Color transparency in electroproduction of the ρ meson at low energies

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 the HERMES experiment, when the coherence length is of the order of the nuclear radius $R_A$. Only at asymptotic high energies, corresponding to large CL, $l_c\gg R_A$, can the CT-CL mixing be eliminated. On the other hand, the net CT effects can be studied in the kinematic range accessible by the experiment at the Continuous Electron Beam Accelerator Facility's large acceptance spectrometer (CLAS) at the Thomas Jefferson National Accelerator Facility, since in that case the CL is much smaller than the nuclear radius. Using light-cone QCD dipole formalism, we investigate manifestations of CT effects in the electroproduction of vector mesons. Motivated by expected data from the CLAS experiment, we predict the $A$ and $Q^2$ dependence of nuclear transparency for ${\rho }^0$ mesons produced incoherently off nuclei. We demonstrate that in the CLAS kinematic region, the CL effects are weak enough to keep the photon energy at such values as to obtain maximal photon virtualities keeping optimal statistics of the data. This has a clear advantage over a standard investigation of net CT effects fixing CL.

Figure 1

$Q^2$ dependence of the nuclear transparency ${\mathrm{Tr}}_A^{\mathrm{inc}}$ for exclusive electroproduction of ρ mesons on ${}^{207}\mathrm{Pb}$ target. The dotted lines represent the CL fixed at $l_c=1.0$ and $5.0\mathrm{fm}$. The solid lines represent predictions at the fixed mean photon energy $\langle \nu \rangle =13\mathrm{GeV}$ corresponding to HERMES experiment.

Figure 2

$Q^2$ dependence of the nuclear transparency ${\mathrm{Tr}}_A^{\mathrm{inc}}$ for exclusive electroproduction of ρ mesons on nuclear targets ${}^{12}\mathrm{C}$, ${}^{56}\mathrm{Fe}$, and ${}^{207}\mathrm{Pb}$ (from top to bottom). The top and bottom dotted line with respect to each solid line represents predictions fixing CL at $l_c=0.9$ and $0.5\mathrm{fm}$, respectively. The solid lines represent predictions at the fixed mean photon energy $\langle \nu \rangle =3.5\mathrm{GeV}$ corresponding to the CLAS experiment at JLab.