Δ Excitations in Nuclei

  • T. Udagawa
  • P. Oltmanns
  • F. Osterfeld

Abstract

It has been known for sometime that the peak position of the Δ (1232) -resonance spectra observed in the (p,n)-1, (3He,t)-2, and (\(\vec d\),2p)-3charge-exchange reactions at intermediate energies is systematically shifted downward for a target with A ≥ 12 as compared to the peak position for a proton target. In contrast to this, in the case of γ-absorption4 and inelastic electron scattering experiments,5 the Δ-peak does not show such a pronounced displacement. The electromagnetic probes excite the Δ transversely, i.e., by the transition operator \(\vec S \times \vec q\vec T\) (\(\vec S\) and \(\vec T\) are the spin and isospin transition operators, respectively.), while the hadronic probes measure both the transverse (TR) and the longitudinal (LO) spin-isospin response. It has thus been speculated that the shift of the Δ-peak would be due to a nuclear medium effect in the isovector spin LO(\(\vec S \cdot \vec q\vec T\))channel.6,7 That is, if the delta particle-nucleon hole (ΔN -1) interaction becomes strongly attractive at large momentum transfers \(\left| {\vec q} \right| \approx 1 - 2\) fm-1 in this channel, then this attraction might lead to a lowering of the Δ mass produced in the target. Along this line of reasoning, no shift of the Δ-peak position is to be observed with the electromagnetic probes. Recently, we have made some realistic calculations of the spectra of the 12C(p,n) and (3He,t) reactions at the Δ-excitation region, and succeeded to show that the LO response is indeed shifted downwards in energy and that this shift is caused by the energy-dependent π-exchange interaction.8

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Copyright information

© Springer Science+Business Media New York 1991

Authors and Affiliations

  • T. Udagawa
    • 1
  • P. Oltmanns
    • 2
  • F. Osterfeld
    • 2
  1. 1.Department of PhysicsUniversity of TexasAustinUSA
  2. 2.Institut für KernphsikForschungszentrumJülichGermany

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