular distributions

taken from mt-scaling Bratkovskaya et al. PLB
424(1998)
16
CC 2AGeV vs PLUTO simulation
Transverse momentum of ee- pair
Invariant mass of ee- pair
17
CC 2AGeV vs HSD v2.5. vacuum
HSD v2.5 vacuum, Giessen E. Bratkovskaya, W.
Cassing
p0
D
h
r
w
18
CC 2AGeV comparison with URQMD transport model
Transport calculation (vacuum result) UrQMD
Frankfurt M. Bleicher, D. Schumacher
fails to describe the high mass region
19
CC 2AGeV comparison with models
vacuum results

• Included calculations
• PLUTO evt generator
• UrQMD Frankfurt
• M. Bleicher, D. Schumacher
• HSD Gießen (v2.5)
• E. Bratkovskaya, W. Cassing

20
Di-electrons from CC collisions at E 1AGeV
21
CC 1AGeV
preliminary

• Combinatorial background (CB)
• from like-sign pairs
• CB
• Signal S- Nee- - CB-

not efficiency corrected !
22
CC 1AGeV
preliminary
"full" PLUTO cocktail
"full" cocktail reproduces almost the data
but ? Dalitz ? ? ? from mt scaling ... ?
transport calculations ? elementary reactions ?
? Dalitz currently measured in pp 1.25 GeV
23
Summary

• CC 2 AGeV analyzed
• Efficiency corrected di-electron spectrum
• Comparison with transport models
• Preliminary results from CC 1 AGeV
• pp 2.2 GeV exclusive eta production
• High momentum resolution achieved (s 3.5)
• Form factor measurement feasible

24
Outlook

• Ongoing data analysis
• ArKCl 1.757 AGeV
• CC 1 AGeV
• Current beam time in May 2006 pp 1.25GeV
• pp 3.5 GeV pd, pA and pA at var. energies
• Upgrade of TOF subsystem with RPC (2007)
• ? NiNi AuAu runs
• Feasibility studies for HADES operation at
  FAIR-GSI ? 2-8 AGeV runs

25
The Collaboration

• Bratislava (SAS, PI), Slovakia
• Catania (INFN - LNS), Italy
• Cracow (Univ.), Poland
• Darmstadt (GSI), Germany
• Dresden (FZR), Germany
• Dubna (JINR), Russia
• Frankfurt (Univ.), Germany
• Giessen (Univ.), Germany
• Milano (INFN, Univ.), Italy
• Munich (TUM), Germany
• Moscow (ITEP,MEPhI,RAS), Russia
• Nicosia (Univ.), Cyprus
• Orsay (IPN), France
• Rez (CAS, NPI), Czech Rep.
• Sant. de Compostela (Univ.), Spain
• Valencia (Univ.), Spain
• Coimbra (Univ.), Portugal

26

• Backup slides

27

• DLS comparison

28
A reminder The controversial DLS result
? DLS puzzle!?
DLS at the Bevalac (1987-1995)
29
The DLS puzzle CC_at_1AGeV
ee- invariant mass spectrum
?, ? in-medium width 200-300 MeV
2841- 82 signal pairs
Data R.J. Porter et al. PRL 79(97)1229
Model E.L. Bratkovskaya et al. NP
A634(98)168, BUU, vacuum spectral
function
Recent theoretical progress Shekhter et al.,
Phys. Rev C 68, 014904 (2003)
30
Phase space coverage HADES vs. DLS
HADES
DLS
mid-rapidity
mid-rapidity
A direct comparison of HADES and DLS di-electron
results is not feasible !!!
31
Comparison with the DLS results

• generated events processed by the full
• HADES analysis including
• detector (in)efficiency
• reconstruction (in)efficiency

32
CC 1AGeV
preliminary
HADES data vs PLUTO sim.
simple event generator only p0 , ? !
only p0 , ? is not sufficient !
33

• Hadron analysis

34
Hadron identification in CC 2 AGeV
p
y 0.1-0.2 0.2-0.3 0.8-0.9 0.9-1.0 1.6-1.
7
p
Data in good agreement with TAPS/KaoS results
35
Pion spectra (12C12C 2 AGeV)
2 parameter fit

• data in good agreement with TAPS/KaoS results!

36
Hadron analysis two-particle correlations in CC
2AGeV
K0s ? pp-
? ? p p-
37

• pp
• Electromagnetic transition form factors

38
Meson form factors
QED
VDM
39

• Physics

40
Dominant Source of ee- - Pairs

p0 - production
CC, E 2AGeV 0,8 p0 / coll. CC, E 1AGeV
0,3 p0 / coll. ct 25 nm
TAPS Averbeck, Z.Phys. A359 (1997)
p0 - decay
p0 --gt gg
BR 1,2
BR 98,8
p0 -gt e e- g
p0 Dalitz - decay in target
41
Meson modification
Model
shift of in-medium mass broadening of resonances
Density
Temperature
42
Dilepton Sources in HI Collisions
HADES
43
Analysis Strategy
44
Offline Track Selection
Tracking Quality Matching
RICH-Signal
e-


Pre-Shower Condition
Velocity vs Momentum

45
Categories of ee- pairs
Next step
Combine
single e and e- from one event systematically
into pairs
2 Sources of Background Pairs with 1
misidentified partner (fake pairs) Pairs from
different sources (combinatorial pairs)
Analysis Cuts
Pairs from same emitting source (mesons, )
46
Fake Pairs
close pair (resolved)
close pair uncorr. track
noise ring
Cluster in TOF
Fake pairs (a,b,c,d) and pairs with opening
angle ?olt9 deg. are removed before
signal S- calculation.
47
Background rejection strategy

• "Conversion rejection"
• C1,2 "close pair candidates"
• Remove the track if a non-fitted
• track candidate is within 10o

