Analysis of Dileptons in Ar+KClR

1. Dileptons in Ar+KClR. Holzmann for the e+e- Team Sep05 run: 1.756 GeV/u Ar on KCl target • 2.2 · 109 LVL1 events inspected (10 Tbyte!) • 3 independent lepton analyses finished (hard cut, Bayesian, MVA) Combined e+e- mass spectrum (HC+Bayes+MVA)/3 : • π0 Dalitz • continuum • omega peak

2. MVA Hard Cut e- e+ optimization (by Manuel Lorenz) Optimized MVA Status 2009 After optimization of MVA pid:

3. Omega production gaus + expo bkg fit → ω yield = (3.9±1.6) ·10-8 30 - 40 counts! • Corrected for • BR = 7.16 ·10-5 • acceptance = 0.29 (for T = 85 MeV) • ► MLVL1(ω) = (6.7±2.7) ·10-3 Systematic error: - background ? - medium effects ?

4. Comparison with statistical model THERMUS calculation (T, μB and RC fit to HADES data) ω and  described in statistical model, but…

5. Statistical model fits (at 1.76 GeV/u) • Manuel’s fit to Ar+KCl: T = 75, μB = 790 MeV [to be written and published] • Cleymans parametrization: T = 67, μB = 763 MeV [Phys. Rev. C73 (2006) 034905] • Andronic parametrization: T = 63, μB = 746 MeV [Phys. Lett. B673 (2009) 142] • Averbeck fit to TAPS data: T = 80, μB = 710 MeV [Phys. Rev. C67 (2003) 024903]

6. / ratio: OZI in Ar+KCl vs. NN • Using yields from •  → K+K-: (2.6 ± 0.7) ·10-4 •  → e+e- : (6.7 ± 2.7) ·10-3 • ► • >> R/ in NN and πN reactions ! • Impact of other channels besides NN and πN ? • (e.g. ρN, ρΔ, …) >> OZI allowed

7.  production in BUU Schade, Wolf & Kämpfer, in Phys. Rev. C 81 (2010) 034902 exotic ch. standard ch.

8. retuned HSD (Nov09 release) • M/Mπ0 = 0.0019 (exp = 0.0020) • for M>0.65 GeV: ω+ρ = 70%

9. Continuum pairs in Ar+KCl • How to characterize the continuum? • Look at many observables! • yield: dN/dm • transverse mass: dN/dmt • rapidity: dN/dy • polar angles: dN/dθcm • helicity: dN/dα • Look at their systematics vs. Ebeam and Apart See my Sesimbra talk 2009

10. “Excess” vs. beam energy and system size Yield/Apart • baryonic contrib. in Ar+KCl >> C+C • scales with Ebeam like π production • scales with Apart stronger than linear • ≈ <Apart>1.4 Compare A+A with N+N, but properly normalized. π0 and η from TAPS (min. bias) e+e-continuum pairs: HADES (LVL1) DLS (min. bias)

11. Preparing a “reference” for Ar+KCl Definition of a ”reference” based on pp and np data: Compare excess over η in Ar+KCl with excess over η in reference x2.5 - 3 • η contributions subtracted ! • yield normalized to M(π0) ►► Excess over NN!

12. Characterizing the excess: dn/dmt A reminder: dn/dmt of a thermal source given by ► Use 1/mt3/2 dn/dmt to extract slope parameters

13. Definition of mass slices mid high low mostly 3-body ! mostly 2-body ?

14. A+A Transverse-mass spectra: Mee<0.15 GeV(efficiency and acceptance corrected) A+A vs. 1.25 GeV N+N ► Low-mass bin is well described by π0 Dalitz ► 1.25 GeV/u NN reference

15. A+A Transverse mass: 0.15<Mee<0.50 GeV A+A vs. 1.25 GeV N+N ► Mid-mass bin: T of baryonic component changes only weakly with Ebeam ! ► 1.25 GeV/u NN reference … ?

16. Transverse mass: Mee>0.50 GeV ► Large slopes: Tee >> Tchem = 75 MeV

17. 3-body (!) 2-body (?) decays More mass cuts in ArKCl… Mee = 0.35 – 0.50 0.50 – 0.65 >0.65 GeV ► Indeed dominant, according to transport

18. Comparison with in-medium HSD HSD vs. Ar+KCl at 1.76 GeV/u • low-mass bin is well described, except for mt >800 MeV • at mid-mass, slope is fine, but no selectivity • high-mass slope reproduced by neither ρ/ω, nor Δ !

19. Systematics of slopes at 1.76 GeV/u π0 and η from TAPS Why are slopes of ρ/ω and  so different ?

20. Summary • ω yield in Ar+KCl: • reproduced by statistical models and (updated) transport • R/ >> R/(NN) >> OZI allowed • dN/dmt vs. Mee: • low-mass region is well understood • mid-mass not very selective → origin of excess remains unclear • high-mass has surprisingly large T>>Tchem ► continue our systematics towards large A+A

21. Thanks to the (still active) e+e- team: • Filip Krizek • Martin Jurkovic • Manuel Lorenz • Tatyana Galatyuk • Gosia Sudol … and let’s get those Au+Au data in!

22. Some more details…

23. dn/dmt in an isotropic thermal source Relativistic Boltzmann distribution: ► Integrating over θ and ɸ this gives: ► Integrating over pz: ► Integrating over mt:

24. mt scaling vs. statistical models ► Integrating dn/dmt over mt gives full yield: ► mt scaling: ► equivalent togrand-canonical statistics in the classical limit (see e.g. Hagedorn 1965)

25. 100 40 0.15<M<0.50 η Pair dN/dmtfrom thermal sources in Pluto(T = 40 –100 MeV) ω π0 Δ ρ M<0.15 ρ M>0.50

26. Pluto mt acceptances 0.15<M<0.50 mt M<0.15 M>0.50

27. Effect of Δ contribution on acceptance Compare acceptances for different cocktail compositions (M>0.50 GeV): T=125

28. Cleaning up high-momentum tracks(by lowering the pe cut) Pair mt acceptances: ► This lowers the acceptance at high mt considerably

29. Effect of p cut on slopes pe< 1.3 GeV/c pe< 0.9 GeV/c ► small reduction of slopes only

30. y = y0 ± 0.1 all y Paradise Lost or Stranger Than Paradise ? Vector mesons dominate in acc. Δ dominates in acc. ► slopes decrease, still remain high

31. Comparison of dσ/dmt and dσ/dEcm dσ/dE makes use of longitudinal component as well, but need to go to AA cm !

32. Ar+KCl vs. C+C excess y = y0 all y ► After subtraction of first-chance part, T2 < T1

33. Attempt to reproduce the HSD Δ tail

34. y = yo 1/Mt2 dN2/dMtdy π0 η Mt [GeV/c2] Vector meson mt scaling beyond π0 and η • Mid-rapidity π0 and η data • from TAPS: • 1.5 GeV/u Ar+Ca • 2.0 GeV/u Ca+Ca • HADES dileptons with Mee>0.5: • 1.756 GeV/u Ar+KCl • y = yo± 0.1 (0.76 – 0.96) • <BRee> = 6 ∙10-5 Consistency with M(ω) ?

35. CM helicity distributions Compatible with pseudoscalar π0 Large B’ points to polarized excess! Statistics!!!

36. CM polar distributions M<0.15 anisotropy consistent with π± distributions Large 0.15<M<0.50 anisotropy requires excess to have A2 > 0! M>0.50 ??? Statistics!!!