Dilepton production in p+p collisions at 3.5 GeV experimental results and their interpretation Anar Rustamov for the HADES collaboration GSI Helmholtzzentrum für Schwerionenforschung

Outline A. Rustamov, GSI, Darmstadt, Germany, Apr 18, HADES spectrometer Dilepton mass spectra Baryon Dalitz Decays Comparison to models PYTHIA UrQMD summary

HADES spectrometer 3 A. Rustamov, GSI, Darmstadt, Germany, Apr 18, 2011  Acceptance  φ ~ 2 π  15 o < θ < 85 o  pair ~ 30%  Momentum resolution  Magnet: Tm  MDC: 24 drift chambers  σ m ~ 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

Invariant mass spectra Combinatorial background (CB) CB = N ++ + N - - uncorrelated correlated Not efficiency corrected. Inside HADES acceptance  Close partner cut  Momentum cut 80 < P [MeV/c] < 2000  Track fitting quality cut A. Rustamov, GSI, Darmstadt, Germany, Apr 18,

Conversion to cross section units Elastic scattering Kinematic constraints: Kammerud et al. Phys. Rev. D 4 (1971), 5 A. Rustamov, GSI, Darmstadt, Germany, Apr 18,

Invariant mass spectra Signal CB high S/CB ratio Number of pairs:  All : ~ 6.1*10 4  M <0.15 : ~ 5.5*10 4  0.6 < M < 0.82 : ~ 450 σ ω ~ 16 MeV efficiency corrected. Inside HADES acceptance. normalized to the p+p elastic scattering A. Rustamov, GSI, Darmstadt, Germany, Apr 18,

Interpretation of the spectra decay part (calculated within QED): V         R N A. Rustamov, GSI, Darmstadt, Germany, Apr 18, x3 exclusive cocktail with inclusive cross sections! production part: unknown inclusive production cross sections not known production mechanisms use different models

Baryon contribution Contribution from Baryon Dalitz decays for N*(1535), N*(1440), N*(1520) resonances from S. Teis et al. Z. Phys. A 356, (1997). from M. Zetenyi and Gy. Wolf, Heavy Ion Phys.17:27-39,2003 A. Rustamov, GSI, Darmstadt, Germany, Apr 18,

Δ width parametrization Manley cutoff: Phys. Rev. D 45, 11, 1992 (used in this report) Moniz cutoff: Ann. of Phys. 154, 99, 1984 A. Rustamov, GSI, Darmstadt, Germany, Apr 18,

N-Delta transition vertex e-e- Δ ** e+e+ 3 different form factors (analogous to Sachs form factors for the nucleon) at the photon point (q 2 = 0) G M (0) = 3 G E (0) ≈ 0 C C (0) – not defined (current conservation) (time like photon)  no data available for the time like form factors  the branching ratio is not measured M. Krivoruchenko et al. Phys.Rev.D 65, , M. Zetenyi and Gy. Wolf, Heavy Ion Phys.17:27-39,2003 A. Rustamov, GSI, Darmstadt, Germany, Apr 18,

Two–component VDM type model alternatively one could use Extended Vector Dominance Model  only ground states (ρ, ω, ϕ ) vector mesons are used  only ρ (isovector) is relevant in case of Δ  reproduces simultaneously nucleon space-like and time-like as well as N-Δ space like form factors. analytic continuation space like time like Q. Wan and F. Iachello, Int. J. Mode. Phys. A20, 1846 (2005) M. I. Krivoruchenko et. al, Ann. of Phys. 296, 299 (2002) A. Rustamov, GSI, Darmstadt, Germany, Apr 18,

VDM form factor  Note: We use modified ρ propagator, taking into account its width (shifts down the peak position in the form factor)!  How important is the intrinsic part at our energies?  In our q 2 range the VDM part dominates However, correct asymptotic behavior with complete picture A. Rustamov, GSI, Darmstadt, Germany, Apr 18,

Particle Production, UrQMD, PYTHIA N *, Δ N N N N m A. Rustamov, GSI, Darmstadt, Germany, Apr 18, UrQMD PYTHIA BW mass dependent BW fixed mass PYTHIA Changes I introduced Mass dependant total width ! Δ - from its π+N decay channel ρ – from its ππ decay channel + additional tunes (GiBUU tunes for HADES) to suppress mainly vector meson production.

