Dileptons from Resonance Matter and the DLS Puzzle :

Dileptons from Resonance Matter 

and the DLS Puzzle : 

Recent results from HADES 

Béatrice Ramstein 

Institut de Physique Nucléaire d’Orsay, France

Outline 

On related topics, topics, 

this morning … B. Kämpfer 

V. Metag’s 

Introduction: motivations and role of Δ resonance 

The DiLepton Spectometer puzzle: 

comparison of HADES and DLS C+C 1 AGeV dilepton 

spectra 

Discussion of HADES C+C, pp and pn data 

briefly…some 

briefly some other results 

Conclusion and outlook 

ISHIP 2008, GSI Beatrice Ramstein, IPN Orsay, France 2

The Collaboration 

SIS 

� Catania (INFN - LNS), Italy 

� Cracow (Univ.), Poland 

� Darmstadt (GSI), Germany 

� Dresden (FZD), Germany 

� Dubna (JINR), Russia 

� Frankfurt (Univ.), Germany 

� Giessen (Univ.), Germany 

� Milano (INFN, Univ.), Italy 

� München (TUM), Germany 

� Moscow (ITEP,MEPhI,RAS), Russia 

� Nicosia (Univ.), Cyprus 

� Orsay (IPN), GSI 

France 

� Rez (CAS, NPI), Czech Rep. 

� Sant. de Compostela (Univ.), Spain 

� Valencia (Univ.), Spain 

� Coimbra (Univ.), LIP, Portugal 


Motivations of HADES experiment 

RHIC 

SPS 

AGS 

hadrons 

QGP 

SIS‐100 

SIS‐300 

SIS‐18 

KEK,JLAB,TAPS 

From B.J. Schäfer, J. Wambach priv. communication 

Instrumental challenge! 

– Low dilepton pair yields from vector 

meson 

~ 1 reaction per 10 5 -10 6 ! 

– Conversion: 

main source π 0 � γγ (BR 99%) 

– Combinatorial background 

Di leptons : penetrating probes 

� medium modifications of vector meson 

properties (ρ,ω) 

� electromagnetic structure of hot and dense 

baryonic matter 

� ρ/ρ o ~ 1-3, T < 100 MeV 

� N π /A part ≈ 10% 


Specificity of 1-2 1 2 A GeV energy range 

� long life time of the dense system 

(up to 15 fm/c) τ ρ ~ 1.3 fm/c, τ ω ~ 23 fm/c 

� dominance of Δ resonance 

UrQMD calculations Au+Au 1 AGeV 

� resonance matter (Δ33 ) ρ/ρ0 

1-2 GeV 

↔ 

Δ 

tot 

ISHIP 2008, GSI Beatrice Ramstein, IPN Orsay, France 5 

π 

N -1 

ρ/ρ 0 

ρ/ρ 0 

n,p 

Δ 

15 fm/c 

Δ Δ 

π 

N -1

! 

inconsistent formula can 

be found in litterature 

Δ Dalitz decay: decay: 

what is known? known? 

exact QED calculation : 

3 amplitudes: e.g. Magnetic, Electric and Coulomb 

( ) ( ) ( ) ( ) ⎟ ⎟ 

2 

⎛ 

⎞ 

2 ⎜ 2 

2 q 

2 

2 

2 2 

m , ⎜ q + 3 q + q 

2 

Δ q G M G E 

C 

+ - 

dΓ(Δ 

→ Ne e ) 

= f 2 dq 

G 

⎜ 

2 

⎝ 

m Δ 

Form factors G M (q 2 ), G E (q 2 ), G C (q 2 ) 

q 2 ≥0 : « Time like « electromagnetic structure of N - Δ transition 

known only at q 2 =0 ( γN → Δ → πN) 

G M (0)=3, G E (0)=0.04, G C (0)=0.2 

radiative decay: Δ→Nγ BR=0.56%±0.06% 


⎠ 

Δ + →pe + e - 

Δ + 

q 2 = M 2 inv (e+ e - ) = M γ* 2 > 0 

Not measured 

p 

γ* 

e - 

Dalitz decay 

Δ + → γp 

Δ + 

q 2 = 0 

p 

γ 

e + 

Radiative decay

Δ Dalitz decay : form factors 

Form factors G M (q 2 ), G E (q 2 ), G C (q 2 ) 

« photon point » G M(0)=3, G E (0)=0.04, G C (0)=0.2 

in transport models : 

constant G M : HSD, IQMD, UrQMD 

extended-VDM extended VDM model: model: 

RQMD 

G M (q 2 ) 

0.6m 2 ρ 

two component Iachello 

model = « VMD like » 

Wan & Iachello, Iachello, 

int. int. 

