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Precision Studies of Light Mesons with COMPASS

Precision Studies of Light Mesons with COMPASS. Bernhard Ketzer Technische Universit ät München f or the COMPASS Collaboration XV International Conference on Hadron Spectroscopy Nara, Japan 5 November 2013. Light Mesons. Quark model: bound state of SU(3) flavor :

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Precision Studies of Light Mesons with COMPASS

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  1. Precision Studies of Light MesonswithCOMPASS Bernhard Ketzer TechnischeUniversitätMünchen for the COMPASS Collaboration XV International Conference on HadronSpectroscopy Nara, Japan 5 November 2013

  2. Light Mesons • Quark model: • bound state of • SU(3)flavor: • color singlets • Quantum numbers: • measured: IG (JPC) • quark model: 2S+1LJ • S=S1+S2 , J=L+S L S1 S2 q q

  3. Light Mesons • Quark model: • bound state of • SU(3)flavor: • color singlets • Quantum numbers: • measured: IG (JPC) • quark model: 2S+1LJ • S=S1+S2 , J=L+S Allowed combinations L S1 S2 q q

  4. Light Mesons = + + Molecule / 4 quarks Allowed combinations + Hybrids “Forbidden” combinations Glueballs + ...

  5. Light Mesons

  6. Light Mesons [R.J. Jaffe, PRD 15, 267 )1977]

  7. The COMPASS Experiment MuonWall SM2 E/HCAL E/HCAL MuonWall SM1 Dipole magnets Tracking detectors RICH El.-mag. calorimeter Hadronic calorimeter Muon identification Target RICH Beam • Two-stage spectrometer • large angular acceptance • broad kinematical range • ~250000 channels • > 1000 TB/year 50 m [COMPASS, P. Abbon et al., NIM A 577, 455 (2007)]

  8. The COMPASS Experiment • Diffractive dissociation • Isovectorstates: • Hybrids, exotics • Scalar states as isobars • Photoproduction • Polarizabilities: • Radiativecouplings: Talks by M. Krämer, S. Huber (Fri, l8-4) Talks by T. Schlüter S. Uhl • Central production: • Scalar states • Glueballs A. Austregesilo, (Tue, l4-3)

  9. 3p Final State - Kinematics

  10. 3p Final State - Kinematics

  11. Isobar Model

  12. Isobar Model

  13. 2-D Partial Wave Analysis • PWA of angular distributions in 20 MeV mass bins and 11 t’ bins • 80 waves with positive reflectivity • 7 waves with negative reflectivity • Flat wave added incoherently • Well-known states as isobars

  14. 2-D Partial Wave Analysis PWA of angular distributions in 20 MeV mass bins and 11 t’ bins not used

  15. 2-D Partial Wave Analysis PWA of angular distributions in 20 MeV mass bins and 11 t’ bins • 2. Mass-dependent c2 fit to spin density matrix from step 1 • 6 waves • Parameterized by Breit-Wigner • Coherent non-resonant background • Waves included: 21 distributions x 100 mass bins x 11 t’ bins, 352 free parameters

  16. Fit of Spin-Density Matrix Intensities Phase differences Low

  17. Fit of Spin-Density Matrix Intensities Phase differences High

  18. 1++0+r(770)p S 11 slices Incoherent sum

  19. 1++0+r(770)p S 11 slices Incoherent sum

  20. New a1 - 1++0+ f0(980)pP 11 slices Incoherent sum

  21. New a1(1420) - Phase Differences fit range Low

  22. Results of Fit to Spin-Density Matrix • precise determination of resonance parameters • mass and width ranges from systematic studies with different fit models • final result and syst. error still under study

