Nuclear Transparency to Large-AngleppElastic Scattering

Abstract

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-...

Diffraction in QCD

“…In some of them, quasielastic high-p T scattering of electrons [14] and hadrons [15], no unambiguous signal of CT was observed. Those processes turned out to be unsuitable for CT searches [16,17], since the formation length of the hadrons was too short compared to the nuclear size.…”

Cross section measurements of charged pion photoproduction in hydrogen and deuterium from 1.1 to 5.5 GeV

“…Next is the oscillation of the differential cross-section dσ/dt around the scaling value in pp elastic scattering [15]. The third is the anomalous energy dependence of nuclear transparency of the A(p, 2p) process [16].…”

“…The hadron helicity conservation rule [11], another outcome of the same short-distance pQCD framework, does not agree with data in the same energy and momentum transfer region [12,13]. There are a few anomalies beyond the constituent counting rule in the extensively studied pp scattering process [14][15][16]. Recently, the parton orbital angular momentum has been found to play a non-negligible role in the exclusive reactions, which may explain hadron helicity nonconservation and other polarization measurements [17,18].…”

Observation of a Coherence Length Effect in Exclusiveρ0Electroproduction

“…If the ISI and FSI amplitudes factorize from the exclusive scattering amplitude, then T A is the probability that no significant ISI or FSI occur. The transparency has been used to study the space-time dynamics of several exclusive reactions [2,[6][7][8][9]. This paper reports measurements of the nuclear transparency for exclusive incoherent r 0 electroproduction on 2 H, 3 He, and 14 N targets at Q 2 .…”