Investigation of the h -Dalitz Decay with CB/TAPS @ MAMI & CBT Dubrovnik 2008 Henning Berghäuser, Martin Kotulla Investigation of the h -Dalitz Decay with CB/TAPS @ MAMI & Determination of the h - formfactor Outline: Introduction Analysis of experimental data Investigation of Background & Discussion Results & Conclusion II. Physikalisches Institut Gießen

Introduction Question: Is the determination of neutral meson's formfactors possible with calorimeters ( CB/TAPS @MAMI ) ? Aim of analysis: investigation & determination of h -formfactor Decay of interest: h -> e+ e- g (h – Dalitz decay) Branching ratio of h – Dalitz decay is BR = 0.6 %

Introduction: Experiment - CB/TAPS in Mainz @ MAMI Photons generated via Bremsstrahlung (680-825) MeV Target: LH2 4p-solid angle nearly totally covered ! Charged particles can be detected using PID & VETO detectors Three beamtimes in 2004; 1*107 h produced

Main problem: separation of electrons/positrons & charged pions Analysis of experimental data: Particle-identification in CB with dE/E-Cuts Main problem: separation of electrons/positrons & charged pions dE/E-Plot of the CB: electrons/positrons of higher energies are within the same area as are the charged pions. Data dEPID / MeV ENaI / MeV Simulation: h->p+p-g Simulation: h->e+e-g Sim Sim dEPID / MeV dEPID / MeV ENaI / MeV ENaI / MeV

Analysis of experimental data Separation possible by different clustersizes ? -> electrons & charged pions cause different clustersizes -> but: is difference large enough for a clear separation? Simulation: h->p+p-g Simulation: h->e+e-g Result: - some depletion of pions possible by clustersize cut ..... but - a complete separation has to be realized by testing the kinematics

Analysis of experimental data Reaction of interest: g p -> h p -> e+ e- g p Exclusive analysis: proton is required reduce background by applying cuts: momentum balance - in x-direction - in y-direction - in z-direction opening angle of the lepton pair ... -> to exclude events stemming from conversion decays Invariant mass spectrum - no cuts except time cut h

Analysis of experimental data applied cuts

Analysis of experimental data: invariant e+e-g mass no cuts after cuts Within the mass range from (510 – 590) MeV 74 events survive the cuts!

2D-Plot: m(e+e-) vs. m(e+e-g) Analysis of experimental data: 2D-Plot: m(e+e-) vs. m(e+e-g) Data Data After all cuts Sim Sim

2D-Plot: m(e+e-) vs. m(e+e-g) Analysis of experimental data: 2D-Plot: m(e+e-) vs. m(e+e-g) Sim Sim Investigated/simulated background channels: h-channels: h->p+p-g, h->p+p-p0, h->3p0, h->2g other reactions: gp->p0p, gp->p+n, gp->2p0p, gp->p0p+n, gp->p+p-p

Analysis of experimental data: Cross section The cross section within the mass range from (20 – 490) MeV has been determined to: sexp. = ( 7.52 +/- 2.10 ) m barn QED (20 MeV – 490 MeV): sOED = 6.23 m barn VMD (20 MeV – 490 MeV): sVMD = 6.65 m barn

Analysis of experimental data: transition formfactor of the h-Dalitz decay Determination of the transition formfactor of the h-Dalitz decay: Fh = (1-q2/Dh2)-1 bh = dFh/dq2 |q2=0 = 1/ Dh2 Experiment: bh = ( 1.99 +/- 0.51 ) GeV-2 with Dh = (708 +/- 186) MeV Theory (Landsberg): bh=1.8 GeV-2 Other Experiments: SND@VEPP-2M bh = ( 1.6 +/- 2.0 ) GeV-2 (Statistic: ~110 counts) M.N. Achasov et all. , Physical Letters B 504 (2001)

Conclusion: achievements and what has to be done next? Formfactors of neutral mesons can be measured with Crystal Ball and TAPS despite of the fact that the calorimeters provide only a limited e+e-/p+p- separation. The h-Dalitz transition formfactor has been determined (unfortunately low statistic) What has to be done next? Analyse more/newer data to improve statistic Improve simulation of background channels Mmeasure w-Dalitz decay and determine w-formfactor

Analysis of the h-Dalitz decay Projections on the mass(e+e-g)-axis Projection on the mass(e+e-)-axis