  23. (pp)S Wave • Systematic study: • New a1(1420) couples exclusively to f0(980) • Influence of isobar parameterization? Ambiguities? Artefact? • Goal: • Determine dynamics of isobars with from data • Obtain model-independent isobar amplitude • Method: • Replace fixed parameterization of 2-body amplitude (e.g. AMP for (pp)S wave or Flatté/BW for f0(980)/f0(1500)) by set of free (complex) parameters in 2-body mass bins with dm=40 MeV (10 MeV around 980 MeV) • No separation into several isobars • Amplitude for isobars determined from data for three • Combined phase information:

  24. Correlation m3pvs m2p Low High

  25. (pp)S Amplitude in 0-+0+ (pp)Sp S Intens. Phase wrt

  26. (pp)S Amplitude in 1++0+ (pp)Sp P Intens. Phase wrt

  27. State of the Art Lattice QCD ? [J. Dudeket al., Hadron Spectrum Collaboration, Phys. Rev. D 83, 111502 (2011)]

  28. Primakoff Reactions • Contributions at very low • Diffraction: • only for • Primakoff: •  (quasireal photon exchange) • radiativewidthsofresonances

  29. Radiative Widths of a2 and p2 • Theory: •  M2 transition • VMD model: • Relativistic quark model: • Covariant oscillator quark model: •  E2 transition • Covariant oscillator quark model: [Rosner 1981] [Aznauryan 1988] [Ishida 1989, Maeda 2013] [Maeda 2013] • Previous experiments: • May et al. • E272: • SELEX •  no publishedresultyet [E.N. May et al., PRD 16, 1983 (1977)] [S. Cihangiret al., PLB 117, 119 (1982)] [V.V. Molchanov et al., PLB 521, 171 (2001)]

  30. PWA in Bins of

  31. Extraction of Radiative Widths Primakoffproduction and decay of broad resonance (mass m0, spin J): Radiative width: Integrate differential cross section: Experiment: determinePrimakoff cross section

  32. Radiative Widths COMPASS results (preliminary): First measurement!

  33. Pion Polarizabilities • Describe response of composite system to external e.m. fields •  LO deviation from point-like particle • electric polarizability • magnetic polarizability Chiral PT prediction: [Gasser et al., Nucl. Phys. B 745, 84 (2006)]

  34. Pion Polarizabilities • Describe response of composite system to external e.m. fields •  LO deviation from point-like particle • electric polarizability • magnetic polarizability Chiral PT prediction: [Gasser et al., Nucl. Phys. B 745, 84 (2006)] Experiments: p Compton scattering PLUTO DM1 DM2 Mark II Lebedev Mami A2 Serpukhov

  35. Pion Polarizabilities • Describe response of composite system to external e.m. fields •  LO deviation from point-like particle • electric polarizability • magnetic polarizability Chiral PT prediction: [Gasser et al., Nucl. Phys. B 745, 84 (2006)] Experiments: p Compton scattering PLUTO DM1 DM2 Mark II Lebedev Mami A2 Serpukhov COMPASS

  36. Measurement at COMPASS • Exclusive reaction • Momentum transfer •  60k events • Control measurements with Extract polarizability from cross section ratio , assuming

  37. Polarizability Results COMPASS preliminary result:

  38. Polarizability Results COMPASS preliminary result:

  39. Conclusions • COMPASS has collected large data samples on diffractive and • photoproduction of light mesons • New analysis method established: 2D-PWA in bins of and • Identify resonant and non-resonant contributions to partial waves • Precise extraction of resonance parameters • New model-independent method to determine isobar amplitude • First application: wave • Information on scalar isobars, e.g. f0(980) • Strong dependence on motherwaveand • No unique parameterization yet • Possibility to measure phases in decays

  40. Conclusions II • New axial vectormesonobserved: • Phase difference close to 180° withrespecttoreferencewaves • Phase behavioroppositetocoupling of and •  Strong component? • Precision determination of parameters of “known” resonances

  41. Conclusions III • Radiativewidths • New measurement for • First determination of • New measurement of pion polarizability •  In agreement with ChPT More results to come in the near future…

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