Bochum 1998 – wissenschaftliches Programm
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HK: Hadronen und Kerne
HK 61: Electromagnetic Probes VIII, Properties of Nucleon, Pion
HK 61.1: Gruppenbericht
Donnerstag, 19. März 1998, 14:00–14:30, B
The Origin of Deformation of the Nucleon — •A.J. Buchmann1, E. Hernández2, and Amand F"a"sler1 — 1Institut f"ur Theoretische Physik, Universit"at T"ubingen, D-72076, T"ubingen — 2Grupo de Fisica Nuclear, Universidad de Salamanca, E-37008 Salamanca, Spain
There is now unambiguous experimental evidence that the nucleon is deformed [1]. QCD predicts tensor forces between quarks. Consequently, all baryons should be deformed. However, one needs an unrealistic D-state probability of PD ∼ 30 % in order to explain the new data. This is in contrast to all constituent quark model calculations which give PD ∼ 0.5%. Here, we show that two-body exchange currents between quarks lead to a N→ Δ transition quadrupole moment of the right sign and magnitude even if the valence quark wave functions have no D-state admixtures. We derive a parameter-free relation between the N → Δ transition quadrupole moment and the neutron charge radius: QN → Δ= 1/√2 rn2 [2]. We find that the measured E2(C2)-amplitude is predominantly due to a two-quark spin-flip transition induced by gluon and pion exchange currents. Thus, the deformation of the nucleon lies mainly in the pion and gluon degrees of freedom, and not in the valence quark distribution. This interpretation is new for the constituent quark model.
[1] R. Beck et al., Phys. Rev. Lett. 71 (1997) 606; G. Blanpied et al. , Phys. Rev. Lett. 79 (1997); F. Kalleicher et al., Z. Phys. A, submitted
[2] A. J. Buchmann, E. Hernández and Amand Faessler, Phys. Rev. C55 (1997) 448