Measurement of the η-mass from the reaction γp pη in threshold region

Transcription

1 Measurement of the η-mass from the reaction γp pη in threshold region A. Nikolaev for the Crystal MAMI und A collaborations Helmholtz Institut für Strahlen- und Kernphysik, University Bonn CB Collaboration Meeting '07 1

2 Measurement of the η-mass at MAMI Measurment of the η-mass from the reaction: Relation between η-mass m η and threshold energy E γ thr : γ + p p + η m η = m m m p : proton mass p + p + m p E thr γ c Recently published measurements: A. Duane et al.: Phys. Rev. Lett. 3 (197) π - p n neutr. F. Plouin et al.: Phys. Lett. B76 (199) dp 3 He η B. Krusche et al.: Z. Phys. A351 (1995) γp pη A. Lai et al.: Phys. Lett. B533 (00) η 3π 0 M. Abdel-Bary et al.: Phys. Lett. B 619 (005) dp 3 He Χ E. Ambrosino et al.: Acta Phys. Slov. 56 (006) φ γη PDG 006 η-mass MAMI KLOE

3 Crystal Ball/TAPS detector Particle identification detector (PID) γ-beam Crystal Ball 67 NaI crystals TAPS 510 BaF crystals Crystal Ball detector: γ(p,p)η (η γ) γ(p,p)η (η 3π 0 ) Determination of the E γ und Ө γ Particle identification detector: Cylinder of 4 plastic detectors 30 x x 0. cm Identification of charged particles Forward wall detector TAPS: 510 BaF crystals 510 thin plastic veto detectors BaF detector Lightguide BaF scintillator Plastic scintillator (Veto) Photomultiplier Photomultiplier

4 Photons tagging facility (tagger) Incident electron beam from MAMI with energy E 0 and σ(e 0 ). Scattered electron with energy E e- and σ(e e- ) (microscope tagger). Energy of the photon is determined (tagged) from: Eγ = E0 Ee Properties σ ( E γ ) = σ ( E0) + σ ( Ee σ(e 0 ) = 68 kev (A. Jankowiak, K.-H. Kaiser) ) Electron beam energy Photon energy range Energy resolution Tagging rate 883 MeV MeV MeV 10 8 γ/s

5 Microscope tagger 96 plastic scintillator fibers (3x mm). 1/3 overlap of fibers with neighbor. Overlap regions define microscope channels µch (191 channels). Focal plane position 1 mm Aluminium shield Energy resolution is 0.3 MeV per microscope channel (µch). Microscope tagger covers electron energies MeV. With beam energy E 0 =883 MeV this corresponds to photons in energy range MeV (η-threshold ~707 MeV)

6 Energy calibration of the microscope tagger Direct energy calibration: Direct position measurement of the MAMI beam with energies E 0 = MeV, 195. MeV and 10. MeV in the same magnetic field as in the experiment (B exp = T). Scan beam through microscope varying the dipole magnetic field at fixed beam energy E 0 : Increase tagger dipole magnetic field B cal in small steps. Measure new position of the electron beam: E = E e 0 B B exp cal

7 Energy calibration of the microscope tagger Equivalent electron energy / MeV 10 Red: MeV, Green: MeV, Cyan: MeV, Blue: MeV, Purple: MeV, Black: MeV, MeV T E γ =707 MeV 10. MeV T 195. MeV T Microscope channel Blue arrows show direct position measurements of the electron beam of energies 180.1, 195. and 10. MeV. Standard deviation of data from fit line # Energy, MeV σ, kev ± ± ± ± ± ± σ mean 77.1 σ ( thr E γ ) = (68 kev) + (77 σ(m η ) = 65 kev kev) = 103 kev

8 Absolute energy measurement of the MAMI beam E = e c B 73 R 73 R 73 Dipole No.1 Turn 90 XYMO90 Dipole No. B dipole magnetic field R 73 radius of the 73 rd turn Turn 73 XYMO73 B = T ± 0.13 mt D = ± 0.4 mm D E E out = Ein in + z E, z = 90 E = 180 MeV E 73 = ± MeV (FWHM) RTM Extraction energy value of the electron beam is extrapolated: E 90 = E 73 + E 90 = (883.X ± 0.068) MeV (σ) A. Jankowiak, K.-H. Kaiser

9 Data analysis Identification of the η-meson. Preliminary η-mesons yield. Simulation of the CB/TAPS efficiency. Determination of the photon flux. Preliminary cross section of the γ(p,p)η. Determination of the η-meson mass

10 Identification of the η-meson uncharged decay modes of the η- meson: η γ (39.38 ± η 3π 0 (3.51± 0.6)% 0.8)% γ (with or without proton) invariant mass distribution. Counts γ(p,p)η (3π 0 ) Mean = 549 MeV FWHM = 38 MeV + Data + Simulation 3π 0 (6γ, with or without proton) invariant mass distribution. Cut on the invariant mass of the η-meson. Cut on the proton missing mass. Cut on the coincident events: CB and hit in microscope tagger. hits Invariant mass / MeV Coincidence between CB and microscope tagger FWHM = 1.7 ns Sigma = 0.7 ns prompt random Microscope time, ns

