Dielectron Spectroscopy at 1-2 AGeV with HADES Stefano Spataro for the collaboration

Probes for nuclear matter Physics Motivations Heavy-ion collisions at relativistic energies (12 AGeV) p n ++  K  T0, B0 r e+ e- Hadron properties inside the nuclear matter M,  Light vector mesons Probes for nuclear matter 2.9·10-4 K+K- 44 1019  7.1·10-5  +- 0 23 783  4.5·10-5   1.3 769  Branching ratio e+e- Main decay Mean life [fm/c] Mass [MeV/c2] Mesons LEPTONS NO strong final interaction Very low probability

Physics Motivations CERES/NA45 DLS Models failed to explain data DLS Ca+Ca 1AGeV full spectral function R.Rapp et al., Nucl. Phys.. A617 (1997) 472 DLS C+C 1AGeV Models failed to explain data 0.15 < M < 0.4 GeV/c2 Explained by medium effects

The HADES Spectrometer Physics program @ GSI p+p, p+A, +p, +A, A+A @ 18 AGeV Geometry Six sectors on a hexagonal geometry: 2 (azimuthal angle) 18° <  < 85° (polar angle) Side view Tracking Superconducting Magnet (Bmax=0.7T) 24 multi-wire drift chambers sy  100 mm, Me+e/M1.5% at  mass Particle identification RICH: Cherenkov detector TOF (>45°): tof ~ 150ps TOFINO (<45°): tof ~ 450ps Pre-Shower: 18 pad chambers Online dilepton trigger

HADES Experimental Runs Heavy-Ion collisions in-medium effects of vector mesons at moderate baryon densities Commissioning and physics run Target: 2 x 2.5% interaction length Low resolution (M~9% @  mass) 650 Mevents C+C@2 AGeV November 2002 Published: PRL 98 (2007) 052302 C+C@1 AGeV August 2004 Target: 3 x 2% interaction length 2500 Mevents ~ DLS energy analysis almost completed Ar+KCl@1.75 AGeV September 2005 Target: 4 x 1.5% interaction length 1200 Mevents ongoing analysis

C+C @ 2 AGeV PRL 98 (2007) 052302 Event generator PLUTO : Normalization: ½(+ + -) yield M < 150 MeV/c2: ~23000 counts M ≥150 MeV/c2: ~ 2000 counts (Mee) ~ 9% @ Mee~0.8 GeV/c2 Event generator PLUTO : thermal source (T=80MeV)  polar angle distribution from charged  analysis η (TAPS data) ρ, ω: m -scaling Δ scales with  Cocktail A: ° + η + ω (long lived) systematic errors: 15 % - efficiency correction 10 % - combinatorial background 11 % - 0 normalization 18% 21% Cocktail B: A + Δ(Ne+e-) + ρ (short lived)

C+C @ 1 AGeV Experimental data Event generator PLUTO : Normalization: ½(+ + -) yield M < 150 MeV/c2: ~17780 counts M ≥150 MeV/c2: ~ 646 counts (Mee) ~ 9% @ Mee~0.8 GeV/c2 Event generator PLUTO : thermal source (T=55MeV)  polar angle distribution from charged  analysis η (TAPS data) ρ, ω: m -scaling Δ scales with  PRELIMINARY systematic errors: 10 % - efficiency correction 10 % - combinatorial background 11 % - 0 normalization 22–25 % Cocktail A: ° + η + ω (long lived) Cocktail B: A + Δ(Ne+e-) + ρ (short lived)

+ Comparison 1 and 2 AGeV Yield above cocktail A (long lived) PRELIMINARY Agreement below 150 MeV/c2 Large relative excess at higher masses (same shape) Neutral Meson production (TAPS) + Pair excess in HADES/DLS Excess (beyond ) scales like °

Ar+KCl @ 1.75 AGeV Lepton analysis: ongoing PRELIMINARY Lepton analysis: ongoing Hadronic analysis: strangeness 30% total statistics PID: also dE/dx in MDC Background: mixed event (Ks0,) PRELIMINARY PRELIMINARY

