1. Recent results from exclusive ppppe+e- reactions with HADES
Side View
START
Béatrice Ramstein
Institut de Physique Nucléaire
France

2. High Acceptance Di-Electron Spectrometer experiments at GSI
Main goal: search for vector meson medium modifications
in 1-2 AGeV range
Side View
START
Why elementary reaction experiments?
Reference for heavy-ion measurements
Test of models in « vacuum » conditions
Selectivity to different dilepton sources
(exclusive channels)
30’’
Béatrice Ramstein, IPN Orsay, France
GDR 2010

3. Outline Introduction: motivations of HADES experiments
in elementary reactions
Discussion of recent HADES pp and dp (quasi-free pn) data E=1.25 GeV, E=2.2 GeV
Exclusive hadronic processes (, production)
Inclusive dilepton production
Exclusive leptonic channels: ppppe+e-
Conclusions - perspectives of pion induced reactions
Béatrice Ramstein, IPN Orsay, France
GDR 2010

4. HADES 2nd generation dilepton spectrometer Side View FW  < 7°
START
Acceptance: Full azimuth, polar angles 18o - 85o
Pair acceptance  0.35
Particle identification:
RICH, Time Of Flight, Pre-Shower (pad chambers & lead converter) ( also MDC (K))
Trigger:
1st Level: charged particle multiplicity (~10 kHz)
2nd Level: single electron trigger (~2.5 kHz)
Momentum measurement
Magnet: ∫Bdl = Tm
MDC: 24 Mini Drift Chambers
Leptons: x~ 140  per cell, p/p ~ 1-2 %
M/M ~ 2% at  peak FW
 < 7°
Consists of 6 sectors, Regarder slide de Gosia. Pair acceptance de DLS?
1’10 tot 8’10
Béatrice Ramstein, IPN Orsay, France
GDR 2010

5. HADES results in C+C reactions at 1 and 2 AGeV
HADES data
G.Agakichiev et al., Phys. Rev. Lett 98, (2007) N
IQMD
C+C 2 A GeV
HSD
1/Np0dN/dM
Me+e- (GeV/c2)
M.Thomere et al.
PRC75,064902(2007)
G. Agakishiev
et al., PLB 663
(2008) 43
M. Sudol PhD
E.Bratkovskaya et al.
NPA 807(2008)214
dilepton spectra reproduced by different transport models with no (or small) medium effects  end of DiLepton Spectrometer puzzle
but with different yields for the individual dilepton sources !
specific role of NN  NNe+e- (Bremsstrahlung)
  Ne+e- (Dalitz decay) 1’
Béatrice Ramstein, IPN Orsay, France
GDR 2010

6. NN Bremsstrahlung and  Dalitz decay
NN  NN* NNe+e-
  Ne+e- (BR from QED = not measured )
nucleon graphs
 graphs
Mee(GeV/c2)
q2 = M*2 > 0 N
N, 
Quantum
interference
Time-Like electromagnetic form factors
Effects of VDM formfactors
Kaptari&Kämpfer, NPA 764 (2006) 338
1’30
no FF
 pp dominated by  graphs
 nucleon graphs (Bremsstrahlung) larger in pn than in pp collisions

7. Elementary reactions measured by HADES
 Dalitz decay : pp E=1.25 GeV
NN Bremsstrahlung: dp (pn) E=1.25 AGeV
°,  production pp E= 2.2 GeV
/ω production: pp E= 3.5 GeV
E<  production threshold
HADES
Forward Wall
2 m
Specific instrumental tools:
LH2 target, p/d beams,
Forward Wall d
spectator proton
7 m
( 0.5–7)
Béatrice Ramstein, IPN Orsay, France
GDR 2010

8. Step 1: hadronic channels
pp elastic scattering  tracking, efficiencies, resolutions
pppp°, pppp  consistency of hadronic and leptonic reconstructions
pppn+, pppp°, pppp  check of resonance model ( dilepton cocktail)
Béatrice Ramstein, IPN Orsay, France
GDR 2010

