Wolfgang Kühn, Universität Giessen Recent Results from HADES Physics Motivation Low mass lepton...

Wolfgang Kühn, Universität Giessen

Recent Results from HADES

Physics Motivation Low mass lepton pair enhancements

SPS and DLS data The HADES detector at GSI Darmstadt Experimental Results

12C+12C and the DLS Puzzle p+p, p+d Ar+KCl

Summary and Outlook


Electromagnetic Probes – Penetrating Probes

Experiments at different s probe different regions of phase diagram

RHIC ee pairs and sQGP

SPS ee pairs transition

region

SIS ee pairs dense

hadronic matter HADES

Color Superconductor

Quark Matter

Hadron Resonance Gas

Nuclear Matter

SIS

DLS, HADES

CERES, NA60

PHENIX, STAR

B

T

TC~170 MeV

940 MeV 1200-1700 MeVbaryon chemical potential

tem

per

atu

re

thermal freeze out

chemical freeze out


CERES: Low-mass dilepton enhancement

Central N-N collisions exhibit a strong enhancement of low-mass dilepton production as compared to p-N reactions (CERES, HELIOS, NA60)

A large fraction of the excess is probably due to pion annihilation, which does not contribute in p-N reactions

There is no quantitative description of the data using vacuum properties of vector mesons

Possible explanation: in-medium modifications of vector meson properties

Broadening and/or mass shift of the rho meson

Nucl-ex/0203004

Nucl-ex/0203004Nucl-ex/0203004


The DLS@BEVALAC - Puzzle

DLS Experiment observed large

excess of di-electrons when

compared to dominant known

sources

Pion, eta Dalitz decays

Effect much stronger than at SPS

energies

Visible also in light ion induced

reactions

For many years, no conclusive

theoretical explanation available

Experimental problem?

New physics?

Not well understood old physics?


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

Pair acceptance 0.35

Particle identification:

RICH: CsI solid photo cathode, C4F10 radiator, Time Of Flight: Scintillator paddles MUL limitation <450, RPC from 2009Pre-Shower: pad chambers & lead converter2' Level single leg electron trigger (Me>=1)

Momentum measurementMagnet: B = 0.36 Tm + MDC: 24 Midi Drift Chambers, single-cell resolution 140 m

Side view


Hadron Identification• Momentum vs velocity (Momentum vs velocity (ββ) + dE/dx in MDC & TOF: ) + dE/dx in MDC & TOF:

• example: Ar + KCl @ 1.75 A GeVTOF: >450

• particle mass spectra with dE/dx (TOF & MDC) selection

kaon identifcation


Electron Identification Spatial correlations

RICH rings ↔tracks PID : e+, e-

β vs momentum correlation

PreShower condition

DATA

C + C @ 2AGeV

• Hadron contamination < 5% (p>1000 MeV)

e-

e+

velocity vs. momentum

with RICH condition


pp0ppe+e-

ppppe+e-

Nexp= 2726 ± 53,SIM=2677

Nexp= 195 ± 19, SIM=196

p+p @ 2.2 GeV : Exclusive 0/ reconstruction

Exclusive reactions Clean and

consistent reconstruction of hadronic and Dalitz decays for 0,

Cross check for detector simulations

totalsignalbackground M 548 MeV/c2

/M 3.5 %

pppp pp+-°

HADES70±8 b


LOW mass excess : "DLS" puzzle"


Inclusive e+e- pairs from C+C collisions

• Normalization N(0)= ½(N+ + N-)

• Cocktail A: 0 + η + ω long-lived component (0/ constraint by TAPS data)

• Cocktail B: Cocktail A + Δ(Ne+e-) + ρ short lived component

A. Agakichiev Phys.Rev. Lett 98(2007) 052302 A. Agakichiev accepted by Phys. Lett. B


Comparison of DLS and HADES Acceptance

mid-rapidity mid-rapidity


Comparison to DLS C+C @1.04 GeV

DLS Data: R.J. Porter et al.:

Phys.Rev.Lett. 79 (1997) 1229

J. Carroll – presentationInternational Workshop on Soft Dilepton ProductionAugust 20-22,1997, LBNL

Very good agreement between both experiments

Full confirmation of DLS effect


Beam energy dependence of the excess

TAPSTAPS

Photon dataR. Averbeck et al., TAPS Col., Z.Phys. A 359 (1997) 65R. Holzmann et al., TAPS Col., Phys.Rev. C 56 (1997) R2920

Enhancement scales like pions !Enhancement scales like pions !

• Baryon resonance decays ? Baryon resonance decays ?

• N-N bremsstrahlung ?N-N bremsstrahlung ?


Important contributions from Δ Dalitz decay and Bremsstrahlung

Cruical input: Elementary reactions

pp, pn

Comparison with Up-to-Date Transport Calculations

IQMD: M. Thomère et al.

Phys.Rev.C75 064902 (2007)HSD: E.L. Bratkovskaya and W. Cassing arXiv:0712.0635v1 and private communication

New treatment of Bremsstrahlung: L.P. Kaptari and B. Kämpfer, Nucl.Phys. A 764 (2006) 338


preliminary

Online spectrum (data from May 2007 )

very preliminary

PhD thesis T.Galatyuk

pp and pn 1.25 GeV

Not efficiency corrected Not corrected for trigger bias Analysis in progress

New precise input of elementarycross sections for model

calculations


Vector meson production


HADES pairs from pp @ 3.5 GeV

Preliminary (not efficiency corrected, no normalization)Preliminary (not efficiency corrected, no normalization)

preliminary

online calibration

~70% statistics

e+e- signal (tot~20 nb! ) ~35 MeV/c2

• Reference for p+A (2008)


Signal Pairs from Ar+KCl

PreliminaryPreliminary• No efficiency correction

Ne+e- = 114.000 pairs (~7k for M>M

0 )

• e+e- decay seen for the first time at SIS/Bevelac energies !

Me+e- > 0.4 GeV/c2


Summary and Outlook

Low mass enhancement : DLS pair excess has been confirmed by the HADES CC data :

No puzzle any more (at least not from the experimental point of view)

Combined CC, pp and pn data @1.25 GeV indicate that most of the excess originates from p-n reactions

Strong isospin dependence of pair production New HADES data on elementary reactions provide important input for

the understanding of HI data Implications for low mass lepton continuum in a baryon rich

environment Importance of resonance decays Correct description of Bremsstrahlung

Interference between these processes ???

Vector meson production e+e- decay observed for a first time at SIS/Bevalac energies

pp @ 3.5 GeV and Ar+KCl @ 1.75 AGeV


HADES runs: Outlook & plans for next years

2008 2009 2010 2011 2012

p+A @ 3.5 GeV

upgrade: RPC, DAQ

Ni+Ni

+N,A

Au+Au

SIS

SIS

Subject to Approval

8 AGeV

FAIR


The collaborationSISSIS 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.), LIP, Portugal

Wolfgang Kühn, Universität Giessen Recent Results from HADES Physics Motivation Low mass lepton...

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