NSTAR2011, Jefferson Lab, USA May 17-20, 2011 Mitglied der Helmholtz-Gemeinschaft Tamer Tolba for the WASA-at-COSY collaboration Institut für Kernphysik Forschungszentrum Jülich Double-Pion Production in Proton-Proton Interactions at T p = 1.4 GeV

NSTAR2011, JLab USA, May 18, 2011Slide 2Tamer Tolba, IKP, FZ Jülich Outline - Physics Motivation - Previous Work - WASA Detector at COSY - Data Analysis - Experimental Results * Total Cross Section * Differential Cross Sections - Summary and Outlook

NSTAR2011, JLab USA, May 18, 2011Slide 3Tamer Tolba, IKP, FZ Jülich Double pion production in pp collisions is dominated by resonance production. Relative strengths of the production mechanisms strongly momentum dependent. pp → ppππ high energiesnear threshold pp→pN*(1440) p π π ∆ (1232) π p π pp→∆ ∆(1232) p π * L. Alvarez-Ruso, E. Oset, and E. Hernández, Nucl. Phys. A633, 519 (1998). (Valencia model) Double pion production in pp collision

NSTAR2011, JLab USA, May 18, 2011Slide 4Tamer Tolba, IKP, FZ Jülich Goal: Study 2π 0 σ total and production mechanism at T p =1.4 GeV (√s ~ 2.48 GeV) pp→ppπ 0 π 0 Absence of differential cross sections at high energies (i.e. T p > 1.3 GeV) Exclusive studies from threshold up to T p = 1.3 GeV Previous work

NSTAR2011, JLab USA, May 18, 2011Slide 5Tamer Tolba, IKP, FZ Jülich WASA Detector at COSY Proton selection Inv. mass γ 1 γ 2 vs. Inv. mass γ 3 γ 4 Peak at π 0 - π 0 mass p p  p p   π0π0 π0π0 Central Detector (CD): -Covers scattering angle θ = 20° – 169°. - Energy, momentum reconstruction and PID for charged particles and photons. Forward Detector (FD): - Covers scattering angle θ = 3° – 18°. - Energy, momentum reconstruction and PID for charged hadrons. - Used to tag mesons (via missing- mass technique). COSY

NSTAR2011, JLab USA, May 18, 2011Slide 6Tamer Tolba, IKP, FZ Jülich Main selection criteria: Demanding 1 or 2 protons in the forward detector and 4γ in the central detector. The full phase space is covered Data Analysis Geometrical acceptance of protons in the forward detector and photons in the central detector. Central Detector Forward Detector θ γ vs. E γ θ p vs. E p MC simulations Background free

NSTAR2011, JLab USA, May 18, 2011Slide 7Tamer Tolba, IKP, FZ Jülich Total cross section pp → ppπ 0 π T p = 1.4 GeV Experimental Results σ tot = (324±61) μb Preliminary  Total error: - Statistical error is negligible. - Systematical error dominated by absolute normalization ~ 18%

NSTAR2011, JLab USA, May 18, 2011Slide 8Tamer Tolba, IKP, FZ Jülich Differential Cross T p = 1.4 GeV M Δ(1232) Modified Valencia model * Preliminary * T. Skorodko, et al., Phys. Lett. B 695 (2011) 115

NSTAR2011, JLab USA, May 18, 2011Slide 9Tamer Tolba, IKP, FZ Jülich No strong N*(1440) contribution (pole position = 1360 MeV/c 2 ) Indication for ΔΔ->pπ 0 pπ 0 MΔMΔ MΔMΔ M N*(1440) = 1.36 GeV/c 2 Preliminary

NSTAR2011, JLab USA, May 18, 2011Slide 10Tamer Tolba, IKP, FZ Jülich Summary and Outlook  Double pion production in pp→ppπ 0 π 0 at T p = 1.4 GeV.  Results: - Total and differential cross section.  Reaction Dynamics: - The production mechanism is dominated by double Δ(1232). - No significant contribution from N*(1440).  Outlook: - Extension to higher energies: contribution of higher resonances. - Study pp→ppπ + π - : contribution of ρ(770). - Investigations on pn and pd reactions: include mixed final states (π - π 0,π + π 0 ).

NSTAR2011, JLab USA, May 18, 2011Slide 11Tamer Tolba, IKP, FZ Jülich Spares

NSTAR2011, JLab USA, May 18, 2011Slide 12Tamer Tolba, IKP, FZ Jülich - Toy-model:  Tune the MC simulations to match the data.  GIN phase space generator. * Provides 4-vector momentum for each particle in the final state at the interaction point. * Provides individual weight for each event. * Availability to simulate resonances production by changing the generated according to the expected production mechanism.  Correction terms added to the generated weight: * 2Δ propagators, only Breit-Wigner width. (Risser, T. and M.D. Shuster, Phys. Lett. B 43(1973) 68). * Angular distributions for p and π in CM frame. * Inv. mass 2π 0. * Inv. mass2 pπ 0. Monte Carlo Models

NSTAR2011, JLab USA, May 18, 2011Slide 13Tamer Tolba, IKP, FZ Jülich - Valencia-model (Alvarez-Ruso et. al., Nucl. Phys. A 633(1998) 519).:  Model describing NN→ππNN reaction.  Studies both resonance and prompt (non-resonance) terms from near threshold up to 1400 MeV nucleon incident energy.  Chiral Lagrangian terms involving nucleons and pions.  Plus terms involving the excitation of the Δ(1232) and N*(1440) resonances.

NSTAR2011, JLab USA, May 18, 2011Slide 14Tamer Tolba, IKP, FZ Jülich - Kinematical Fit (KFit) with constraints:  total (E,P) conservation = 4  2 x Inv. mass π 0 = 2  Number of degrees of freedom (NDF) = 3  1 particle in final state is treated as unmeasured.  KFit used as combinatorial selection tool for the best γ-pair forming π 0.  The γ-pair combination of minimum χ 2 KFit was chosen as best γ-pair forming π 0. - data - MC simulation

NSTAR2011, JLab USA, May 18, 2011Slide 15Tamer Tolba, IKP, FZ Jülich Data – MC Comparison After KFit

NSTAR2011, JLab USA, May 18, 2011Slide 16Tamer Tolba, IKP, FZ Jülich Scale from luminosity determination  Use the pp→ppη(→γγ and →3π 0 ) cross section as reference channel.  σ pp→ppη (T p = 1400 MeV) = (9.8 ± 1) μb. (Chiavassa, PL 322B, (1994) 270). Errors generated from acceptance correction to different models  Different models (Toy-model, Valencia-model and Phase space) lead to different acceptance results. Edge Effects  Selection on the scattering angles of protons and photons in the FD and CD. Errors generated from confidence level of the KFit  Different confidence level selections. - Systematic Errors Error ~ 5% ~ 1% ~ 4% ~ 18% Total 19% - Statistical Errors ~ % Total Error

NSTAR2011, JLab USA, May 18, 2011Slide 17Tamer Tolba, IKP, FZ Jülich Differential cross sections

NSTAR2011, JLab USA, May 18, 2011Slide 18Tamer Tolba, IKP, FZ Jülich

NSTAR2011, JLab USA, May 18, 2011Slide 19Tamer Tolba, IKP, FZ Jülich

NSTAR2011, JLab USA, May 18, 2011Slide 20Tamer Tolba, IKP, FZ Jülich

NSTAR2011, JLab USA, May 18, 2011Slide 21Tamer Tolba, IKP, FZ Jülich MC-Toy

NSTAR2011, JLab USA, May 18, 2011Slide 22Tamer Tolba, IKP, FZ Jülich