9. exclusive pion production channels measured by HADES
absolute normalisation
from elastic scattering
good agreement
with resonance model
Teis et al. ZPA 356(1997) refinements
in both isospin channels
Consistent picture at
E=1.25 GeV and 2.2 GeV
important check for dilepton analysis
(° and resonances are dilepton sources !)
T. Liu et al. arXiv: [nucl-exp]
PhD IPN Orsay
Béatrice Ramstein, IPN Orsay, France
GDR 2010

10.  and  production : acceptance corrected results
 production: 1 exchange
 decay: cos2
pp  pn+
+ angular dist.
in (+ ,p) ref. frame
pp  pn+
Neutron angular
distribution in CM p n
D++ qn p+
D++
qp+ p
Total cross sections compared to systematics
pppp
Erreur sur sec. Eff eta
Béatrice Ramstein, IPN Orsay, France
GDR 2010

11. Step 2: inclusive e+e- analysis
Béatrice Ramstein, IPN Orsay, France
GDR 2010

12. Results for pp reaction at 1.25 GeV
PLUTO ROOT-based event generator I. Fröhlich et al, arxiv: ;
I. Fröhlich et al,arxiv:
resonance model: Teis et al. Zeit. Phys.A356(1997) refinements
° Dalitz decay  ° = 4.5 mb
branching ratio ° →e+e- 1.2 %
 Dalitz decay : =3/2  ° = 6.75 mb
branching ratio  →Ne+e- (QED: )
Time-like N-  transition electromagnetic form factors
subm. to PLB
[nucl-ex]
Iachello and Wan
0.6m2
GM(q2) D+ p e+
e-- *
q2 = M2inv(e+e-) = M*2 > 0
GM(0)~3
Faire une animation, enelver OBE
Resonance model results:
° Dalitz
Dalitz)+ effect of Iachello FF
Béatrice Ramstein, IPN Orsay, France
GDR 2010

13. two component model: fit of parameters to existing data
elastic nucleon form factors
4 parameters a2, , g8 ,v
N- magnetic transition form factors
2 additionnal parameters: a’,g10
best fit
a’/a= 1.27
g10/g8=1.28
GMp/pFd
GMN-/3GD
GEp /Fd
30 ’’ (tot 12’05)
GMn /nFd
GEn
Béatrice Ramstein, IPN Orsay, France
GDR 2010

14. Quasi-free « pn » dilepton spectra
Comparison to resonance model
° Dalitz decay  ° = 8.6 mb
branching ratio ° →e+e- 1.2 %
 Dalitz decay : =3/2  ° = 12.9 mb branching ratio  →Ne+e- (QED : )
New features with respect to pp reactions:
participant neutron Fermi momentum
( Paris potential)
 contribution
subm. to PLB
[nucl-ex]
 Dalitz
° Dalitz
 Dalitz (BR ~ ) + +
effect of Iachello FF
Ne mettre queresonance model
Additionnal sources with respect to pp?
NN Bremsstrahlung is absent !!
 Check with full One Boson Exchange calculations
Béatrice Ramstein, IPN Orsay, France
GDR 2010

15. pp/pn reactions at 1.25 GeV:
Comparison to simulations with OBE differential cross sections :
R. Shyam & U.Mosel, PRC67 (2003)
L.P. Kaptari, B. Kämpfer, NPA 764 (2006) 338
Kaptari & Kämpfer
Shyam & Mosel
Faire une animation, mettre seulement OBE
pp OK with Shyam & Mosel
overestimated by K&K
HADES np data: a new puzzle ??
Béatrice Ramstein, IPN Orsay, France
GDR 2010

