N ov 1 99 9 The ∆ - excitation in the nuclear charge longitudinal response
نویسندگان
چکیده
The presence of a resonant structure corresponding to the ∆ exci-tation in the longitudinal response to an e.m. probe is investigated. It is shown that many-body effects could significantly increase the relativistic contribution suggested by M.Ericson and coworkers 1 Short introduction Few years ago Chanfray et al.[1] showed that the ∆(3, 3) excitation is present also in the longitudinal response of a nucleus to an electromagnetic (e.m.) probe. The key issue in [1] is elegant and simple. Let us assume that a nucleon at rest is excited to a ∆ through a M1 excitation. The above is purely transverse but since the longitudinal/transverse separation is frame-dependent, a (relativistic) boost is able to give, in a moving frame, a longitudinal component too. Since in a nucleus the nucleons are moving thanks to the Fermi motion, then the conversion of a single (moving) nucleon to a ∆ contains necessarily a (small) longitudinal component.
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