1. LOGO The η -mass measurement with the Crystal Ball at MAMI A. Nikolaev for the Crystal Ball @ MAMI and A2 Collaborations Helmholtz Institut für Strahlen- und Kernphysik, University Bonn

2. A.Nikolaev, BonnCB Meeting, Basel 10/5/062 The  mass measurement at MAMI  The  production threshold measurement from γp→pη.  The mass of  is calculated from relation: where m p is the proton mass, E  thr – production threshold.  Recently published measurements:

3. A.Nikolaev, BonnCB Meeting, Basel 10/5/063 Crystal Ball/TAPS detector  Crystal Ball detector:  672 NaI crystals  measures E γ and Ө γ TAPS 510 BaF 2 detectors Particle identification detector Crystal Ball 672 NaI detectors  Particle identification detector:  24 plastic bars of size 30 cm x 2 cm 0.2 cm  marks charged particles  Forward wall detector TAPS:  510 BaF 2 single plastic detectors with individual vetos

4. A.Nikolaev, BonnCB Meeting, Basel 10/5/064 The tagging facility  The energy E  of the photon is determined (tagged) by:  Incoming MAMI electron beam energy E 0 is known with an accuracy of    = 68 keV.  Scattered electron energy E e- is known with an accuracy of   e  = 78 keV.

5. A.Nikolaev, BonnCB Meeting, Basel 10/5/065 Tagger microscope detector  An array of 96 plastic scintillator fibres (3 mm x 2 mm).  Each single fibre overlaps by 1/3 with its neighbor. The overlap region defines microscope detector channel μch (191 channels in total).  The energy resolution is 0.3 MeV per microscope channel μch.  Tagger microscope is positioned to cover electron energies 153 to 209 MeV. At a beam energy of 883 MeV this corresponds to tagged photons between 674 MeV and 730 MeV ( η threshold ~707 MeV).

6. A.Nikolaev, BonnCB Meeting, Basel 10/5/066 Tagger microscope energy calibration  Direct calibration:  Direct position measurement of the MAMI electron beam for the energies E e- = 180.1 MeV, 195.2 MeV and 210.2 MeV using the same dipole magnetic field as in the experiment ( B cal = B exp = 1.049 T).  Scan beam through microscope varying the dipole field:  Increase tagger dipole magnetic field B cal in small steps.  Measure new electron beam position supposing equivalent electron beam energy for a given field setting B cal :

7. A.Nikolaev, BonnCB Meeting, Basel 10/5/067 Tagger microscope energy calibration Blue arrows show direct position measurements of the electron beam of energies 180.1, 195.2 and 210.2 MeV. 180.1 MeV 1.049 T 210.2 MeV 1.055 T 195.2 MeV 1.049 T Expected η threshold

8. A.Nikolaev, BonnCB Meeting, Basel 10/5/068 Total energy measurement uncertainty  Standard deviation of data points from fit: was calculated for each scan individually:  Average uncertainty:   = 0.27  ch = 78 keV (electron energy E e - ).  Add electron beam energy E 0 determination uncertainty of MAMI 68 keV (  )  (threshold) = 103 keV  (m  ) = 65 keV

9. A.Nikolaev, BonnCB Meeting, Basel 10/5/069 The  mass beamtimes  Two beamtimes for the  mass measurement:

10. A.Nikolaev, BonnCB Meeting, Basel 10/5/0610 Identification of  meson   mesons are identified via their uncharged decay modes.  Events with 2 photons and 6 photons (with or without proton) are investigated.  Photons are identified with Crystal Ball/TAPS detector.  Particle identification detector serves as a veto for charged particles in CB.  TAPS crystals have individual veto detectors for marking charged particles.   →2γ and  →3π 0 decays are analyzed independently. Particle identification detector (PID) Crystal Ball 672 NaI crystals TAPS 510 BaF2 crystals with individual vetos

11. A.Nikolaev, BonnCB Meeting, Basel 10/5/0611 Identification of photons and protons  Crystal Ball  Uncharged particles = photons.  Charged particles: ΔE(PID) vs. E CB : cut the proton.  TAPS forward wall  Vetos: charged particles.  Pulse shape analysis: cluster energy deposition in short E short vs. in long E long integration interval.  Time of flight: E TAPS cl. vs. ( t TAPS cl. – t CB photons ). protons protons, neutrons protons photons

12. A.Nikolaev, BonnCB Meeting, Basel 10/5/0612 CB time walk correction  Investigate γp→pπ 0 with γγp final state.  For each of 672 NaI crystals:  E CB hit vs. ( t CB photons – t PID proton )  fit the peaks with t(E) function.  7-13 Dec 2004 #4924-5088:  faulty crystals #: 168, 183, 194, 205, 221, 352.  11-20 Jan 2005 #5200-5452:  faulty crystals #: 41, 168, 205. Microscope FWHM = 1.7 ns Sigma = 0.7 ns Ladder FWHM = 2.3 ns Sigma = 1.0 ns

13. A.Nikolaev, BonnCB Meeting, Basel 10/5/0613 Invariant mass distributions  2  (with or without proton) final state invariant mass distribution.  3  0 ( 6  with or without proton) final state invariant mass distribution.  Cut on invariant mass distribution.  Time coincidence peak between tagger microscope and detector: cut the coincident events.  Cut on lost proton missing mass. Mean = 568 MeV Sigma = 20 MeV Mean = 138 MeV Sigma = 8 MeV Mean = 558 MeV Sigma = 16 MeV FWHM = 1.7 ns Sigma = 0.7 ns η π0π0

14. A.Nikolaev, BonnCB Meeting, Basel 10/5/0614 Tagger dipole field / Jan 2005  NMR = 1.049057±0.000535 T (± 0.01 MeV in E e ). Very good!