J. Mod. Mod. 

Phys. A 

20(2005) 1846 1846 

dΓ/dM 

MΔ =1232 MeV/c2 GM =3.0 

GM (q2 ) Iachello 

M Δ =1480 MeV/c 2 

(E th =1.25 GeV) 

M Δ =1830 MeV/c 2 

(E th =2.2 GeV) 

Δ Dalitz decay 

branching ratio 

BR ~ 4.2 10 -5 

M ee (GeV/c 2 ) 


The DiLepton Spectrometer puzzle 


A prominent result in the 1-2 1 2 AGeV energy range: 

Dilepton Spectrometer at the Bevalac 

The puzzle ! 

Calculation: E.L.Bratkovskaya et al. 

Phys. Lett. B445 (1999) 265 

Observed excess remained 

unexplained over years 

Porter et al., Phys. Rev. Lett 79 (1997) 

Calculation: Ernst et al. 

Phys. Rev. C58 (1998) 447 

Calculation: C. Fuchs et al. 

Phys. Rev. C68 (2003) 014904 


1 AGeV

Acceptance: Full azimuth, polar angles 18 o -85 o 

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) 

2' Level: single electron trigger (~2.5 kHz) 

Momentum measurement 

Magnet: ∫Bdl = 0.1-0.34 Tm 

MDC: 24 Mini Drift Chambers 

Leptons: Δx~ 140 μ per cell, Δp/p ~ 1-2 % 

ΔM/M ~ 2% at ρ mass 

HADES 

2 nd generation dilepton 

spectrometer 

START 

Side View 


FW

Dielectron DATA analysis: analysis 

normalization to π0 (Nπo (N = ½ (N π- + N π+)) )) 

Comb. Background (CB) subtraction 

Mee ee < 150 MeV/c 2 Like-sign Like sign pairs 

Mee ee > 150 MeV/c 2 event mixing 

Signal: ~ 18400 counts 

Me+e- ≥ 150 MeV/c2 : ~ 646 counts 

Efficiency correction 

Measured rates span over 5 

orders of magnitude 

Systematic errors: 

20% efficiency correction 

10% combinatorial background 

15% π 0 normalization 

22% - 27% Total 


HADES and DLS Phase Space Coverage 

mid-rapidity 

HADES 

DLS 

mid-rapidity 

Method for HADES/DLS comparison: 

� HADES yield extrapolated to full phase space. 

� DLS acceptance filter applied 


C+C 1 A GEV : result of HADES / DLS comparison 

HADES/DLS M e+e- spectra HADES/DLS pt spectra 

HADES: G. Agakishiev et al., PLB 663 

(2008) 43 

DLS: J. Carroll – presentation 

International Workshop on Soft Dilepton Production 

August 20-22,1997, LBNL 


DLS data confirmed → dilepton enhancement has to be explained !!

Comparison of HADES Data with Simulated Dilepton Cocktails 

PLUTO event generator I. Froehlich et al., al. arXiv:0708.2382 

Excess in the intermediate mass region 

�Cocktail A: π 0 + η + ω 

= “long-lived” components only 

� π 0 → e + e - γ (Dalitz) , η → e + e - γ (Dalitz) 

� ω→ e + e - (direct) , ω →e + e - π 0 (Dalitz) 

� π 0 /η constrained by TAPS data 

Cocktail B: Cocktail A + Δ + ρ 

= “short-lived” 

“short lived” sources added 

� Δ → e + e - p (Dalitz decay) 

N(Δ) =3/2 N( π 0 ) 

� ρ → e + e - (direct) 


Cocktail A: π0 Excess yield scales like π 

+ η + ω 

= “long-lived” components only 

0 

(related to Δ in cocktail) 

π 

Δ regeneration 

Δ N → 

← 

NN 

Comparison of HADES Data with Pluto Cocktails 

N -1 

Δ 

π 

Δ 

Excess in the intermediate 

Need for transport 

mass region 

models 


N -1

New theoretical calculations !! 

1997: transport models failed to explain DLS data 

2007: DLS and HADES agree and are compatible with new transport model 

calculations 

Calculation: E.L.Bratkovskaya et al. 

