Tamer Tolba for the WASA-at-COSY collaboration

1. Double-Pion Production in Proton-Proton Interactions at Tp = 1.4 GeV Tamer Tolba  for the WASA-at-COSY collaboration Institut für Kernphysik Forschungszentrum Jülich

2. Outline • Physics Motivation • Previous Work • WASA Detector at COSY • Data Analysis - Experimental Results • * Total Cross Section • * Differential Cross Sections • - Summary and Outlook

3. near threshold high energies pp→∆ ∆(1232) p π p π Double pion production in pp collision pp → ppππ pp→pN*(1440) p π π ∆ (1232) π p π * L. Alvarez-Ruso, E. Oset, and E. Hernández, Nucl. Phys. A633, 519 (1998). (Valencia model) Double pion production in pp collisions is dominated by resonance production. Relative strengths of the production mechanisms strongly momentum dependent.

4. Absence of differential cross sections at high energies (i.e. Tp > 1.3 GeV) Exclusive studies from threshold up to Tp = 1.3 GeV Goal: Study 2π0σtotal and production mechanism at Tp =1.4 GeV (√s ~ 2.48 GeV) Previous work pp→ppπ0π0

5. p  p π0 p π0    p WASA Detector at COSY 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 Inv. mass γ1γ2 vs. Inv. mass γ3γ4 Peak at π0- π0 mass Proton selection • Central Detector (CD): • Covers scattering angle θ = 20° – 169°. • - Energy, momentum reconstruction and PID for charged particles and photons.

6. Data Analysis MC simulations MC simulations θγvs. Eγ θp vs. Ep Central Detector Forward Detector Geometrical acceptance of protons in the forward detector and photons in the central detector. Main selection criteria: Demanding 1 or 2 protons in the forward detector and 4γ in the central detector. The full phase space is covered Background free

7. Experimental Results Total cross section pp → ppπ0π0 @ Tp = 1.4 GeV Preliminary • Total error: • Statistical error is negligible. • Systematical error dominated by absolute normalization ~ 18% σtot = (324±61) μb

8. Differential Cross Sections @ Tp = 1.4 GeV Modified Valencia model* Preliminary MΔ(1232) *T. Skorodko, et al., Phys. Lett. B 695 (2011) 115

9. No strong N*(1440) contribution (pole position = 1360 MeV/c2) MN*(1440) = 1.36 GeV/c2 Preliminary Indication for ΔΔ->pπ0 pπ0 MΔ MΔ

10. Summary and Outlook • Double pion production in pp→ppπ0π0 at Tp = 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-π0,π+π0).

11. Spares

12. Monte Carlo Models - 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 2π0. * Inv. mass2 pπ0.

13. - 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.

14. - Kinematical Fit (KFit) with constraints: • total (E,P) conservation = 4 • 2 x Inv. mass π0 = 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 π0. • The γ-pair combination of minimum χ2KFit was chosen as best γ-pair forming π0. - data - MC simulation

15. Data – MC Comparison After KFit

16. Total Error Error - Statistical Errors ~ 10-3% - Systematic Errors • Scale from luminosity determination • Use the pp→ppη(→γγ and →3π0) cross section as reference channel. • σpp→ppη (Tp = 1400 MeV) = (9.8 ± 1) μb. (Chiavassa, PL 322B, (1994) 270). ~ 18% • Errors generated from acceptance correction to different models • Different models (Toy-model, Valencia-model and Phase space) lead to different acceptance results. ~ 4% • Edge Effects • Selection on the scattering angles of protons and photons in the FD and CD. ~ 1% • Errors generated from confidence level of the KFit • Different confidence level selections. ~ 5% Total 19%

17. Differential cross sections

21. MC-Toy