Göttingen 1997 – wissenschaftliches Programm
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HK: Hadronen und Kerne
HK 6: Elektromagnetisch wechselwirkende Sonden I
HK 6.5: Vortrag
Montag, 24. März 1997, 16:00–16:15, HS G
The N → Δ Quadrupole Transition — •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
QCD predicts tensor forces between quarks. Consequently, baryons should be deformed. Experimentally, the C2 and E2 multipoles of the process γ + N → Δ are clearly nonzero [1]. In the single-quark transition model this is usually interpreted in terms of small D-state admixtures in the nucleon and/or Δ wave function. Here, we show that two-body exchange currents lead to a N→ Δ transition quadrupole moment of the right sign and magnitude even if the 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. Our main result is that the measured E2(C2)-amplitude is predominantly due to a two-quark Δ S=2 spin-flip transition (exchange currents) rather than due to D-waves in the nucleon (deformation). We obtain for the E2/M1 ratio E2 /M1 =−0.035, consistent with a recent dispersion-theoretical analysis [3].
[1] G. Beck, Baryons 95, Santa Fé, (1995); F. Kalleicher, PhD thesis, Univ. Mainz, 1993
[2] A. J. Buchmann, E. Hernández and Amand Faessler, nucl-th/9610040; Phys. Rev. C55 (1997) Vol.1
[3] O. Hanstein, D. Drechsel and L. Tiator, nucl-th/9605008