HSD Giessen 

Nucl. Phys. A634 (1998) 168 

E. Bratkovskaya and W.Cassing, nucl-th/0712.0635 

1/N 0 dN/dM [1/GeV /c 

π 2 

] 

10 -2 

10 -3 

10 -4 

10 -5 

10 -6 

10 -7 

10 -8 

C+C, 1.0 A GeV 

no medium effects 

HADES 

HADES DLS 

0.0 0.2 0.4 0.6 0.8 

M [GeV/c²] 

HSD: 

π 0 

Dalitz 

η Dalitz 

Δ Dalitz 

ω Dalitz 

ω 

ρ 

Brems. NN 

Brems. πN 

All 

dσ/dM [μb/(GeV c 2 

)] 

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 

M [GeV/c²] 

new treatments of Bremsstrahlung: R. Shyam & U.Mosel, PRC67 (2003) 065202 

L.P. Kaptari, B. Kämpfer, NPA 764 (2006) 338 


10 2 

10 1 

10 0 

10 -1 

10 -2 

10 -3 

C+C, 1.04 A GeV 

no medium effects 



π 0 Dalitz 

η Dalitz 

Δ Dalitz 

ω Dalitz 

ω 

ρ 

Brems. NN 

Brems. πN 

All

Soft photon approximation 

γ* 

γ* 

Full OBE: meson exchange currents 

R. Shyam & U.Mosel, PRC67 (2003) 065202 

+ 

N,Δ 

Kaptari&Kämpfer, NPA 764 (2006) 338 

Gauge invariance → contact terms 

NN Bremsstrahlung: 

Amplitude calculated for real photon 

+ phase space correction 

pn Bremsstrahlung by Kaptari & Kämpfer 

factor 4 larger than SPA 

Same result for pp Bremsstrahlung, Bremsstrahlung, 

but still 

much smaller than pn 

Δ graphs also present 


BUU Barz et al. 

More transport models results .... 

RQMD: E.Santini et al 

UrQMD Bleicher et al. IQMD Thomere et al. 

Δ Dalitz 

pn 

C+C 2 AGeV 

pn Bremsstrahlung and Delta Dalitz decay 

Important issues for understanding 

intermediate mass dilepton yield 

Need for elementary pp, pn reactions 


pp reaction at 1.25 GeV: GeV: 

Δ Dalitz decay 

LH2 target, normalisation to elastic scattering 

(syst.error ~ 30 %) 

preliminary 

E=1.25 GeV < η threshold 

pp Bremsstrahlung is small 

Pluto simulations 

1) pp→pΔ pp + 

σ(pp pp→pΔ + ) = 6 mb 

~ 3/2 σ(pp pp→pπ° ) 

Δ Dalitz decay from QED vertex 

a) G M=3 =3 

b) Iachello GM (q 2 ) 

2) dσ/dM d dM from OBE model (Kaptari ( Kaptari & Kämpfer) K mpfer) 

with assumption on γ* * angular distribution 

a) Delta graphs only 

b) all graphs ( bremstrahlung +FSI) 

Dalitz decay from simple model + Iachello G M (q 2 ) OK 

BR ~ 4.2 10 -5 sensitivity to N-Δ electromagnetic structure ! 

G M (q 2 ) 

Even better 

in exclusive 

e + e - p events 


p 

p 

Δ + 

p 

p 

γ* 

e -- 

e +

Dilepton spectra in OBE : 

a « hot » and controversial subject 

------ Kaptari and Kämpfer 

___ Shyam and Mosel 

Δ from our model: 

pp→pΔ pp→p + ~ 6 mb 

np →nΔ →n + ~ 12 mb 

Δ Dalitz decay : GM =3 

p 

p 

G M (q 2 ) 

Δ + 

p 

p 

γ* 

e -- 

e + 

……..… news from last week ………………. 

arXiv:nucl-th 08110739 Shyam and Mosel 

« Dilepton production in nucleon-nucleon revisited » 

including Δ contribution 

Dilepton spectra in 4π acceptance 

M(GeV/c 2 ) 


dp at 1.25 GeV: GeV: 

pn Bremstrahlung sensitivity 

selection of np reactions: spectator 

proton in Forward Wall ( 0.5°–7°) 

Test of pn bremsstrahlung contribution 

Fermi momentum → η contribution 

normalisation to elastic scattering 

(syst.error ~ 30 %) 

PRELIMINARY 

PRELIMINARY 

HADES np data: a new puzzle ?? 

PLUTO simulations 

1) σ (np np→nΔ+) = 12 mb ~ 3/2 σ(np np→np npπ°) 

η production (COSY/CELSIUS) added 

2) Kaptari and Kämpfer full OBE 

CC scales as ½(pp+pn) !! 

no enhancement ? 

