We report a first measurement of the spin-correlation parameter A zz in neutron-proton scattering at 67.5 MeV. The results, obtained in the angular range 105° < 0 cm . ^ 170° with typical accuracies of 0.008, are highly sensitive to the l S\-l D\ mixing parameter e\. A phase-shift analysis based on the current world data yields a value of e\ significantly higher than predicted by modern potential models.PACS numbers: 21.30.+y, 13.75.Cs, 13.88.+e, 25.10.+S One of the long-standing problems in our understanding of the fundamental nucleon-nucleon interaction is the poor knowledge of the isoscalar tensor force. This force mixes states with different angular momenta L= S J±\ (with J the total angular momentum). It is directly related to the 3 S\-3 D\ mixing parameter €\ in neutronproton scattering and the Z)-state admixtures of light nuclei.Since the binding energies of few-nucleon systems are highly sensitive to the isoscalar tensor force, the poor knowledge of €\ has important implications concerning the accuracy of binding-energy calculations, and the question of whether nuclear binding energies can quantitatively be understood in terms of two-nucleon forces. Exact calculations of the triton binding energy show that without three-body forces the measured binding energy can only be reproduced for a weak tensor force. 1 The binding energy of nuclear matter as well depends strongly on the isoscalar tensor interaction. x For the study of non-nucleonic degrees of freedom of nuclei a precise knowledge of S-D transitions is also a prerequisite. The contribution of mesonic degrees of freedom to electromagnetic form factors of the A = 2,3 systems-the observables most sensitive to these degrees of freedom-is of nearly equal size, but of opposite sign as the effects of the S-D transition. A determination of the former requires reliable information on the latter.The problem 2,3 with e\ is a twofold one: (a) only higher-order spin observables such as spin-correlation or spin-transfer parameters in n-p scattering are sensitive to €\\ these quantities are difficult to measure; (b) these observables in most cases are also sensitive to the other poorly determined quantity, the l P\ phase. As a result, the values of e\ and ] P\ which have been extracted from phenomenological phase-shift analyses (PSA) exhibit large uncertainties. A more accurate determination of e\ in the energy region relevant for nucleons in nuclei ik}l 2m -35 MeV) thus would be of high interest.Condition (a) requires precise experiments in which the spins of at least two of the reaction partners are measured. Recently, measurements have been performed for the spin-correlation coefficient A yy between 18 and 50 MeV. 4 The present Letter reports the first measurement below 400 MeV of the spin-correlation parameter A zz (0), the observable most sensitive to €\. 2 Condition (b) calls for the measurement of complementary quantities to pin down other phases, especially X P\. In order to reduce the amount of ambiguity in a PSA we have measured the differential cros...

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