Investigating the Onset of Color Transparency with CLAS

Abstract

One of the fundamental predictions of QCD is the phenomenon of Color Transparency (CT), which was first described more than two decades ago [2, 3]. CT describes the process by which a color singlet object with a reduced transverse size has a vanishingly small interaction cross section when propagating through a nucleus. Such a Point Like Configuration (PCL) can be created in a scattering experiment at high momentum transfer (Q). See one of the many review articles for details [4, 5, 6]. Experimentally the onset of CT can be observed as a rise in the nuclear transparency, TA = σA AσN with increasing Q. Many experiments have looked for a signal of CT but only a few have observed one. For an overview of the experimental status see [7]. The reaction used in this experiment is incoherent electro-production of ρ mesons off nuclei, which offers many advantages. It is expected that the onset of CT occurs at a lower Q for a meson (qq) compared to a hadron (qqq)[8]. Also, since the ρ is a vector meson, the production mechanism is well understood by the Vector Meson Dominance model (VMD) as the fluctuation of a photon (which has the same quantum numbers as a vector meson) into a (qq) pair. The photon at high virtuality Q is expected to produce a pair with small ∼ 1/Q transverse separation. CT then manifests itself as a vanishing absorption of this (qq) pair as it propagates through the nucleus [9, 10]. The dynamical evolution of the small size (qq) pair to a normal sized vector meson is controlled by the time scale called the formation time. In the rest frame of the nucleus this is given by tf = 2ν m′2 V −mV , where V ′ represents the first excited state of the meson and V represents the ground state. This formation time needs to be sufficiently long to be able to observe CT.