Nuclear transparency in90c.m.°quasielasticA(p,2p)<

Abstract

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.

Puzzles in Hadronic Physics and Novel Quantum Chromodynamics Phenomenology

“…Color transparency is a clear manifestation of the role of gauge interactions in hadron physics. In fact, pQCD color transparency has also been clearly observed (20) in quasi-elastic proton scattering, in which the effective number of interacting protons in the target increases with p T . However, this observation is true only at energies below √ s ∼ 5 GeV; in fact, color transparency disappears at the same energy and angles that show the anomalous spin-spin correlation (Figure 2c).…”

“…(b) The s −10 scaling of the measured elastic pp → pp scattering cross section at a fixed center-of-mass angle. (c) Transparency ratio in quasi-elastic pp scattering in nuclei(20). References to the experiments and theory fit can be found in Reference 20.…”

Using branching processes in nuclei to reveal dynamics of large-angle, two-body scattering

“…• One can use high energy proton beams to look for production of two back to back protons with the same invariant energies as the ones studied at BNL [5] to check whether oscillations of the transparency would be present for invariant energies of the proton pair matching BNL invariant energies.…”

“…Nevertheless, after many years and many studies the momentum transfer range of applicability of pQCD has not been firmly established. A series of experiments at Brookhaven National Laboratory (BNL) measured the nuclear transparency of nuclei in quasielastic scattering process near 90 • in the pp center of mass, see the summary in [5]. An increase of nuclear transparency was observed between p inc N = 5.9 GeV/c and p inc N = 9.5 GeV/c, indicating that freezing of nucleonic configurations becomes possible for p N ≥ 8 GeV/c.…”

The nuclear transparency effect of π−-mesons in p+12C- and d+12C-interactions at 4.2A GeV/c

“…The increase in the NT indicates the appearance of CT as compared to the predictions of traditional nuclear physics expectations. A lot of efforts have been made to search for the CT effect, including A(p, 2p) [1,11,12,13,14] and A(e, e p) [15,16,17,18,19,20] reactions with protons as a probe for studying the effect. Mesons were used on the premise that at large Q 2 the values of r ⊥ are significantly smaller than the size of the nucleon.…”