Andrei Afanasev, Workshop on Photon-Hadron Physics with GlueX, 3/6/08

Proton-Antiproton Photoproductionon a Nucleon

Andrei Afanasev

Hampton University and Jefferson Lab

Andrei Afanasev, Workshop on Photon-Hadron Physics with GlueX, 3/6/08

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

Andrei Afanasev, Workshop on Photon-Hadron Physics with GlueX, 3/6/08

Proton-antiproton spectroscopy

Andrei Afanasev, Workshop on Photon-Hadron Physics with GlueX, 3/6/08

Recent Claims for P-antiP states

Andrei Afanasev, Workshop on Photon-Hadron Physics with GlueX, 3/6/08

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.

Andrei Afanasev, Workshop on Photon-Hadron Physics with GlueX, 3/6/08

Cross Section Data on pppp

From Bodenkamp et al, PL 133B, 275 (1983); also claimedobservation of X(2020) signal in p anti-p invariant mass

Andrei Afanasev, Workshop on Photon-Hadron Physics with GlueX, 3/6/08

Andrei Afanasev, Workshop on Photon-Hadron Physics with GlueX, 3/6/08

Andrei Afanasev, Workshop on Photon-Hadron Physics with GlueX, 3/6/08

CLAS Data vs Phase Space(comparison by A. Sibirstev)

Enhancement at smallerinvariant mass

Andrei Afanasev, Workshop on Photon-Hadron Physics with GlueX, 3/6/08

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

pp0

Andrei Afanasev, Workshop on Photon-Hadron Physics with GlueX, 3/6/08

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

Andrei Afanasev, Workshop on Photon-Hadron Physics with GlueX, 3/6/08

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)

Andrei Afanasev, Workshop on Photon-Hadron Physics with GlueX, 3/6/08

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.

Andrei Afanasev, Workshop on Photon-Hadron Physics with GlueX, 3/6/08

Belle Collab., PL B621, 41 (2005)

•Hard exclusive kinematics•Compared with Kroll et al. calculations

•Also γγ→pion and kaon pairsFrom BELLE (2006)

Andrei Afanasev, Workshop on Photon-Hadron Physics with GlueX, 3/6/08

Belle (2005)

Andrei Afanasev, Workshop on Photon-Hadron Physics with GlueX, 3/6/08

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

Andrei Afanasev, Workshop on Photon-Hadron Physics with GlueX, 3/6/08

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

Andrei Afanasev, Workshop on Photon-Hadron Physics with GlueX, 3/6/08

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