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

2. Introduction
Question: Is the determination of neutral meson's formfactors possible with calorimeters ( ) ?
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 %

3. Introduction: Experiment - CB/TAPS in Mainz @ MAMI
Photons generated via Bremsstrahlung ( ) 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

4. 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

5. 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

6. 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

7. Analysis of experimental data applied cuts

8. 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!

9. 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

10. 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

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

12. 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 = ( / ) GeV-2 with Dh = (708 +/- 186) MeV
Theory (Landsberg): bh=1.8 GeV-2
Other Experiments: bh = ( 1.6 +/- 2.0 ) GeV (Statistic: ~110 counts) M.N. Achasov et all. , Physical Letters B (2001)

13. 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

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