Spin correlations, QCD color transparency, and heavy-quark thresholds in proton-proton scattering.

The strikingly large spin-spin correlation ANN observed in pp elastic scattering at plab = 2.5 GeV/c and 11.75 GeV/ c and the unexpected energy dependence of absorptive corrections to quasi-elastic proton-proton scattering in a nuclear target can be interpreted in terms of two J = L = S = 1, B = 2 resonance structures associated with the strange and charmed particle production thresholds, interfering with a perturbative QCD background. The results provide support for the “color transparency” phenomenon predicted in perturbative QCD away from resonances or heavy quark thresholds. Submitted to Physical Review Letters * Work supported by the Department of Energy, contract DE-AC03-76SF00515. One of the most serious challenges to quantum chromodynamics is the behavior of the spin-spin correlation asymmetry ANN = &+$T%#j measured in large momentum transfer pp elastic scattering.’ At Pl& = 11.75 GeV/c and 8,, = 7r/2, ANN rises to N SO%, corresponding to four times more probability for protons to scatter with their incident spins both normal to the scattering plane and parallel, rather than normal and opposite. Moreover, the data show a striking energy and angular dependence not expected from the slowly-changing perturbative QCD predictions.2 The onset of this apparently new phenomenon3 at s N 23 GeV2 appears to signal new degrees of freedom or exotic components in the two-baryon system. In this letter we propose an explanation of (1) th e o b served spin correlations, (2) the deviations from fixed-angle scaling laws, and (3) th e anomalous energy dependence of absorptive corrections to quasi-elastic pp scattering in nuclear targets: in terms of a simple model based on two J = L = S = 1 broad resonances (or threshold enhancements) interfering with a perturbative QCD quark-interchange background amplitude. The structures in the pp + pp amplitude may be associated with the onset of strange and charmed thresholds. If this view is correct, large angle pp elastic scattering would have been virtually featureless for pl&, 2 5 GeV/c, had it not been for the onset of heavy flavor production. As a further illustration of the threshold effect, we also show the effect in ANN due to a narrow 3F3 pp resonance at ,/s = 2.17 GeV (pi& = 1.26 GeV/ c associated with the pA threshold. ) The perturbative QCD analysis5 of exclusive amplitudes assumes that large momentum transfer exclusive scattering reactions are controlled by short distance quark-gluon subprocesses, and that corrections from quark masses and intrinsic transverse momenta can be ignored. The main predictions are fixed-angle scaling laws6 (with small corrections due to evolution of the distribution amplitudes,