• Pair rejection cuts
• C3 opening angle lt9o
• Remove both tracks
• from the sample

48
Combinatorial Pair Background
Dominant Sources
true pair !
g
e e-
Gamma conversion p0 -gt g g
g -gt ee-
Dalitz decay of neutral mesons p0 -gt gee-
? -gt gee-
Proton-Neutron Bremsstrahlung pn -gt N N
ee-
Dalitz decay of D resonance D -gt
N ee-
g

Pion annihilation pp-
-gt ee-
p0
false pair !
e e-
2 resolution scenarios

49
Cut Performance

• Singles
• e / e- candidates ( RICH
  Tracking )
• PreShower, Beta vs Mom

• Pairs
• 0 All pairs
• 1,2,3 Combine only tracks with individual hits
  in each detector
• 4 (Close pair rejection, preliminary)
• 5 Opening angle larger 9
• 6 Good segment fit

12C12C 2 AGeV Simulation
Example
cut number
50

• Trigger

51
Fast 2nd level trigger
up to 20 kHz LVL1Fast multiplicity trigger
full event information digitised.up to 450
MB/sec, comparable to large RHIC experiments!
on-line selection of electron candidates
Suppression 10 - 100
LVL2 triggered events are transported to mass
storage
52
Online Electron Selection
LVL1 Trigger
LVL2 Trigger
Online RICH IPU META IPUs
Hadron dominated
e/e- signal
53

• Runs

54
Production Runs

• November 2002 CC _at_ 2 AGeV , ? 200 106
  Events (LVL1LVL2)
• - Target 2 x 2.5 int. length
• - Trigger MMeta gt 3 (LVL1)
• LVL2 ? 10x enhanced pair statistics
• _at_ 92 ee- pair efficiency comp. to
  LVL1
• - up to now only inner Mini-Drift Chambers in
  analysis dp/p 8
• - 4 sectors with high mass resolution
• February 2004 p p _at_ 2 GeV , ? 420 106
  Events
• - calibration, efficiency studies
• September 2004 CC _at_ 1 AGeV , ? 500 106
  Events (LVL1LVL2)
• - Target 1 / 3 segment(s) -gt 6 int.
  length
• - Trigger MMeta gt 3 (LVL1)
• LVL2 ? 17x enhanced pair
  statistics
• _at_ 90 ee- pair efficiency comp. to
  LVL1
• - up to now only inner Mini-Drift Chambers in
  analysis dp/p 8
• - all sectors with high mass resolution

55
Production Runs II

• September 2005 ArKCl _at_ 1.757 AGeV , ? 800
  106 Events
• May 2006 p p _at_ 1.25 GeV , ongoing

56
Simulation CaCa, E 2AGeV
2 weeks of beam time Mult (LVL1) gt 20 ? ee- gt
15 degrees
CaCa, E 2AGeV
(c.f. HADES Proposal 2004)
57

• Photos, names, .

58
The HADES Set-up at GSI
59
The HADES Collaboration
GSI
60
Di-electrons from CC at E 2AGeV2006-05-11
High Acceptance Di-Electron Spectrometer
61
CC 2AGeV ee- experimental results
in HADES acceptance

• efficiency corrected spectra
• - detector efficiency
• - reconstruction efficiency
• normalized to the pion yield

12C12C 2AGeV
in HADES acceptance
62
CC 2AGeV comparison to RQMD Tübingen
RQMD D. Cozma, C. Fuchs
63
CC 2AGeV vs PLUTO, HSD, URQMD (vacuum
results)
64
CC 2AGeV vs HSD v2.5. vacuum
65
CC 2AGeV vs URQMD
66
CC 2AGeV vs BUU (B. Kaempfer et al.)
vacuumin-medium
67
CC 2AGeV vs PLUTO simulation
Transverse momentum of the di-electron
68
CC 2 AGeV PLUTO simulation
69
CC 2AGeV vs PLUTO
Rapidity
70
PLUTO simulation
Reconstructed signalTrue signal
71
CC 2AGeV, URQMD vs PLUTO
Open symbols URQMD Full symbols PLUTO
simulation
p0
h
72
Signal to Background ratio
CC 2AGeV
73
Background in CC 2AGeV
Reconstructed combinatorial background (LS -
method) Simulated combinatorial background
74

• Older data slides on CC 2AGeV

75
CC 2AGeV comparison with transport models
vacuum results

• included transport
• calculations
• RQMD Tübingen
• C.Fuchs, D. Cozma
• UrQMD Frankfurt
• M. Bleicher, D. Schumacher
• HSD Gießen (v2.5)
• E. Bratkovskaya, W. Cassing

fail to describe the high mass region
76
CC 2AGeV comparison with transport models
filtered with the HADES acceptance
Mee lt 140 MeV/c2
Mee gt 140 MeV/c2

• RQMD in-medium calculation
• collisional broadening
• extended VDM decoherence
• Brown-Rho scaling of VMs
• See Phys. Rev. C68 (2003) 014904 for details.

77
CC 2AGeV ee- experimental results
simple event generator only p0 , ? !
p0 , ? cross section well known!
only p0 , ? is not sufficient !
78
CC 2AGeV ee- experimental results
"full" PLUTO cocktail
fails to describe the data !
79
Physics goals and Experimental Concept
Hadron properties in nuclear medium Minv , G ,
spec. funct. ? r , mB , T

• USE
• (p, p, A) A collisions at SIS GSI,
  0 ? r ? 3 r0 , 0 ? T ? 80 MeV
• Dielectron two-body decays of light Vector Mesons
  r, w, f
• High resolution spectroscopy of ee- -
  pairs, no final state interaction !

c?
?10-15 fm/c
e
e-