16. Step 3: exclusive ppppe+e- analysis
Béatrice Ramstein, IPN Orsay, France
GDR 2010

17. exclusive analysis : ppppe+e- at 1.25 GeV using pe+e- events (i)
pppe+e-X missing mass MX
Selection of ppppe+e- channel is efficient mp
simulation
HADES
data
MX(GeV/c2)
Mee>140 MeV/c2
preliminary
B.Ramstein Hadron 2009 proceedings
Dilepton mass distribution
in very good agreement with
° and  Dalitz decay
consistent with inclusive dilepton analysis
pp p+ppe+e-
ppp+pp°ppe+e-
200 events
for Me+e- > 140 MeV/c2
preliminary
0.2
Béatrice Ramstein, IPN Orsay, France
GDR 2010

18. exclusive analysis (ii) ppppe+e- at 1.25 GeV
reconstructed by (p,e+,e-)
Mee>140 MeV/c2
No acceptance corrections !
preliminary
(p,e+,e-) invariant mass (GeV/c2)
2 indistinguishible protons
(p1,e+,e-) p p2 e+ D+
q2=M2inv(e+e-)=M2* 1 e-
(p2,e+,e-)
acceptance corrected
simulation of
 production + Dalitz decay
(cf hadronic channels)
cos(CMpe+e-)
preliminary
dominance of ppp + p pe+e-
room for pp Bremsstrahlung contribution?
Béatrice Ramstein, IPN Orsay, France
GDR 2010

19. exclusive analysis ppppe+e- at 1.25 GeV: helicities
 Dalitz decay: +  p g* pe+e-:
only transverse g*
(if Coulomb interaction is neglected)
helicity angle    * N e+ e-
Helicity angle distributions *  e+e-
d/dΩe~ 1+cos2
True helicity from pe+e- events:
* in  ref. frame
acceptance corrected
fit 1+B cos2
B=1.110.32
cos
preliminary
First measurement of  Dalitz decay!
Dalitz decay branching ratio in agreement
with QED value ( ) within ~20%
Helicity angle distribution consistent
with 1+cos2
Béatrice Ramstein, IPN Orsay, France
GDR 2010

20. pp  pp e+ e-  at 2.2 GeV: an “easier case” exclusive reconstruction p°,h  ge+e-
ppe+e- detected
+ miss.mass cut 
Comparison with resonance model
Teis et al, ZPA356 (97) 421
D1232 
N1440
N1535
D1600
acceptance corrections
p/h  g g*  ge+e-
Helicity distributions g*  e+e-
d/dΩe~ 1+cos2
agrees with QED predictions
and analysis check for  case !!
Béatrice Ramstein, IPN Orsay, France
GDR 2010 20

21. Outlook: future HADES experiments
Actual upgrade of HADES
Medium effects with Heavy systems (Ag+Ag E=1.65 AGeV, Au+Au E=1.25 AGeV)
Perspectives of pion beam experiments ( > 2011)
GSI pion beams
p~ GeV/c
second resonance region
expected intensities 106/s
acceptance p/p =8%
Dilepton spectroscopy in A AND N induced experiments
A : medium effects
N : sensitivity to coupling of vector mesons with higher lying resonances
(S11, S31, D13, D33) (subthreshold / production )
HADES at FAIR: E ~ 8 AGeV
Béatrice Ramstein, IPN Orsay, France
GDR 2010

22. Conclusions: HADES experiments
Inclusive spectra in pp/pn and heavy-ion reactions:
critical tests for transport models
a consistent description of dilepton production in all systems is needed !! ↓
Prerequisite to medium effect investigations
Ar+KCl
future Ag+Ag and Au+Au reactions
Not mentionned: very fruitful strangeness production program:
« Deep sub-threshold - production in Ar+KCl reactions at 1.76 AGeV » PRL 103 (2009)
«  decay: a relevant source for K- production at SIS energies »
Specificity of elementary reactions:
Sensitivity to time-like electromagnetic form factors of nucleon and resonances
Selectivity on dilepton sources (  Dalitz decay,  Dalitz decay,  production,…)
Very new ppppe+e- exclusive analysis !
First measurement of  Dalitz decay
helicity distributions to be investigated in dp and heavy-ion reactions
perspectives of pion beam experiments ( 2011)
Béatrice Ramstein, IPN Orsay, France
GDR 2010

