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HK: Physik der Hadronen und Kerne

HK 44: Elektromagnetische und hadronische Proben VII

HK 44.5: Vortrag

Donnerstag, 22. März 2001, 15:45–16:00, C

Virtual Compton Scattering on a Proton as a probe of the spontaneously broken Chiral Symmetry of QCD — •T.R. Hemmert¹, B.R. Holstein², G. Knöchlein³ und D. Drechsel³ — ¹Physik-Department T39, TU München — ²Physics Dept., UMASS, Amherst MA, USA — ³Inst. f. Kernphysik, Universität Mainz

The pioneering experiment on Virtual Compton Scattering (VCS) off a Proton e p→ e^′p^′γ at MAMI is now analyzed [1]. The extracted values for the response functions are in good agreement with the leading order predictions from chiral perturbation theory [2]. In this contribution we want to discuss how the physics of the spontaneously broken chiral symmetry of QCD manifests itself in VCS via the concept of “generalized polarizabilities” (GPs) of the nucleon, introduced in [3]. Chiral effective field theories demand that there exists a low energy regime where the momentum-dependence of the GPs is governed by the mass of the pion—the Goldstone boson of the low energy QCD, with a small mass due to the additional explicit breaking of chiral symmetry via the mass terms of the u,d quarks in the QCD Lagrangian. We attempt to give an inuitive picture of the physics behind the GPs by discussing their multipole content, associated Feynman diagrams and their Fourier-transform. At higher values of momentum transfer in VCS, the low energy contribution from the chiral “pion-cloud” should be gradually overwhelmed by standard resonance physics. The surprising find of the MAMI experiment was that, nevertheless, even at the relatively large momentum transfer of Q²=0.33GeV², the chiral physics still gives a good description for the measured response functions ! We attempt to give an explanation of this fact by analyzing the leading order contribution of the Δ(1232) resonance to the GPs [4].

[1] J. Roche et al., Phys. Rev. Lett. 85 (2000) 708

[2] T.R. Hemmert et al., Phys. Rev. Lett. 79 (1997) 22

[3] P.A.M. Guichon, G.Q. Liu and A.W. Thomas, Nucl. Phys. A591 (1995) 606

[4] T.R. Hemmert et al., Phys. Rev. D62 (2000) 014013