PRELIMINARY 


2001/2 

2004 

2004 

2005 

2006 

2007 

2007 

2008 

HADES experimental program 

C + C 2 AGeV 

C + C 1 AGeV 

p + p 2.2 GeV 

pp elastic, 

exclusive ppe+e- ppe+e (π°,η) 

Ar + KCl 1.75 AGeV 

p + p 1.25 GeV 

exclusive ppe+e- ppe+e (π°,Δ) 

p + p 3.5 GeV 

d + p 1.25 AGeV 

p+Nb 3.5 GeV 

1 PRL (e+e-) + 1 submitted (hadron) 

1 PLB (e+e-) + 1 submitted (hadron) 

Analysis finished (draft) 

Validation of detector performance, 

measures of π° and η Dalitz decays (helicity 

distribution), “free” cocktail 

Analysis ongoing dielectron spectra, medium 

effects 

inclusive analysis finished (draft) 

Δ Dalitz decay first measurement 

ω line shape and cocktail (π, Δ, η, ω, ρ, φ...)! 

Contribution of pn Bremstrahlung / Δ Dalitz decay 

Reference for medium effects 


First peaks at 3.5 GeV 

ω→e + e - decay observed for a first time at SIS/Bevalac energy 

Cocktail normalized to π° (no abs. norm.) 

pp at 3.5 GeV 

Δ 

η 

Mass resolution in the 

ω region < 2,7 % 

Apr 2007 

ρ 

π0 ω 

PRELIMINARY 

φ 

pNb at 3.5 GeV 

Oct 2008 

On-line pNb spectrum 

PRELIMINARY 


Hadronic channels: channels: 

high quality by-products 

by products !! 

M nπ+ 

pp→pnπ + E=1.25 GeV 

Δ production mechanism 

Submitted to EPJA 

C+C 

1 and 2 AGeV 

π ± mt spectra 

Δ ++ M pπ+ 

pn FSI 

pp elastic scattering 

Data normalisation and consistency checks 

First φ mt spectrum at SIS 

Ar+KCl 

1.75 AGeV 


SQM08

Exclusive reconstruction of the π,η → γe + e- Δ1232 

π 

N1440 N1535 Δ1600 

Yields and invariant mass 

agree with resonance model 

Teis et al, ZPA356 (97) 421 

Refinements: 

Δ Decay ang. Distr. (Wicklund 

et al.) 

N(1440) production angle 

( Aichelin et al.) 

ppe + e - detected 

η 

π,η → γ γ* γ → γe + e- Helicity distributions γ* * → e + e- dσ/d /dΩe~ ~ 1+cos 2θ (QED prediction, first measurement ! 


Conclusions and outlook 

HADES is in a very productive period 

C+C data have confirmed DLS data and triggered new theoretical 

calculations which are much closer to data 

E=1.25 GeV : 

pp data as expected from π ° and Δ contributions 

pn data unexplained 

critical test for theoretical inputs 

check contribution of pn Bremsstrahlung and ΔΔ Dalitz decay 

A lot of other promising data (exclusive channels, strangeness, …) 

Focus on « high precision hadronic physics tests » 

�� electromagnetic structure of hadrons 

�� Inclusive production of mesons (off-shell (off shell effects) effects 

Necessary (and interesting !) step before studying medium 

effects….. effects….. 


2001/2 

2004 

2008/9 

2004 

2009-2011 2009 2011 

2011 

2005 

> 2013 

2006 

2007 

2007 

2008 

A program for the future 

C + C 2 AGeV 

C + C 1 AGeV 

p + p 2.2 GeV 

Ni + Ni, Au+Au 

pp elastic, 

π + 

elastic, 

+ N, π + A 

exclusive ppe+e- ppe+e (π°,η) 

Ar + KCl 1.75 AGeV 

p + p 1.25 GeV 

exclusive ppe+e- ppe+e (π°,Δ) 

p + p 3.5 GeV 

d + p 1.25 AGeV 

p+Nb 3.5 GeV 

1 PRL (e+e-) + 1 submitted (hadron) 

1 PLB (e+e-) + 1 submitted (hadron) 

Upgrade RPC, DAQ, MDC1 

Analysis finished (draft) 

Validation of detector performance, 

measures of π° and Planned η Dalitz decays (helicity 

distribution), “free” cocktail 

Analysis ongoing dielectron spectra, medium 

Hades goes 

effects 

goes FAIR ( 8 AGeV) AGeV 

effects 

inclusive analysis finished (draft) 

Δ Dalitz decay first measurement 

2010 Planned 

ω line shape and cocktail (π, Δ, η, ω, ρ, φ...)! 

Contribution of pn Bremstrahlung / Δ Dalitz decay 

Reference for medium effects 


Vielen Dank

Dileptons from Resonance Matter and the DLS Puzzle :

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