The Q^2-Dependence of Nuclear Transparency for Exclusive $\rho^0$ Production

The propagation of QCD color through atomic nuclei is studied via a new analysis using a geometric model of semi-inclusive deep inelastic scattering. The experimental data were previously published by the HERMES Collaboration and consisted of the multiplicity ratio observable (2007) and the transverse momentum broadening observable (2010). We perform a simultaneous fit of these two observables to estimate (1) the color lifetime of the quark, (2) quark energy loss, (3) the $\h...

In the last years much work has been done to learn about the properties of the $\rho$-meson in nuclear matter. In a calculation of the in-medium spectralfunction of the $\rho$ we find that it is strongly modified by baryonic resonances, especially the $D_{13}(1520)$, through its coupling to resonance-hole states. In order to test the predictions of a model for the in-medium properties of the $\rho$-meson so far mostly heavy-ion collision have been used. We argue that the cohe...

We argue that study of the cross section of coherent photo(electro) production of vector mesons off nuclear targets provides an effective method to probe the leading twist hard QCD regimes of color transparency and perturbative color opacity as well as the onset of black body limit (BBL) in the soft and hard QCD interactions. In the case of intermediate energies we use the Generalized Vector Dominance Model to take into account coherence effects for two distinctive limits - t...

We introduce a data analysis procedure for color transparency experiments which is considerably less model dependent than the transparency ratio method. The new method is based on fitting the shape of the A dependence of the nuclear cross section at fixed momentum transfer to determine the effective attenuation cross section for hadrons propagating through the nucleus. The procedure does not require assumptions about the hard scattering rate inside the nuclear medium. Instead...

The first measurement of the coherent photoproduction of $\rho^{0}$ vector mesons in ultra-peripheral Xe-Xe collisions at $\sqrt{s_{\mathrm{\rm NN}}}= 5.44$ TeV is presented. This result, together with previous HERA $\gamma$p data and $\gamma$-Pb measurements from ALICE, describes the atomic number ($A$) dependence of this process, which is particularly sensitive to nuclear shadowing effects and to the approach to the black-disc limit of QCD at a semi-hard scale. The cross se...

This dissertation explores two main topics: 1) Color Transparency and quasi-elastic knockout reactions involving pions and $\rho$ mesons; and 2) determination of the $J/\psi$-nucleon scattering amplitude and scattering length via $J/\psi$ electroproduction on the deuteron. It is shown that at the energies available at the COMPASS experiment at CERN, Color Transparency should be detectable in the reaction $\pi+A\to \pi+p+(A-1)^*$ (proton knockout). It is also shown that Color ...

Nuclear effects for J/Psi production in pA collisions are controlled by the coherence and color transparency effects. Color transparency onsets when the time of formation of the charmonium wave function becomes longer than the inter-nucleon spacing. In this energy regime the effective break-up cross section for a c-cbar dipole depends on energy and nuclear path length, and agrees well with data from fixed target experiments, both in magnitude and energy dependence. At higher ...

Exclusive $\rho^0$-meson electroproduction is studied by the HERMES experiment, using the 27.6 GeV longitudinally polarized electron/positron beam of HERA and a transversely polarized hydrogen target, in the kinematic region 1.0 GeV$^2$<Q$^2$<7.0 GeV$^2$, 3.0 GeV<W<6.3 GeV, and -t'<0.4 GeV$^2$. Using an unbinned maximum-likelihood method, 25 parameters are extracted. These determine the real and imaginary parts of the ratios of several helicity amplitudes describing $\rho^0$-...

Interference between vector mesons electroproduced at different longitudinal coordinates leads within Glauber approximation to a $Q^2$-dependence of nuclear effects similar to what is expected to be an onset of color transparency (CT). We suggest such a mapping of the photon energies and virtualities, which eliminates the undesirable $Q^2$-variation and allows to measure a net CT effect. We develop a multichannel evolution equation, which for the first time incorporates CT an...

We introduce a data analysis procedure for color transparency experiments which is considerably less model dependent than the transparency ratio method. The new method is based on fitting the shape of the A dependence of the nuclear cross section at fixed momentum transfer to determine the effective attenuation cross section for hadrons propagating through the nucleus. The hard scattering cross section is then determined directly from the data. We apply this procedure to the ...