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Proton-Antiproton Photoproduction on a Nucleon

Proton-Antiproton Photoproduction on a Nucleon. Andrei Afanasev Hampton University and Jefferson Lab. Plan of Talk. Available data on p-pbar photoproduction Theoretical interpretation and problems addressed Near-threshold p-pbar interactions; hadronic spectroscopy

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Proton-Antiproton Photoproduction on a Nucleon

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  1. Proton-Antiproton Photoproductionon a Nucleon Andrei Afanasev Hampton University and Jefferson Lab

  2. Plan of Talk • Available data on p-pbar photoproduction • Theoretical interpretation and problems addressed • Near-threshold p-pbar interactions; hadronic spectroscopy • Cross-channels for hard exclusive reactions; time-like form factors • Motivation for new measurements

  3. Proton-antiproton spectroscopy

  4. Recent Claims for P-antiP states

  5. BES Data Interpretation • BES Collab (2003): Near-threshold enhancement may be interpreted as a JPC= 0-+ or 0++ state with M≈1850MeV and width <30 MeV • Sibirtsev et al., PRD71, 054010 (2005); Haidenbauer et al, PRD74, 017501 (2006); p-pbar invariant-mass enhancement can be reproduced by the final-state p-pbar interactions; but other interpretations are not ruled out • Entem, Fernandez, PRD75, 014004 (2007): final-state 3P0 Figure from Haidenbauer et al.

  6. Cross Section Data on From Bodenkamp et al, PL 133B, 275 (1983); also claimed observation of X(2020) signal in p anti-p invariant mass

  7. CLAS Data vs Phase Space(comparison by A. Sibirstev) Enhancement at smaller invariant mass

  8. Theoretical estimates for the cross section(Afanasev, in progress) • Pion-exchange in t-channel • Constraints on the sub-process amplitude obtained from annihilation at rest; BR (=1.5% measured at CERN PS) assumed energy-independent

  9. Comparison with CLAS datafrom B. Stokes, APS Meeting Talk, Nashville, TN, Oct. 2006

  10. Theoretical Progress • Theory work in progress: Afanasev, Sibirtsev • Small invariant masses of p-pbar: spectroscopy, low-energy p-pbar interactions • Experimental requirements: PWA analysis to identify quantum numbers of states responsible for threshold enhancement=> need good energy and angular resolution, high statistics; higher beam energy provides kinematic separation of t-channel exchange; linear polarization for t-exchange naturality meas. • Large invariant masses: cross-channels for hard exclusive reactions, GPDs; reggeized baryon exchange • Experiment: higher energies, angular resolution, luminosity (to compensate for small cross sections)

  11. Hard Exclusive Reactions • 1973: Constituent counting rules: Brodsky, Farrar; Matveev, Muradyan, Tavhelidze • 1979: QCD for exclusive reactions: Efremov, Radyushkin; Brodsky, Lepage • 1996: GPDs: Ji, Radyushkin, et al.

  12. Belle Collab., PL B621, 41 (2005) • Hard exclusive kinematics • Compared with Kroll et al. • calculations • Also γγ→pion and kaon pairs • From BELLE (2006)

  13. Belle (2005)

  14. Nucleon Time-Like Form Factors • Process e+e-→p+pbar, most recently studied by BABAR Collab, PRD73, 012005 (2006) • Scanned through a broad range of p-pbar invariant masses

  15. Compton Scattering on a Pion • Coriano, Radyushkin, Sterman, Nucl.Phys.B405:481-506,1993, QCD sum rules for γπ→γπ • Afanasev, hep-ph/9808291; predicted scaling violations in γπ→γπ using pion GPDs from hep-ph/9808291

  16. Summary • Photoproduction of p-pbar pairs allows studies of • p-pbar interaction near threshold; hadronic spectroscopy • Access cross-channels of hard exclusive reactions on a nucleon • Compton scattering on a pion at large transverse momenta: a test of the mechanism of hard exclusive scattering

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