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					    Photoproduction of neutral pion pairs off the
  proton with the Crystal-Barrel-Detector at ELSA




Michael Fuchs                           October 14th, 2005
           Introduction ◦ Experiment ◦ Selection ◦ Acceptance ◦ Results ◦ Summary



➢   Introduction
➢   Experiment
➢   Selection
➢   Acceptance
    ➢   Problems
    ➢   “Alternative” method
➢   Results
    ➢   Total cross section
    ➢   Differential cross sections
    ➢   Dalitz-Plots
➢   Summary and Outlook
     Introduction ◦ Experiment ◦ Selection ◦ Acceptance ◦ Results ◦ Summary


                     Missing resonances




Nucleon resonances                                Löring, et.al. EPJA10, 309-346 (2001)
            Introduction ◦   Experiment ◦ Selection ◦ Acceptance ◦ Results ◦ Summary


                        Crystal-Barrel@ELSA Setup




➢   Tagging System                              ➢   Crystal Barrel
    ➢   Energy: incoming g                          ➢   Energy, direction: decay g
    ➢   Range: 25 – 95 % of Ee-                     ➢   1380 CsI(Tl) crystals
➢   Inner Detector                                  ➢   Covers 98% of 4p
    ➢   Charged particle ident.
             Introduction ◦ Experiment ◦   Selection ◦ Acceptance ◦ Results ◦ Summary



➢   Cuts on reconstructed information
    ➢   Tagging System
         ➢   Coincident hit in
             MWPC and Scintillator

    ➢   Inner detector
         ➢   At least: 2 of 3 layers
             → Charged Particle

    ➢   Crystal-Barrel
         ➢   4 or 5 PEDs

    ➢   Invariant masses
         ➢   Two invariant masses of two photons:
             m(gg) in interval: 134.97 ± 16.00 MeV
               Introduction ◦ Experiment ◦   Selection ◦ Acceptance ◦ Results ◦ Summary

➢   Cuts on kinematic fit
    ➢   E, p conservation
          ➢   Confidence Level (PS) >1%
    ➢   Final Cuts
          ➢   CL (g p → p p0p0) > 10%
          ➢   Matching Protons: (pInner & pkin.Fit)
          ➢   CL (gp → pp0p0) > CL (gp → pp0)

        Invariant pp0-mass (@ 3.2 GeV)


                                                                 ➢   Decay of baryon
                                                                     resonances in:
                                                                      ➢     (1232) p
                                                                      ➢   D13 (1520) p
                                                                      ➢     X (1660) p
   Introduction ◦ Experiment ◦   Selection ◦ Acceptance ◦ Results ◦ Summary


          Dalitzplots: m2 (pp0) vs m2 (pp0)
1800 < √s < 2000 MeV                                   2000 < √s < 2200 MeV




     Clear bandlike structures in Dalitzplot
                        ↓
     Cascading decays of baryon resonances
             Introduction ◦ Experiment ◦ Selection ◦   Acceptance ◦ Results ◦ Summary



➢   Problems:
    ➢   Areas with missing Acceptance
         ➢   Extrapolation necessary

    ➢   MC events
         ➢   Distribution by phase space
         ➢   Dynamics of reaction not included

    ➢   3 particle final states
         ➢   5 independent variables
         ➢   Automatic Integration


➢   Solution:
    ➢   Use MC-events with correct reaction dynamics
              Introduction ◦ Experiment ◦ Selection ◦   Acceptance ◦ Results ◦ Summary


➢   Weighting procedure
    ➢   Preparation
         ➢   Use
              ➢   Original distr. (PS-MC)
              ➢   Target distr. (Correct dynamics)
         ➢   Create
              ➢   Normalized weights: wnorm                        Original distribution (yellow)
    ➢   Copy                                                       Target distribution (orange)
         ➢   By event
         ➢   Generate random number: r
         ➢   If: wnorm < r
              ➢   Copy whole event
                  in new dataset

➢   Next Step:
    ➢   Weight factors using PWA
                                                                    Normalized weights: wnorm
             Introduction ◦ Experiment ◦ Selection ◦   Acceptance ◦ Results ◦ Summary



➢   Partial wave analysis
    ➢   Determination of resonance properties
         ➢   Masses
         ➢   Partial widths
         ➢   Coupling constants