15. A.Nikolaev, BonnCB Meeting, Basel 10/5/0615 Tagger dipole field / Dec 2004  NMR = 1.049527±0.015 T (± 2.78 MeV in E e ).  Need to refine data with faulty NMR!

16. A.Nikolaev, BonnCB Meeting, Basel 10/5/0616 Microscope channel efficiency  Investigate γp→pπ 0 with 2 photons and 1 proton final state using tagger.  Prompt - random time windows.  In microscope region (tagger channels 55-85) “pion yield” looks constant.  Investigate the same with microscope.  Microscope channels have different efficiencies!  Use “pion yield” distribution as microscope channels efficiency to correct the η yield! Microscope region pion yield in microscope pion yield in tagger

17. A.Nikolaev, BonnCB Meeting, Basel 10/5/0617 Eta yield for reaction γp→pη (η→2γ) Fit range: 0-75 μch Without acceptance correction! Fit function: f(x)=a 0 +a 1 x+a 2 x 2 +a 3 x 3 η→2γ EγEγ

18. A.Nikolaev, BonnCB Meeting, Basel 10/5/0618 Eta yield for reaction γp→pη (η→3π 0 ) Without acceptance correction! Fit function: f(x)=a 0 +a 1 x+a 2 x 2 +a 3 x 3 η→3π 0 Fit range: 0-75 μch EγEγ

19. A.Nikolaev, BonnCB Meeting, Basel 10/5/0619 Very preliminary Result dependence on fit range η→2γη→3π 0  1. η→2γ: m η =547.683 ± 0.013 fit MeV  2. η→3π 0 : m η =547.743 ± 0.013 fit MeV Fit range: 0 to 75 μch 10 to 75 μch 20 to 75 μch 30 to 75 μch 40 to 75 μch 50 to 75 μch 60 to 75 μch 70 to 75 μch Fit range: 0 to 75 μch 10 to 75 μch 20 to 75 μch 30 to 75 μch 40 to 75 μch 50 to 75 μch 60 to 75 μch 70 to 75 μch

20. A.Nikolaev, BonnCB Meeting, Basel 10/5/0620 Conclusions  New threshold data on γp → pη.  The η identified by η → γγ and η → 3π 0.  Energy calibration for E e - (tagger microscope) done.  Energy calibration E 0 (Mainz Microtron) final checks for systematical error underway.

21. A.Nikolaev, BonnCB Meeting, Basel 10/5/0621 MAMI energy determination uncertainty B = 1.3260 T ± 0.13 mT ΔE = ~7.81 ± 0.02 MeV D 73 = 3768.45 ± 0.4 mm RTM 3 D 73 turn 73 Cavity 2 turn 90 Cavity 1 ΔEΔE E 73 = 751.649 ± 0.107 MeV Extrapolate to end energy: E out = 883.X ± 0.16 MeV (FWHM) σ Eout = 68 keV E out = E in + z·ΔE, z = 90E in = 180 MeV Dipole 1Dipole 2 where e is electron charge, B is dipole magnetic field, D - electron track diameter.

22. A.Nikolaev, BonnCB Meeting, Basel 10/5/0622 Proton missing mass distributions  p → p   p→p  (  → 2  )  p→p  (  → 3   ) Microscope Main tagger Missing mass / MeV hits

23. A.Nikolaev, BonnCB Meeting, Basel 10/5/0623 Photon beam position Jan 2005

24. A.Nikolaev, BonnCB Meeting, Basel 10/5/0624 The  mass beamtimes  Two beamtimes for the  mass measurement:

25. A.Nikolaev, BonnCB Meeting, Basel 10/5/0625 MAMI energy measurements

26. A.Nikolaev, BonnCB Meeting, Basel 10/5/0626 Result dependence on fit range η→2γη→3π 0  1. η→2γ: σ = 0.023 MeV (in m η )  2. η→3π 0 : σ = 0.025 MeV (in m η )  Function fits the points very good! 0.1 MeV Fit range: 0 to 75 μch 10 to 75 μch 20 to 75 μch 30 to 75 μch 40 to 75 μch 50 to 75 μch 60 to 75 μch 70 to 75 μch Fit range: 0 to 76 μch 10 to 76 μch 20 to 76 μch 30 to 76 μch 40 to 76 μch 50 to 76 μch 60 to 76 μch 70 to 76 μch

27. A.Nikolaev, BonnCB Meeting, Basel 10/5/0627 Result dependence on fit range η→2γη→3π 0  1. η→2γ: σ = 0.023 MeV (in m η )  2. η→3π 0 : σ = 0.100 MeV (in m η )  Function fits the points good! 0.1 MeV Fit range: 0 to 75 μch 10 to 75 μch 20 to 75 μch 30 to 75 μch 40 to 75 μch 50 to 75 μch 60 to 75 μch 70 to 75 μch Fit range: 0 to 75 μch 10 to 75 μch 20 to 75 μch 30 to 75 μch 40 to 75 μch 50 to 75 μch 60 to 75 μch 70 to 75 μch