Dilepton production with HADES at 1-3.5 GeV kinetic energy Anar Rustamov GSI Helmholtzzentrum für Schwerionenforschung

Outline dilepton production main physics goal experimental approach HADES spectrometer pair spectra cross sections comparison with models PLUTO HSD UrQMD pA data AA data summary

Dilepton spectra Sources of dileptons: Drell-Yan process charmonimum A. Drees Sources of dileptons: Drell-Yan process charmonimum QGP dileptons resonances particles from freeze-out stage low mass part: study of Chiral symmetry restoration

Physics motivation r + ... Hadrons in medium Chiral symmetry: SU(2)L x SU(2) R r N-1 N*(1520) + ... Spontaneously broken in vacuum by Partial Restoration in medium (melting of ) QCD sum rules QCD part: dispersion relation T. Hatsuda, S.H. Lee, Phys. Rev. C 46 (1992) R34 Probing the matter with vector mesons

Experimental approach Mass distribution of the hadrons inside medium: p1, p2: 4-momenta Hadrons should be short lived (ρ, ω, Φ) p1, p2 should be detected undistorted (leptons) ρ(m) - spectral function, Г - width Experimental challenge: Mass [MeV] ct [fm] Г/Гtot->ee r 770 1.3 4.7×10-5 w 782 23.4 7.07×10-5 F 1020 44.4 2.97×10-4 small branching ratios high background

HADES spectrometer Acceptance Momentum resolution φ ~ 2 π 15o < θ < 85o pair ~ 30% Momentum resolution Magnet: 0.1-0.34 Tm MDC: 24 drift chambers σm~ 2% at ρ/ω region Particle identification RICH Time of flight Pre-Shower MDC (for hadrons) Trigger LVL1- charged particle mult. LVL2- single electron trigger

Pair spectra for pp at 3.5 GeV Not efficiency corrected. Inside HADES acceptance Close partner cut Momentum cut 80 < P [MeV/c] < 2000 Track fitting quality cut θpair > 90 Combinatorial background (CB) uncorrelated correlated CB = N++ + N- -

Absolute normalization Elastic scattering Kinematic constraints: Kammerud et al. Phys. Rev. D 4 (1971), 5

Pair spectra Efficiency corrected. Inside HADES acceptance Signal σω ~ 16 MeV/c2 CB high S/CB ratio preliminary Number of pairs: All : ~ 6.1*104 M <0.15 : ~ 5.5*104 0.6 < M < 0.82 : ~ 451 θpair > 90

Cross sections Inclusive cross sections from pair spectra σπo = 15 mb ± 4mb from Consistent with Resonance model S. Teis et al. Z. Phys. A 356, 421-435 (1997) pp → π + X pp → π - X pp → π o X (HADES) lines are from HSD preliminary H. Weber et al., PRC 67 (2003) 014904 and references therein

Resonance model ~ 13 mb Δ (1232) sum over N* states higher 3/2 states S. Teis et al. Z. Phys. A 356, 421-435 (1997) and refernces therein

Cocktail simulation with PLUTO particle production phase space ( but 1 π exchange for the Δ ) Fröhlich et al, arxiv:0708.2382 particle decays form factors mass dep. Width cross sections [mb] πo : 15 Δ : from isospin coeff. η : 1.04 ω : 0.28 ρ : 0.36 preliminary

ρ/ω cross sections Cross sections were obtained from simulated cocktail by changing the ρ/ω ratio until simulation fits the data W. Cassing, E. Bratkovskaya, Physics Reports 308 (1999) 65 -233

Comparison with HSD preliminary preliminary

Comparison with UrQMD preliminary preliminary

Cocktail subtracted data πo, η and ω contributions are subtracted Particle production HSD Lund String Model UrQMD Through Resonances W. Cassing, E. Bratkovskaya , Physics Reports 308 (1999) 65 -233 preliminary K. Schmidt, et al. Phys. Rev. C 79, 064908 (2009) N*, Δ N m Missing higher resonances in PLUTO Overestimation of ρ in UrQMD HSD from new calculations

pNb data at the same kinetic energy of protons ( 3.5 GeV ) Signal Online spectra: preliminary calibrations preliminary alignment pNb data pair momenta in ρ/ω region shape of ρ/ω as a function of their momenta can be studied

AA collisions normalization Enhancement factor ( 0.15 < M < 0.5) F(2.0) = 1.9 ± 0.2(stat) ± 0.3(sys) ± 0.3( sys) F(1.0) = 6.8 ± 0.6(stat) ± 1.3(sys) ± 2.0( sys) PRL 98, 052302 (2008) PLB 663 (2008) 43-48

Excess yield DLS data HADES data π0 and η mutlt. from TAPS data Excess scales with: Ebeam: like π production Apart : more than linear

Excess yield Facto -> factor Remove one “of” thesis: T. Galatyuk, K. Lapidus thesis: T. F. Krizek, S. Lang, M. Jurkovic …. C+C data reproduced (within 20%) by superposition of NN interactions Ar+KCl data overshoots the NN data by a factor of ~2.5; 20 20

Summary I Meson production is investigated in pp reactions at Tkin = 3.5 GeV For the first time inclusive cross sections for the π0, η, ω and ρ mesons are reconstructed Comparison of the data to transport models is shown The investigation of production mechanisms of the vector mesons are ongoing Reference spectra for the pNb run at the same beam kinetic energy is obtained

Summary II Excess observed in DLS is confirmed by HADS experimentally The observed excess scales with energy like pion production and more than linear with Apart The excess in light system ( C+C ) is reproduced by superposition of NN interactions The observed enhancement in DLS data already exists in elementary reactions, and is not connected with any kind of medium effects

Comparison with UrQMD PT distributions for different mass bins preliminary

Pair spectra Particle identification: RICH-MDC matching Alle+e- = Se+e- + CB Particle identification: RICH-MDC matching Time of flight cuts Shower cuts Background rejection: close track cut opening angle cut track fitting quality cut Alle+e- = Se+e- + CB θpair > 90 preliminary Combinatorial background (CB) CB = N++ + N- - uncorrelated correlated

AA data  HADES and DLS data agree ! HADES data in DLS acceptance … DLS Data: R.J. Porter et al.: PRL 79 (1997) 1229  HADES and DLS data agree !

Comparison with the models Normalized to π peak preliminary preliminary W. Cassing, E. Bratkovskaya, Physics Reports 308 (1999) 65 -233 K. Schmidt, et al. Phys. Rev. C 79, 064908 (2009)