14 A. Rustamov, GSI, Darmstadt, Germany, Apr 18, 2011 Data vs. models

Mass Spectra A. Rustamov, GSI, Darmstadt, Germany, Apr 18, UrQMD: K. Schmidt, et al. Phys. Rev. C 79, (2009 ) PYTHIA (production) + PLUTO (decay part) PLUTO: I. Fröhlich et al, arxiv: Fitted to π o

Mass bin selections A. Rustamov, GSI, Darmstadt, Germany, Apr 18,

Pt spectra A. Rustamov, GSI, Darmstadt, Germany, Apr 18, PYTHIA+PLUTOUrQMD

Rapidity spectra A. Rustamov, GSI, Darmstadt, Germany, Apr 18, PYTHIA+PLUTOUrQMD

Angular distributions A. Rustamov, GSI, Darmstadt, Germany, Apr 18, PYTHIA+PLUTOUrQMD

Modeling N - Δ transition form factor A. Rustamov, GSI, Darmstadt, Germany, Apr 18,

Mass spectra (PYTHIA + PLUTO) A. Rustamov, GSI, Darmstadt, Germany, Apr 18, constant Δ FFVDM Δ FF

Pt spectra (PYTHIA + PLUTO) A. Rustamov, GSI, Darmstadt, Germany, Apr 18, constant Δ FFVDM Δ FF

Rapidity spectra (PYTHIA + PLUTO) A. Rustamov, GSI, Darmstadt, Germany, Apr 18, constant Δ FFVDM Δ FF

Angular distributions A. Rustamov, GSI, Darmstadt, Germany, Apr 18, constant Δ FFVDM Δ FF

Extracting cross sections σ ω ~ 16 MeV particle production π multi resonance η, ω, ρ via phase space Δ through 1 π exchange mass dep. Width Fröhlich et al, arxiv: Δ form-factor is fixed at the photon point cross sections in 4π [mb] π: 16 ± 2.6 (from data) Δ: 7.5 fixed from PYTHIA η: 0.93 ± 0.2 (fit to data) ω: 0.25 ± 0.05 (fit to data) ρ: 0.38 ± 0.07 (fit to data) The PDG 2010 value for η  e + e - BR has to be scaled down by a factor of at least 3. normalized to σ elastic A. Rustamov, GSI, Darmstadt, Germany, Apr 18,

Cross sections from PYTHIA cross sections in 4π [mb] π: 18 ± 2.7 (16) Δ: 7.5 η: 1.14 ± 0.2 (0.93) ω: ± 0.04 (0.25) ρ: ± 0.04 (0.38) A. Rustamov, GSI, Darmstadt, Germany, Apr 18,

Summary  For the first time at this energy the inclusive production cross sections for π o, η, ρ and ω mesons have been obtained  Δ resonance has a strong impact on dilepton spectra  Time-like electromagnetic transition form-factors are necessary for its differential decay rate calculation  Data exhibits a clear structure below the ρ meson pole mass  This structure is satisfactorily described by using the form factor model for the N-Delta transition vertex  Sensitivity of data to the η  e + e - branching ratio is observed  Comparison of data to different production mechanisms is performed A. Rustamov, GSI, Darmstadt, Germany, Apr 18,

The HADES collaboration Cyprus: Department of Physics, University of Cyprus Czech Republic: Nuclear Physics Institute, Academy of Sciences of Czech Republic France: IPN (UMR 8608), Université Paris Sud Germany: GSI, Darmstadt FZ Dresden-Rossendorf IKF, Goethe-Universität Frankfurt II.PI, Justus Liebig Universität Giessen PD E12, Technische Universität München Italy: Istituto Nazionale di Fisica Nucleare, Laboratori Nazionali del Sud Istituto Nazionale di Fisica Nucleare, Sezione di Milano Poland: Smoluchowski Institute of Physics, Jagiellonian University of Cracow Portugal: LIP-Laboratório de Instrumentação e Física Experimental de Partículas Russia: INR, Russian Academy of Science Joint Institute of Nuclear Research ITEP Spain: Departamento de Física de Partículas, University of Santiago de Compostela Instituto de Física Corpuscular, Universidad de Valencia-CSIC 17 institutions 120+ members A. Rustamov, GSI, Darmstadt, Germany, Apr 18,

PYTHIA tunes PARJ(11) 0.15 (def: 0.5) prob. for light meson to have spin 1 PARJ(21) 0.25 (def: 0.36) width of the Gaussian for Pt of primary hadrons PARJ(25) 0.63 (def : 1.) extra suppression for the η production PARP(91) (def: 2.) Primordial kt distribution in hadron (width of Gaussian) 29 A. Rustamov, GSI, Darmstadt, Germany, Apr 18, 2011

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Sensitivity to angular distributions Δ through 1 π exchange phase space production A. Rustamov, GSI, Darmstadt, Germany, Apr 18,