We measured simultaneously pp elastic and quasielastic ͑ p, 2p͒ scattering in hydrogen, deuterium, and carbon for momentum transfers of 4.8 to 6.2 ͑GeV͞c͒ 2 at incoming momenta of 5.9 and 7.5 GeV͞c and center-of-mass scattering angles in the range u c.m. 83.7 ± 90 ± . The nuclear transparency is defined as the ratio of the quasielastic cross section to the free pp cross section. At incoming momentum of 5.9 GeV͞c, the transparency of carbon decreases by a factor of 2 from u c.m. Ӎ 85 ± to u c.m. Ӎ 89 ± . At the largest angle the transparency of carbon increases from 5.9 to 7.5 GeV͞c by more than 50%. The transparency in deuterium does not depend on incoming momentum nor on u c.m. . [S0031-9007 (98)07818-1] PACS numbers: 24.85. + p, 25.40. -h, 24.10. -iNuclear transparency is a measure of the initial and final state interactions that the incoming and outgoing protons undergo before and after the main ͑p, 2p͒ reaction. Conventional theoretical calculations of the nuclear transparency within the Glauber picture [1,2] predict that above an incident momentum of approximately 5 GeV͞c the nuclear transparency does not depend on the incoming momentum nor on the pp c.m. scattering angle, u c.m. . The expectation from QCD based models of proton dynamics in hard exclusive interactions is that the initial and final state scattering may be smaller than the Glauber theory would predict. It is also expected that nuclear transparency should increase with incoming momentum reaching an asymptotic value of 1. These QCD phenomena have been referred to as color transparency [3].

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We summarize the results of two experimental programs at the Alternating Gradient Synchrotron of BNL to measure the nuclear transparency of nuclei measured in the A(p,2p) quasielastic scattering process near 90 • in the pp center of mass. The incident momenta varied from 5.9 to 14.4 GeV/c, corresponding to 4.8 < Q 2 < 12.7(GeV /c) 2 . Taking into account the motion of the target proton in the nucleus, the effective incident momenta extended from 5.0 to 15.8 GeV/c. First, we describe the measurements with the newer experiment, E850, which had more complete kinematic definition of quasielastic events. E850 covered a larger range of incident momenta, and thus provided more information regarding the nature of the energy dependence of the nuclear transparency. In E850 the angular dependence of the nuclear transparency near 90 • , and the nuclear transparency for deuterons was studied. Second, we review the techniques used in an earlier experiment, E834, and show that the two experiments are consistent for the Carbon data. E834 also determines the nuclear transparencies for lithium, aluminum, copper, and lead nuclei as well as for Carbon. A determination of the (π + , π + p) transparencies is also reported. We find for both E850 and E834 that the A(p,2p) nuclear transparency, unlike that for A(e,e'p) nuclear transparency, is incompatible with a constant value versus energy as predicted by Glauber calculations. The A(p,2p) nuclear transparency for Carbon and Aluminum increases by a factor of two between 5.9 and 9.5 GeV/c incident proton momentum. At its peak the A(p,2p) nuclear transparency is ∼ 80% of the constant A(e,e'p) nuclear transparency. Then the nuclear transparency falls back to a value at least as small as that at 5.9 GeV/c, and is compatible with the Glauber level again. This oscillating behavior is generally interpreted as an interplay between two components of the pN scattering amplitude; one short ranged and perturbative, and the other long ranged and strongly absorbed in the nuclear medium. A study of the A dependent nuclear transparency indicates that the effective cross section varies with incident momentum and is considerably smaller than the free pN cross section. We suggest a number of experiments for further studies of nuclear transparency effects.

We measured the high-momentum quasi-elastic 12 C(p, 2p) reaction (at θcm ≃ 90 o C.M.) for 6 and 7.5 GeV/c incident protons. The three-momentum components of both final state protons were measured and the missing energy and momentum of the target proton in the nucleus were determined.The validity of the quasi-elastic picture was verified up to Fermi momenta of about 450 MeV/c where it might be questionable. Transverse and longitudinal Fermi momentum distributions of the target proton were measured and compared to independent particle models which do not reproduce the large momentum tails. We also observed that the transverse Fermi distribution gets wider as the longitudinal component increases in the beam direction, in contrast to a simple Fermi gas model.Quasi-elastic (QE) scattering is a process in which a projectile is elastically scattered from a single bound nucleon in the nucleus, which we call the "target nucleon", while the rest of the nucleus acts as a spectator. Specifically, the QE (p,2p) scattering at large momentum transfer provides a method for measuring the high momentum tails of the nuclear wave function. This fact can be understood by considering the s-scaling law for high momentum transfer hadronic reactions [1]. The elementary pp elastic differential cross section scales as dσ/dt ∼ 1/s 10 for fixed (s/t), where s and t are the Mandelstam variables. Farrar et al.[2] pointed out that this scaling will cause the QE (p,2p) reaction in the nucleus to favor * strongly the scattering from those target nucleons that are moving in the beam direction with large Fermi momentum , because the value of s is reduced under those kinematic conditions. The same reaction also attracted much attention in recent years [3] in connection with the QCD prediction that the nuclear attenuation will vanish at asymptotically large momentum transfer, a phenomenon called "Color Transparency" [4]. Thus, at large but finite energies the QE (p,2p) reaction , with emerging protons of a few GeV/c, is a good tool for experimental studies of the nuclear transparency. Those results will be discussed in a forthcoming publication [5].This letter will concentrate on the identification of QE events and will test the validity of the QE picture up to large Fermi momenta, where it might become questionable. If the QE picture is valid, then over the measured kinematical range it is possible to separate nuclear properties from the nuclear reaction mechanism. We will also present transverse and longitudinal Fermi momentum distributions of the target proton and compare them to independent particle models.We measured the high-momentum transfer quasielastic (p,2p) reaction at θ cm ≃ 90 o on carbon for 6 and 7.5 GeV/c incident protons in a kinematically complete coincidence experiment. The three-momentum components of both high p t final state protons were measured, which yielded the missing energy and momentum of the target proton in the nucleus.The experiment (E850) was performed at the AGS accelerator at Brookhaven National Laboratory with the ...

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