1. Fabien Zehr Double π photoproduction Glasgow 28 March 2006 Double π photoproduction at threshold and in the second resonance region With Crystal Ball and TAPS at MAMI in Mainz

2. Fabien Zehr Double π photoproduction Glasgow 28 March 2006 Motivation 1: Second resonance region Resonance → Δπ →Nππ Resonance → Nρ → Nππ Resonance → Nππ S-wave Focus on P 11 (1440) and D 13 (1520)→ Nρ Final states: p π 0 π 0 n π 0 π + (p π + π - )

3. Fabien Zehr Double π photoproduction Glasgow 28 March 2006 ChPT predicts that pion loops contribution to double π 0 production is dominant at threshold. Small contribution for charged pions. σ threshold (π 0 π 0 ) > σ threshold (π + π 0 ) σ(π 0 π 0 ) < σ(π + π 0 ) σ threshold (π 0 ) < σ threshold (π + ) Motivation 2: Reaction at threshold

4. Fabien Zehr Double π photoproduction Glasgow 28 March 2006 Experimental Setup: Overview 883 MeV electrons Crystal Ball: 720 NaI Crystals with PID and MWPC TAPS: 510 BaF2 Crystals with Vetos Coherent Bremsstrahlung Tagged Photons Lineraly and Circularly Polarized Unpolarized Liquid H target

5. Fabien Zehr Double π photoproduction Glasgow 28 March 2006 Preliminary Analysis: Double π 0 : invariant mass 4 Photons 1 or 0 Protons cut between 125 and 145 MeV ~260 h (1/3 of total statistics)

6. Fabien Zehr Double π photoproduction Glasgow 28 March 2006 Preliminary Analysis: Double π 0 : Missing Mass Counts Missing Mass [MeV] High energies: good η→3π 0 background control 350 – 400 MeV 707 – 820 MeV Low energies: good statistic

7. Fabien Zehr Double π photoproduction Glasgow 28 March 2006 Preliminary Analysis: Double π 0 : Excitation Function Blue: Previous Experiment M. Kotulla et al.: Phys.Lett. B578 (2004) 63-68 Curves: ChPT Calculations V. Bernard et al.: Phys. Lett. B382 (1996) 19-23

8. Fabien Zehr Double π photoproduction Glasgow 28 March 2006 Preliminary Analysis: Double π 0 : Excitation Function Blue: Previous Experiment M. Kotulla et al.: Phys.Lett. B578 (2004) 63-68 Curves: ChPT Calculations V. Bernard et al.: Phys. Lett. B382 (1996) 19-23 Blue: Previous Experiment M. Kotulla et al.: Phys.Lett. B578 (2004) 63-68 Curves: ChPT Calculations V. Bernard et al.: Phys. Lett. B382 (1996) 19-23

9. Fabien Zehr Double π photoproduction Glasgow 28 March 2006 Preliminary Analysis: Excitation function comparison Energy [Mev] Arbitrary Units Red: γp→nπ + π 0 Black: γp→pπ 0 π 0

10. Fabien Zehr Double π photoproduction Glasgow 28 March 2006 Preliminary Analysis: Excitation function comparison Energy [MeV] Arbitrary Units Red: γp→nπ + π 0 Black: γp→pπ 0 π 0 π0π0π0π0 π0π+π0π+

11. Fabien Zehr Double π photoproduction Glasgow 28 March 2006 Preliminary Analysis: Asymmetry Definitions Angle definitions for the double pion reaction. Φ used thereafter is the azimuthal angle of the ππ system. dσ +/- is the differential cross section produced by a right/left handed photon

12. Fabien Zehr Double π photoproduction Glasgow 28 March 2006 Preliminary Analysis: Asymmetry Calculations Observable very sensitive to the details of the model L.Roca Helicity asymmetries in double pion photoproduction on the proton Nucl. Phys. A 748 (2005) 192 Example: circular asymmetry for the double π 0 channel

13. Fabien Zehr Double π photoproduction Glasgow 28 March 2006 Preliminary Analysis: π + π 0 Helicity Asymmetry φ φ φ φ φ φ Asymmetry L. Roca, Private Communication

14. Fabien Zehr Double π photoproduction Glasgow 28 March 2006 Preliminary Analysis: π + π 0 Helicity Asymmetry φ φ φ φ φ φ Asymmetry 575 MeV 650 MeV 730 MeV 810 MeV RED: Complete Model BLUE: Without D 13 →ρN “the observable is [...] very sensitive to small changes in details of the model which, on the other hand, are little relevant in unpolarized observables. Hence, do not be surprised if my calculations do not agree very well with your data.” L. Roca, Private Communication

15. Fabien Zehr Double π photoproduction Glasgow 28 March 2006 Work in progress