The Continuum Spin Response to Intermediate Energy Protons at Low Momentum Transfer
Let us start with some interesting new data. Shown in Fig. 1 are the spin-longitudinal (S L ) and spin-transverse (S T ) spin-flip probabilities for proton scattering from 40Ca at 500 MeV and 580 MeV. They are the thesis data of Andrew Green. The momentum transfer q here is small, about 100 MeV/c. The excitation energies ω, while fairly large, are still in the region where nuclear structure, nuclear collectivity, is expected to be important. The quasielastic peak is not seen at these momentum transfers because it would appear in the region of the giant resonances or even below. But the same one-step scattering mechanism which excites one- particle one- hole states in first order and yields the quasielastic peak at high q is responsible for continuum excitation, including the giant resonances, at low q. The data of Fig. 1 show that the probe plus the nucleus yield approximately equal strengths in the longitudinal and transverse channels. This may not seem surprising. But, if we divide the values of S L /S T from Fig. 1 by the values of S L /S T from isospin averaged NN scattering amplitudes,2 we get the ratios of around 4.0 shown in Fig. 2. These may seem startling, particularly when they are compared to values slightly less than unity measured by Rees et al. at much higher momentum transfer (350 MeV/c) for a very similar ratio.1 Is the long sought pionic (ie, spin-longitudinal) enhancement in nuclei to be found at low q instead of high q? The answer is no, of course, but it will take awhile, even into the next talk by Jochen Wambach, to see why. We first have to review the measurements and interpretation that led us to measure S L and S T .
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