23. The HADES Collaboration
Cyprus:
Department of Physics, University of Cyprus
Czech Republic:
Nuclear Physics Institute, Academy of Sciences of Czech Republic
France:
IPN (UMR 8608), Université Paris Sud
Germany:
GSI, Darmstadt
FZ Dresden-Rossendorf
IKF, Goethe-Universität Frankfurt
II.PI, Justus Liebig Universität Giessen
PD E12, Technische Universität München
Italy:
Istituto Nazionale di Fisica Nucleare, Laboratori Nazionali del Sud
Istituto Nazionale di Fisica Nucleare, Sezione di Milano
Poland:
Smoluchowski Institute of Physics, Jagiellonian University of Cracow
Portugal:
LIP-Laboratório de Instrumentação e Física Experimental de Partículas
Russia:
INR, Russian Academy of Science
Joint Institute of Nuclear Research
ITEP
Spain:
Departamento de Física de Partículas, University of Santiago de Compostela
Instituto de Física Corpuscular, Universidad de Valencia-CSIC
17 institutions members
Béatrice Ramstein, IPN Orsay, France
GDR 2010 23 23

24. Thank You
Béatrice Ramstein, IPN Orsay, France
GDR 2010

25. pp allows to dicriminate
pp/pn reactions at 1.25 GeV: transport model calculations
pp allows to dicriminate
among models
HADES np data:
a new puzzle ??
Faire une animation, mettre seulement OBE
HSD: E.Bratkovskaya et al.NPA 807(2008)214
IQMD: J. Aichelin et al.
Béatrice Ramstein, IPN Orsay, France
GDR 2010

26. Experimental tests of quasi-free (p,n):
Spectator tagging is under control
p momentum
in d reference frame
- QF simulation
Note: absolute normalization!
M < 140 MeV/c2
M > 140 MeV/c2
Scaling factor for massive pairs
d/dp
- QF simulation
Béatrice Ramstein, IPN Orsay, France
GDR 2010

27. DiLepton Spectrometer data (Berkeley
Shekter et al. RQMD
E. Bratkovskaya and W. Cassing nucl-th:
Béatrice Ramstein, IPN Orsay, France
GDR 2010

28. DLS puzzle solved by new calculation of NN Bremsstrahlung ?
Maybe, but a lot of contradictory calculations are pretty good too ….
RQMD: E.Santini et al
BUU Barz et al.
C+C 2 AGeV
UrQMD Bleicher et al.
IQMD Thomere et al.
 Dalitz pn
HADES data:
Phys Rev. Lett 98 , (2007)
pn Bremsstrahlung and Delta Dalitz decay
are important issues for understanding
intermediate mass dilepton yield
Béatrice Ramstein, IPN Orsay, France
GDR 2010

29. Analysis steps : one example
Forward Wall (np selection):
1. Mult > 0
2. search for particle with
1.6 GeV < p < 2.6 GeV
(e+,e-) pair cuts:
1. track and ring quality
2. identification
3. background rejection cuts
4. openangle > 9°.
Combinatorial background: like sign pairs
efficiency corrections:
preliminary
NCB=N++ + N--
Note:
~1500 pairs above °
Nice S/B ratio
normalisation by elastic scattering measurement

30. Role of resonances in dilepton production at 1-2 AGeV
° Dalitz decay: °  e+e- (BR ~ 1.2 %)
R  N° ,  (1232) , N (1440)…
 Dalitz decay:   e+e- (BR ~ 0.6 %)
N(1535)  p
/ω vector meson production
N(1520)  /ω N , ….. /ω  e+e-
Dalitz decay of baryonic resonances:
Both are linked through
electromagnetic form factors
(1232)  Ne+e- (BR ~ )
not measured !
Faessler et al. RQMD
+ NN Bremsstrahlung NN  NN*
 e+e-