11 Preliminary η-yield: γ(p,p)η Number of identified η-mesons versus the photon energy in January 005. Without acceptance correction. N of detected η arbitrary units γ(p,p)η(γ) γ(p,p)η(3π 0 ) PRELIMINARY Photon Energy / MeV Photon Energy / MeV Statistics: Dec-04 Jan-05 Hours 74 h 114 h Current 35 na 1 na Main tagger: Dec-04 Jan-05 η γ η 3 π Microscope tagger: Dec-04 Jan-05 η γ η 3 π

12 Simulation of the CB/TAPS efficiency: γ(p,p)η Distribution of original η Isotropic angular distribution of the original η-mesons. Simulation of both η-decay modes in CB/TAPS tracking programm. Reconstruction of simulated η-mesons. CB/TAPS efficiency CB/TAPS efficiency γ(p,p)η (γ) E γ =707 MeV

13 Determination of the photon flux (in microscope) The cross section for γ(p,p)η (i - microscope channel): where N γ [i] -??? N γ [i] was found from the total cross section for γ(p,p)π 0 : Total cross section σ π 0[i] was found with main tagger. γ(p,p)π 0 total cross section Reconstructed γ-flux ], [ ], [ ] [ 1 ], [ ) det( cm cm cm η i i N i N C i d d θ ε θ θ σ γ η γ η γ η = Ω ], [ ], [ ] [ 1 ] [ ) det( cm cm i i N i C i N cm θ ε θ σ γ π γ π θ π π γ =

14 Preliminary cross sections: γ(p,p)η N of target protons: /µbarn DCS: E γ =715.9 MeV: Jan 005 PRELIMINARY PRELIMINARY Krusche - TAPS This work (η γ) DCS: E γ =715.9 MeV: Jan 005 Krusche - TAPS This work (η 3π 0 ) Total CS: Dec 004 PRELIMINARY Total CS: Jan 005 PRELIMINARY Krusche TAPS This work (η γ) This work (η 3π 0 ) Krusche TAPS This work (η γ) This work (η 3π 0 )

15 Preliminary result η-mass: γ(p,p)η N of detected η arbitrary units γ(p,p)η (γ) γ(p,p)η (3π 0 ) MeV MeV PRELIMINARY MAMI result is in the region of NA48 and KLOE values. m η = ± 0.10 ± 0.07 Very preliminary MeV 0 m m m NA48 η KLOE η COSY η Photon Energy / MeV Photon energy / MeV = ± = ± = ± 0.08 stat. stat. stat. ± ± ± 0.03 syst. syst. syst. MeV/c MeV/c MeV/c Eta Masse / MeV Duane PDG 006 NA48 KLOE SATURNE MAMI COSY MAMI PRELIMINARY Jahr

16 Preliminary cross sections: γ(p,p)η Krusche TAPS: m η = MeV This work (prelim.): m η = MeV Tagger energy calibration in the experiment by B.Krusche probably was ~1.01 MeV too low? Total CS: Dec 004 Total CS: Dec 004 PRELIMINARY Krusche - TAPS points shifted MeV PRELIMINARY Krusche TAPS This work (η γ) This work (η 3π 0 ) Krusche MeV This work (η γ) This work (η 3π 0 )

17 Results η-mass: fit CS with sqrt MeV MeV Total CS: Dec 004 PRELIMINARY σ ( Eγ ) = a0 + a1 ( Eγ a) Full microscope resolution (res. 300 kev) Border of fit range η γ, MeV η 3 π, MeV ± ± ± ± µch 1 channel (res. 1. MeV) Border of fit range η γ, MeV η 3 π, MeV ± ± ± ± MeV MeV Total CS: Jan 005 PRELIMINARY σ ( Eγ ) = a0 + a1 ( Eγ a) Full microscope resolution (res. 300 kev) Border of fit range η γ, MeV η 3 π, MeV 710 MeV ± MeV ± no solution no solution 4 µch 1 channel (res. 1. MeV) Border of fit range η γ, MeV η 3 π, MeV 715 MeV ± MeV ± no solution no solution

18 Results η-mass: fit S /3 S /3 : Dec 004 Dec 004 S /3 : Jan 005 Jan

19 Conclusion Measurement of the η-mass from the reaction γ p p η in the region of production threshold E γ thr. Precise determination of the photon energy E thr γ : E E Eγ = 0 e Absolute calibration of E e - (microscope tagger) is finished. Absolute calibration of E 0 : final checks for systematical error underway. Preliminary value for the η-mass from the Crystal Ball experiment at MAMI: m η = ± 0.10 ± 0.07 Very preliminary MeV

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