HADES Experimental Runs Elementary collisions exclusive study of dilepton sources p+p@2.2GeV January 2004 Target: 5cm LH2 600 Mevents analysis almost completed  production pp  pp Dielectron efficiency p+p@1.25GeV April 2006 3000 Mevents analysis ongoing form factor and BR unmeasured + Dalitz decay pp  p+  ppe+e- p+p@3.5GeV d+p@1.25GeV April 2007 analysis just started Vector mesons direct decay pp  pp(0) ppe+e- pn brehmsstrahlung

p+p @ 2.2 GeV Verify dielectron efficiency Target: LH2 (5 cm length) Beam: 107 p/second Verify dielectron efficiency Hadron and dielectron channel for  EXP: 15.3 ± 1.8 (stat) SIM: 15.6 ± 0.9 (stat) total signal background M 548 MeV/c2 M 549 MeV/c2 /M 2.5 % Hadronic channel  ° bg M 548 MeV/c2 M 546 MeV/c2 /M 3.3 % M° 135 MeV/c2 M 140 MeV/c2 /M 18 % Electromagnetic channel  yield 2322  118 24845  348 ° yield 7580  454 pppppp+-° ppppppe+e- 

p+p @ 2.2 GeV:  Electromagnetic Form Factor QED +- G.Landsberg, Phys.Rep.128 (1985) 301  form factor known check of invariant mass spectrum preliminary ppppppe+e-   form factor: e+e- (~100 counts) Phys. Lett. B 59 (1975) 103-105 Phys. Lett. B 504 (2001) 275 More statistics expected with 3.5 GeV

p+p / n+p @ 1.25 GeV Evidence for additional sources  Form factor ? PRELIMINARY n + p, 1.25 GeV e+e- > 90 ONLINE: ~50% available statistics Excess above cocktail ~2 (pp), 4-5 (np) Event Generator PLUTO: °e+e-, +pe+e- + fixed to ° via N(+)=3/2 N(°) Evidence for additional sources  Form factor ?

preliminary online spectrum p+p @ 3.5 GeV (april 2007) preliminary online spectrum Online spectrum ~70% statistics  peak visible (~35 MeV/c2) excess below  pole mass  line shape Very important reference for p+A (next year)

Summary and Outlook Runs p, d, p - induced A + A Status/Comment 2002 C + C 2 AGeV Published! 2004 C + C 1 AGeV analysis finished p + p 2.2 GeV 2005 Ar + KCl 1,75 AGeV analysis ongoing 2006 p + p 1.25 GeV 2007 p + p 3.5 GeV d + p 1.25 AGeV … online results 2008 p + A 3.5 GeV 2008/9 Upgrade RPC, DAQ 2009 Ni + Ni Planned 2010  + N, A 2011 Au + Au > 2011 Hades goes FAIR (8 AGeV)

Backup slides

The HADES Spectrometer Averaged over 0 < Y < 2 Opening angle > 9° PAIR ACCEPTANCE

Electron identification Track matching Pre-Shower condition RICH pattern e- + + e- e+ velocity vs. momentum Data Monte Carlo e- e+ Momentum * charge [MeV/C]

C+C @ 2 AGeV Combinatorial background: Same event like-sign pairs CB = Mixed event opposite sign pairs

C+C @ 2 AGeV: comparison with models RQMD: M. D. Cozma, C. Fuchs, E. Santini, A. Faessler, Phys. Lett. B 640, 150 (2006) UrQMD: D. Schumacher, S. Vogel, M. Bleicher, nucl-th/06080401. HSD (v2.5): W. Cassing and E. L. Bratkovskaya, Phys. Rep. 308, 65 (1999). “free” spectral function Closer to data than cocktail B But model calculations Undershoot between 200<Mee<500 MeV/c2 Overshoot for Mee> 700 MeV/c2

Hadron analysis (2 AGeV) Anisotropy PRELIMINARY PRELIMINARY π+ π- 1 AGeV **) .137 ± .015 .069 ± .008 N<- +>/Apart 2 AGeV *) Used for the normalization of dilepton spectra, extrapolated to 4 *) 11% syst, error, in agreement with KaoS, TAPS **) preliminary

collisional broadening of ω ( Г(ω) = 200 MeV) Calculation: K. Shekhter, C. Fuchs et al. (Tübingen) Phys. Rev. C68 (2003) 014904 collisional broadening of ω ( Г(ω) = 200 MeV) extended VDM + decoherence BR scaling of VM

HADES/DLS comparison PRELIMINARY PRELIMINARY J. Carroll – International Workshop on Soft Dilepton Production August 20-22,1997, LBNL DLS Data: R.J. Porter et al.: PRL 79(97)1229

P┴ Y Mee < 150 MeV/c2 150 < Mee< 550 MeV/c2 • Data - Simulation