Status of the η - mass measurement with the Crystal Ball at MAMI

1. Status of the η-mass measurement with the Crystal Ball at MAMI A. Nikolaev for the Crystal Ball @ MAMI and A2 Collaborations, Mainz

2. Previous eta mass measurements • A. Duane et al., Phys. Rev. Lett. 32 (1972) • F. Plouin et al., Phys. Lett. B276 (1992) • B. Krusche et al., Z. Phys. A351 (1995) • A. Lai et al., Phys. Lett. B533 (2002) • GEM Collaboration, arXiv:hep-ex/0505006 (2005) CB Collaboration Meeting 29.03.2006

3. Coincidence Electron Ladder Primary Focal Plane Beam e E e DETECTOR Electron Collimator Beam E 0 Target Photon E Radiator g Mainz eta mass experiment • Mainz / MAMI • determine production threshold in eta photoproduction reaction: • Relation between mass and threshold: • Thus eta mass can be found: . the tagger microscope CB Collaboration Meeting 29.03.2006

4. Tagger microscope for the new eta mass experiment CB Collaboration Meeting 29.03.2006

5. Improvements since previous eta mass experiment in Mainz • Precisely known electron beam energy (160 keV, FWHM) and much higher beam stability. • Online monitoring of the electron beam position via two position sensitive cavities. • Higher energy resolution provided by the tagger microscope. • Large acceptance of the Crystal Ball detector. • Thus uncertainty in eta mass measurement is expected to be reduced by a factor 2-4. CB Collaboration Meeting 29.03.2006

6. Crystal Ball detector • Eta mesons are identified via their uncharged decay modes: • Photons from both decays are identified with Crystal Ball detector. • Particle identification detector serves as a veto for charged particles. • 2γ and 3π0 decays are analyzed independently. CB Collaboration Meeting 29.03.2006

7. Invariant mass distribution of the 2-fold and the 6-fold coincident photons in the detector • 2 photons invariant mass distribution. Cut at the eta peak. • 3π0 invariant mass distribution. CB Collaboration Meeting 29.03.2006

8. Proton missing mass distribution CB Collaboration Meeting 29.03.2006

9. Preliminary eta yield • Eta yield versus the microscope fibre. Microscope fibre is used instead of the microscope channel. • Polinomial fit above and linear fit below the threshold. • The intersection of fits is floating too much. CB Collaboration Meeting 29.03.2006

10. Eta yield vs. microscope channel • 96 microscope fibres = 191 microscope channels (96 single channels + 95 overlapped channels). • Microscope overlapped channels are less efficient than single channels. CB Collaboration Meeting 29.03.2006

11. Integrated eta yield vs. microscope fibre(data from Dec. 2004) CB Collaboration Meeting 29.03.2006

12. Integrated eta yield vs. microscope fibre(data from Jan. 2005) CB Collaboration Meeting 29.03.2006

13. Squared eta yield vs. microscope fibre(data from Dec. 2004) CB Collaboration Meeting 29.03.2006

14. Squared eta yield vs. microscope fibre(data from Jan. 2004) CB Collaboration Meeting 29.03.2006

15. Uncertainty for coming eta mass measurement CB Collaboration Meeting 29.03.2006

16. Summary • Determination of the eta mass via the eta production threshold in γ+p→p+η. • Two eta decay modes are investigated: η→2γ, η→3π0. • Error for coming eta mass measurement estimated from tagger microscope calibration is around 0.07 MeV (σ). • The eta production threshold is not yet determined. Thank you! CB Collaboration Meeting 29.03.2006

17. Outline • Previous eta mass measurements. • Mainz eta mass experiment. • Apparatus improvements since the last Mainz experiment. • Experimental method. • Preliminary results. • Summary. CB Collaboration Meeting 29.03.2006

18. Tagger microscope energy calibration The tagger microscope is calibrated in two steps: • The first step is based on direct measurement of the electron beam position with known energy. • The second step is based on scanning the microscope with electron beam varying the tagger dipole field. CB Collaboration Meeting 29.03.2006

19. Tagger microscope calibration – direct Dipole field used in experiment B=1.049T T T T CB Collaboration Meeting 29.03.2006

20. Tagger microscope calibration – beam scan • 139 data points in 6 scans. 1 microscope channel (μch) corresponds to 300 keV. CB Collaboration Meeting 29.03.2006

21. Uncertainty in e-- and γ- energy determination CB Collaboration Meeting 29.03.2006

22. Time difference between double hits in the tagger microscope CB Collaboration Meeting 29.03.2006