Nuclear Transparency in Large Momentum Transfer Quasielastic Scattering

Abstract

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].

Overview of color transparency measurements

“…However, it was surprisingly followed by a drop at higher momenta. As a cross-check, a series of similar experiments were performed afterwards at BNL [9,10], and all confirmed the same behavior [11]. One proposed explanation described this behavior as an interference between the short and long distance amplitudes in the free pp cross section, where the nuclear medium acts as a filter for the long distance amplitudes [12,13].…”

Color transparency: Past, present and future

“…Presence of CT in the pion electroproduction and the change in the transparency of pA → pp(A − 1) process for p N ≥ 8 GeV [49] indicate that experiments with antiprotons of energies above 6 − 8 GeV where a pair of pions is produced in the final state will be able to verify whether the annihilation process is dominated by the contribution from small size configurations, by measuring the transparency ratio;…”

Nuclearρmeson transparency in a relativistic Glauber model

“…At intermediate energies, no sign of CT was observed in A(e, e p) measurements on a variety of nuclear targets and four-momentum transfers Q 2 8 (GeV/c) 2 [8][9][10][11][12][13][14]. The nuclear 12 C(p, 2p) transparencies were studied at Brookhaven National Laboratory (BNL) [15][16][17]. The transparency first shows a rise with increasing incoming proton momentum and drops for momenta larger than 9 GeV/c.…”