Large-angle pp elastic and quasielastic (p,2p) scattering have been simultaneously observed in hydrogen and each of several nuclear targets (Li, C, Al, Cu, Pb) at incident proton momenta of 6, 10, and 12 GeV/c. The nuclear transparency is the ratio of such a cross section in a nucleus to the free pp cross section. The transparency of aluminum increases with incident momentum by more than a factor of 2 from 6 to 9.5 GeV/c and falls significantly between 9.5 and 12 GeV/c. This occurs in a region where the free-proton nucleon-absorption cross section exhibits little energy dependence. QCD predicts an increase in transparency with energy.PACS numbers: 13.75. Cs, 12.38.Qk, 13.85.Dz, 25.40.Ve This Letter describes the first results from a program of study at the Brookhaven National Laboratory Alternating-Gradient Synchrotron which investigates the effects of "color transparency." Quasielastic pp scattering from each of several nuclei is compared to pp elastic scattering in hydrogen at three energies. These data are analyzed with a simple model in which the quasielastic cross section is assumed to factor into the product of three terms, a single-particle nuclear momentum distribution, a free pp cross section, and a factor T which we refer to as the transparency of the nucleus. In the absence of Fermi motion the transparency would beData are presented for pp elastic and quasielastic scattering near 90° cm. (center of mass) at incident proton energies of 6, 10, and 12 GeV/c, corresponding to t [(four-momentum transfer) 2 ] of -4.8, -8.5, and -10.4 GeV 2 .The cross section (da/dt) for pp elastic scattering at large transverse momentum and at fixed cm. angle is characterized by an s [(center-of-mass energy) 2 ] dependence which oscillates around the nominal s~] 0 form predicted by the dimensional scaling law of Brodsky and Farrar. l The form of this energy dependence can be related to the probability of finding protons with all of their quarks confined to a region of space which is proportional to 1A/7. This implies that for large s these initial-and final-state protons are very small.It has been pointed out by Mueller 2 and others that small protons which participate in such processes are characterized by color-charge and color-field distributions confined to ever smaller dimensions as s increases. In high-/ quasielastic scattering this implies that the cross section for soft initial-and final-state interactions with other nucleons in the nucleus will vanish as the energy scale increases. It has thus been predicted that at high energy the transparency of nuclei should approach unity. This is in sharp contrast to a more conventional Glauber picture 3 of absorption in which the transparency would be expected to be energy independent.The apparatus consists of a large-angle magnetic spectrometer with a 4.5° aperture. 4 Large proportional chambers measure the trajectories of recoil tracks opposite the spectrometer. When configured for incident momentum of 10 GeV/c, the spectrometer has Ap/p = 1% and A0 = 1 mr and the recoil-...

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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].