1. Dilepton production with HADES
at GeV kinetic energy
Anar Rustamov
GSI Helmholtzzentrum für Schwerionenforschung

2. Outline dilepton production main physics goal experimental approach
HADES spectrometer
pair spectra
cross sections
comparison with models
PLUTO
HSD
UrQMD
pA data
AA data
summary

3. Dilepton spectra Sources of dileptons: Drell-Yan process charmonimum
A. Drees
Sources of dileptons:
Drell-Yan process
charmonimum
QGP dileptons
resonances
particles from freeze-out stage
low mass part: study of Chiral symmetry restoration

4. Physics motivation r + ... Hadrons in medium
Chiral symmetry: SU(2)L x SU(2) R r
N-1
N*(1520)
+ ...
Spontaneously broken in vacuum by
Partial Restoration in medium
(melting of )
QCD sum rules
QCD part:
dispersion relation
T. Hatsuda, S.H. Lee, Phys. Rev. C 46 (1992) R34
Probing the matter with vector mesons

5. Experimental approach
Mass distribution of the hadrons inside medium:
p1, p2: 4-momenta
Hadrons should be short lived (ρ, ω, Φ)
p1, p2 should be detected undistorted (leptons)
ρ(m) - spectral function, Г - width
Experimental challenge:
Mass [MeV]
ct [fm]
Г/Гtot->ee r
770
1.3
4.7×10-5 w
782
23.4
7.07×10-5 F
1020
44.4
2.97×10-4
small branching ratios
high background

6. HADES spectrometer Acceptance Momentum resolution
φ ~ 2 π
15o < θ < 85o
pair ~ 30%
Momentum resolution
Magnet: Tm
MDC: 24 drift chambers
σm~ 2% at ρ/ω region
Particle identification
RICH
Time of flight
Pre-Shower
MDC (for hadrons)
Trigger
LVL1- charged particle mult.
LVL2- single electron trigger

7. Pair spectra for pp at 3.5 GeV
Not efficiency corrected. Inside HADES acceptance
Close partner cut
Momentum cut 80 < P [MeV/c] < 2000
Track fitting quality cut
θpair > 90
Combinatorial background (CB)
uncorrelated
correlated
CB = N++ + N- -

8. Absolute normalization
Elastic scattering
Kinematic constraints:
Kammerud et al. Phys. Rev. D 4 (1971), 5

9. Pair spectra Efficiency corrected. Inside HADES acceptance Signal
σω ~ 16 MeV/c2 CB
high S/CB ratio
preliminary
Number of pairs:
All : ~ 6.1*104
M <0.15 : ~ 5.5*104
0.6 < M < 0.82 : ~ 451
θpair > 90

10. Cross sections Inclusive cross sections from pair spectra
σπo = 15 mb ± 4mb from
Consistent with Resonance model
S. Teis et al. Z. Phys. A 356, (1997)
pp → π + X
pp → π - X
pp → π o X (HADES)
lines are from HSD
preliminary
H. Weber et al., PRC 67 (2003) and references therein

11. Resonance model ~ 13 mb Δ (1232) sum over N* states higher 3/2 states
S. Teis et al. Z. Phys. A 356, (1997) and refernces therein

12. Cocktail simulation with PLUTO
particle production
phase space
( but 1 π exchange for the Δ )
Fröhlich et al, arxiv:
particle decays
form factors
mass dep. Width
cross sections [mb]
πo : 15
Δ : from isospin coeff.
η : 1.04
ω : 0.28
ρ : 0.36
preliminary

13. ρ/ω cross sections
Cross sections were obtained from simulated cocktail by changing the
ρ/ω ratio until simulation fits the data
W. Cassing, E. Bratkovskaya,
Physics Reports 308 (1999)

14. Comparison with HSD
preliminary
preliminary

15. Comparison with UrQMD
preliminary
preliminary

16. Cocktail subtracted data
πo, η and ω contributions
are subtracted
Particle production
HSD
Lund String Model
UrQMD
Through Resonances
W. Cassing, E. Bratkovskaya ,
Physics Reports 308 (1999)
preliminary
K. Schmidt, et al. Phys. Rev. C 79, (2009)
N*, Δ N m
Missing higher resonances in PLUTO
Overestimation of ρ in UrQMD
HSD from new calculations

17. pNb data at the same kinetic energy of protons ( 3.5 GeV )
Signal
Online spectra:
preliminary calibrations
preliminary alignment
pNb data
pair momenta in ρ/ω region
shape of ρ/ω as a function of
their momenta can be studied

18. AA collisions normalization Enhancement factor ( 0.15 < M < 0.5)
F(2.0) = 1.9 ± 0.2(stat) ± 0.3(sys) ± 0.3( sys)
F(1.0) = 6.8 ± 0.6(stat) ± 1.3(sys) ± 2.0( sys)
PRL 98, (2008)
PLB 663 (2008) 43-48

19. Excess yield DLS data HADES data π0 and η mutlt. from TAPS data
Excess scales with:
Ebeam: like π production
Apart : more than linear

20. Excess yield
Facto -> factor
Remove one “of”
thesis: T. Galatyuk, K. Lapidus
thesis: T. F. Krizek, S. Lang, M. Jurkovic ….
C+C data reproduced (within 20%) by superposition of NN interactions
Ar+KCl data overshoots the NN data by a factor of ~2.5; 20 20

21. Summary I
Meson production is investigated in pp reactions at Tkin = 3.5 GeV
For the first time inclusive cross sections for the π0, η, ω and ρ mesons are reconstructed
Comparison of the data to transport models is shown
The investigation of production mechanisms of the vector mesons are ongoing
Reference spectra for the pNb run at the same beam kinetic energy is obtained

22. Summary II Excess observed in DLS is confirmed by HADS experimentally
The observed excess scales with energy like pion production and more than linear with Apart
The excess in light system ( C+C ) is reproduced by superposition of NN interactions
The observed enhancement in DLS data already exists in elementary reactions, and is not connected with any kind of medium effects

23. Comparison with UrQMD PT distributions for different mass bins
preliminary

24. Pair spectra Particle identification: RICH-MDC matching
Alle+e- = Se+e- + CB
Particle identification:
RICH-MDC matching
Time of flight cuts
Shower cuts
Background rejection:
close track cut
opening angle cut
track fitting quality cut
Alle+e- = Se+e- + CB
θpair > 90
preliminary
Combinatorial background (CB)
CB = N++ + N- -
uncorrelated
correlated

25. AA data  HADES and DLS data agree ! HADES data in DLS acceptance …
DLS Data: R.J. Porter et al.: PRL 79 (1997) 1229
 HADES and DLS data agree !

26. Comparison with the models
Normalized to π peak
preliminary
preliminary
W. Cassing, E. Bratkovskaya,
Physics Reports 308 (1999)
K. Schmidt, et al.
Phys. Rev. C 79, (2009)