September 26, 2002
Exclusive coherent and incoherent electroproduction of the $\rho^0$ meson from $^1$H and $^{14}$N targets has been studied at the HERMES experiment as a function of coherence length ($l_c$), corresponding to the lifetime of hadronic fluctuations of the virtual photon, and squared four-momentum of the virtual photon ($-Q^2$). The ratio of $^{14}$N to $^1$H cross sections per nucleon, known as nuclear transparency, was found to increase (decrease) with increasing coherence length for coherent (incoherent) $\rho^0$ electroproduction. For fixed coherence length, a rise of nuclear transparency with $Q^2$ is observed for both coherent and incoherent $\rho^0$ production, which is in agreement with theoretical calculations of color transparency.
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November 6, 1998
Exclusive incoherent electroproduction of the rho^0(770) meson from 1H, 2H, 3He, and 14N targets has been studied by the HERMES experiment at squared four-momentum transfer Q**2>0.4 GeV**2 and positron energy loss nu from 9 to 20 GeV. The ratio of the 14N to 1H cross sections per nucleon, known as the nuclear transparency, was found to decrease with increasing coherence length of quark-antiquark fluctuations of the virtual photon. The data provide clear evidence of the intera...
January 13, 2012
We have measured the nuclear transparency of the incoherent diffractive $A(e,e'\rho^0)$ process in $^{12}$C and $^{56}$Fe targets relative to $^2$H using a 5 GeV electron beam. The nuclear transparency, the ratio of the produced $\rho^0$'s on a nucleus relative to deuterium, which is sensitive to $\rho A$ interaction, was studied as function of the coherence length ($l_c$), a lifetime of the hadronic fluctuation of the virtual photon, and the four-momentum transfer squared ($...
February 15, 2013
We study the nuclear transparency $T$ for the exclusive reaction $\gamma^*+A\to\rho+p+(A-1)^*$ at incident virtual photon energies $\nu \ge$ 10 GeV %where the final state of the $A-1$-nucleon system is a one-hole state of the initial nucleus, to investigate the separate dependence on the photon virtuality, $Q^2$ and the 4-momentum-transfer-squared to the knocked-out proton, $t$. If the effects of color transparency are included, $T$ shows significant variation with $t$ even f...
March 27, 2008
We study the electroproduction of rho mesons in nuclei at intermediate energies, deriving a treatment of the energy lost by the rho in each step of multiple-scattering. This enables a close match between calculations and the experimental kinematic conditions. A standard Glauber calculation is presented, and then the effects of color-transparency are included. The influence of poor experimental resolution on the extracted transparency is assessed. The effects of $\rho$ meson d...
July 23, 2001
So far no theoretical tool for the comprehensive description of exclusive electroproduction of vector mesons off nuclei at medium energies has been developed. We suggest a light-cone QCD formalism which is valid at any energy and incorporates formation effects (color transparency), the coherence length and the gluon shadowing. At medium energies color transparency (CT) and the onset of coherence length (CL) effects are not easily separated. Indeed, although nuclear transparen...
February 27, 2007
The study of color transparency (CT) in elastic electroproduction of vector mesons off nuclei encounters the problem of the onset of coherence length (CL) effects. The problem of CT-CL separation arises especially at medium energies, corresponding to HERMES experiment, when the coherence length is of the order of the nuclear radius R_A. Only at asymptotic large energies, corresponding to large CL, l_c >> R_A, the CT-CL mixing can be eliminated. On the other hand, the net CT e...
November 2, 1995
The variation of the coherence length $l_c=2\nu/(Q^2+m_V^2)$ in virtual photoproduction of vector mesons off nuclei as a function of the photon energy $\nu$ or virtuality $Q^2$ results in dramatic changes in the values of the nuclear transparency. Color transparency effects can be easily mixed up with the effects of the coherence length in incoherent photoproduction on nuclei.
November 8, 1993
The exclusive production of vector mesons in deep inelastic scattering is a hard scattering process with the well controlled size of quark configurations which dominate the production amplitude. This allows an unambiguous prediction of color transparency effects in the coherent and incoherent production of vector mesons on nuclei. We demonstrate how the very mechanism of color transparency leads to a belated onset of color transparency effects as a function of $Q^{2}$. We con...
May 23, 1994
We predict a strikingly different $A$ and $Q^{2}$ dependence of quasielastic leptoproduction of the $\rho^{0}(1S)$-meson and its radial excitation $\rho '(2S)$ on nuclei. Whereas for the $\rho^{0}$ production nuclear transparency $T_{A}$ decreases monotonically with $A$, for the $\rho'$ nuclear transparency $T_{A}$ can have the counterintuitive nonmonotonic $A$-dependence, having the minimum for light nuclei, and increasing with $A$ for medium and heavy nuclei. Strong enhance...
August 11, 2000
A systematic multiple scattering formalism for vector meson electroproduction from nuclei is developed, with emphasis on the formation, propagation and hadronization scales for quark-gluon fluctuations of the virtual high-energy photon. The theory is compared to HERMES measurements of rho electroproduction on 14N. The nuclear transparency as a function of the vector meson propagation length is well reproduced.