[P. Jain, B. Pire and J. Ralston, "Quantum Color Transparency and Nuclear Filtering", Phys. Rep. 271 67 (1996)]
A(p,2p) Experiments
Brookhaven experiments, A(p,2p):
Early experiments to measure CT with A(p,2p) at Brookhaven by Carroll resulted in a rise of transparency up to 8 GeV, followed by a fall:
[ A.S. Carroll et al. PRL 61 1698 (1988) ]
There were some questions about the interpretation of this data, since only one of the 2 final state protons was momentum analyzed, so the exclusivity of the reaction could not be guaranteed.
Kinematics are pL=6, 10 and 12 GeV, Q2=4.8, 8.6 and 10.4 Gev2 and thus the expansion times are roughly 3.4, 5.7, 6.8 fm, relatively long (Miller). At first it was considered that Color Transparency effects were observed.
A repeat of the experiment by Y. Mardor [ Y. Mardor et al. PRL81 5085 (1998) ] and later extended by A. Leksanov [ A. Leksanov et al. PRL87 212301 (2001) ] did measure a complete final state, and confirmed the earlier results. However, this also meant the fall in the data at higher momentum, which goes against the CT prediction. Color Transparency was not unambiguously observed after all.
There were two explanations proposed for this striking behavior. Ralston and Pire [ J.P. Ralston and B. Pire, PRL61, 1823 (1988) ] explained it as an interference between the short and long distance amplitudes in the free p-p cross section, which causes oscillations where the nuclear medium acts as a filter for the long distance amplitudes.
The other explanation by Brodsky and de Teramond [ S.J. Brodsky and G.F. de Teramond, PRL60, 1924 (1988) ] explains that the unexpected decrease could be related to the crossing of the open charm threshold.
A(e,e'p) Experiments
Experiments trying to measure the CT effect in A(e,e'p) did not see any significant effect.SLAC experiments, A(e,e'p) (NE-18):
[N. Makins et al. PRL 72 1989 (1994)]
Measured the cross section for quasielastic 12C(e,e'p) with momentum transfers of Q2= 1, 3, 5 and 6.8 (GeV/c)2. The outgoing proton momenta are not that large: 1.1, 2.3, 3.5, 4.6 GeV, and corresponding expansion times are 0.65, 1.3, 2.0, 2.6 fm. The failure to observe the CT effect could be due to the rapid expansion of the color singlet (Miller). No rise of transparency with Q2 is observed.[T.G. O'Neill et al., Phys. Lett. B351 87 (1995) ]
Same measurement on 2H, C, Fe, Au. Still no rice in nuclear transparency is seen.
Jefferson Lab, A(e,e'p):
[K. Garrow et al., hep-ex/0109027, Sept 18, 2001]
Exp. in Hall-C, beam energies 3, 4.4, 5.56 GeV, Q2= 3.3, 6.1, 8.1 (GeV/c) 2. H, 2H, 12C, 56Fe targets.
No evidence of the onset of CT was seen in this Q2 range:
Transparency for (e,e'p) quasielastic scattering from D (stars), C (squares), Fe (circles), and Au (triangles). The curves are Glauber calculations by [V.R. Pandharipande and S.C. Pieper, Phys. Rev.C45, 791 (1992)] and [L.L. Frankfurt, W.R. Greenberg, G.A. Miller, M.M. Sargsian, and M.I. Strikman, Z. Phys. A 352, 97 (1995)]
Data sources:
Red small open symbols: BATES [G. Garino et al., Phys. Rev.C45, 780 (1992)]
Green large open symbols: SLAC [N. Makins et al. PRL 72 1989 (1994)] [T.G. O'Neill et al., Phys. Lett. B351 87 (1995) ]
Black small and dark blue large solid symbols: JLAB [D. Abbot et al.,Phys. Rev. Lett. 80, 5072 (1998)] [K. Garrow et al., hep-ex/0109027, Sept 18, 2001]
Meson production and scattering Experiments
The A dependence of J/Psi photoproductionThe Q2 dependence of the t slope of diffractive rho0 production in muon scattering
The yield of non-diffractive rho0 production in deep inelastic muon scattering
The energy and flavor dependence of vector meson production in ep scattering at DESY collider HERA
Coherent Nuclear Diffraction of High Energy Pions into two Jets: Fermilab E791
[Aitila et al., Phys.Rev.Lett.86, 4773 (2001)]Normal pion - nucleus interactions are observed to have cross sections that are proportional to A2/3.
The diffractive dissociation of a pion on a nucleus: pi + A -> two-jets + A, is predicted to have an A dependence of A4/3. The E791 experiment at Fermilab used a beam of 500 GeV pions on Carbon and Platinum.
The transverse momentum, kt, for the di-jets, relates to the invariant mass M2J = k2t/[x(1-x)], Q2~M2J => 4k2t~10 GeV2. (The coherence lenght, the distance before the q-qbar system expands is about 10 fm.)
Since the A depence should vary with kt, the analysis was carried out in 3 regions of kt. It was found that for the highest region, sigma=sigma0Aalpha, gives an alpha = 1.6+/-0.1, consistent with the CT prediction.
HERMES
Coherence Length EffectThe HERMES collaboration at DESY measured e+A ->e+rho0->e+pi+pi- on targets 1H,2H,3He,14N, with a positron beam of 27.5 GeV.Kopeliovich, Nemchick, Shafer, Tarasov, "Color Transparency versus quantum coherence in electroproduction of vector mesons", PhysRev C65, 35201 (2002)
[K. Ackerstaff et al. Hermes Collab. Phys. Rev. Lett. 82, 3015 (1999)]
The data can then be plotted against the coherence length: lc=2nu/(Mrho^2+Q^2):
Data in a) and b) are from HERMES. Data in c): solid circles from HERMES, open circles from E665 at FNAL with muons [Adams PRL74, 1525 (1995)] (horizontal errors are large because of large acceptance in nu), open diamonds from Cornell on 12C with photons. Dashed line is Glauber calculation by Huefner and Kopeliovich [Phys. Lett. B 383, 362 (1996)].
For comments or questions: holtrop(at)jlab.org