    ➢   Version by A. Sarantsev, U. Thoma et. al.
         ➢   Isobar - model
         ➢   Breit-Wigner (or K-Matrix) parametrization
         ➢   Unbinned Maximum-Likelihood fit
               ➢   Event based
               ➢   All correlations correctly taken into account
               ➢   (5 independent variables)

         ➢   No fitting of projections !!
Introduction ◦ Experiment ◦ Selection ◦   Acceptance ◦ Results ◦ Summary

                                                           Comparison of
                                                           Acceptance:

                                                           Phasespace MC




                                                           PWA weighted MC
Introduction ◦ Experiment ◦ Selection ◦ Acceptance ◦   Results ◦ Summary

                  Total Cross Section
                       Introduction ◦ Experiment ◦ Selection ◦ Acceptance ◦              Results ◦ Summary

                             Differential Cross Section: m(pp0)




                                                                  1570 < √s < 1800 MeV
1350 < √s < 1570 MeV
                                         (1232)                                                         (1232)




                                                                  2060 < √s < 2550 MeV
1800 < √s < 2060 MeV




                                       D13(1520)                                                         X(1660)
                       Introduction ◦ Experiment ◦ Selection ◦ Acceptance ◦              Results ◦ Summary

                            Differential Cross Section: m(p0p0)




                                                                  1570 < √s < 1800 MeV
1350 < √s < 1570 MeV




                                                                  2060 < √s < 2550 MeV
1800 < √s < 2060 MeV




                                                                                         f0(980)
Introduction ◦ Experiment ◦ Selection ◦ Acceptance ◦   Results ◦ Summary

          Dalitzplot m2 (pp0) vs m2 (pp0)
              Introduction ◦ Experiment ◦ Selection ◦ Acceptance ◦ Results ◦   Summary

➢   Selection
     ➢   Good identification of: gp→pp0p0

➢   Acceptance
     ➢   MC contains correct dynamics of reaction

➢   Cross Sections
     ➢   Total and differential cross sections up to Eg ~ 3 GeV

➢   Data used for Partial Wave Analysis
     ➢   Search for new baryon resonances
     ➢   Determination of resonance properties:
          ➢   Masses, Partial widths, Coupling constants


➢   Cascading decays of baryon resonances observed via
     ➢   (1232)p                  D13(1520)p                X(1660)p
           Introduction ◦ Experiment ◦ Selection ◦ Acceptance ◦ Results ◦   Summary




➢   Acknowledgements
    The Crystal-Barrel Collaboration
    ➢ U. Thoma


    ➢   E. Klempt
    ➢   A. Sarantsev




               Thank you for your attention!
  Introduction ◦ Experiment ◦ Selection ◦ Acceptance ◦ Results ◦ Summary ◦   Add-Ons



                 Gen MC                        Rec MC                        Acceptance
                                                                             (Phase space)

      4Vec: p, p0, p0
         Eg, ID


PWA
          ID, wnorm

                               wnorm< r          Yes

                   r              No
                                                       Copy
Rndm()




                 Gen MC                        Rek MC                        Acceptance
                                                                         (PWA weighted)
Introduction ◦ Experiment ◦ Selection ◦ Acceptance ◦ Results ◦ Summary ◦   Add-Ons


               Total cross section @ 1.4 GeV
Introduction ◦ Experiment ◦ Selection ◦ Acceptance ◦ Results ◦ Summary ◦   Add-Ons


               Total cross section @ 3.2 GeV
Introduction ◦ Experiment ◦ Selection ◦ Acceptance ◦ Results ◦ Summary ◦   Add-Ons


                  Differential Cross Section
Introduction ◦ Experiment ◦ Selection ◦ Acceptance ◦ Results ◦ Summary ◦   Add-Ons
      Differential cross section: m(pp0) for selected energie bins in Eg
Introduction ◦ Experiment ◦ Selection ◦ Acceptance ◦ Results ◦ Summary ◦   Add-Ons
      Differential cross section: m(p0p0) for selected energie bins in Eg
Introduction ◦ Experiment ◦ Selection ◦ Acceptance ◦ Results ◦ Summary ◦   Add-Ons


              Dalitzplot m2 (pp0) vs m2 (p0 p0)

				
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posted:8/15/2012
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