Abstract
An incoming or outgoing hadron in a hard collision with large momentum transfer gets squeezed in the transverse direction to its momentum. In the case of nuclear targets, this leads to the reduced interaction of such hadrons with surrounding nucleons which is known as color transparency (CT). The identification of CT in exclusive processes on nuclear targets is of significant interest not only by itself but also due to the fact that CT is a necessary condition for the applicability of factorization for the description of the corresponding elementary process. In this paper we discuss the semiexclusive processes A ( e , e ′ π + ) , A ( π − , l − l + ) and A ( γ , π − p ) . Since CT is closely related to hadron formation mechanism, the reduced interaction of ’pre-hadrons’ with nucleons is a common feature of generic high-energy inclusive processes on nuclear targets, such as hadron attenuation in deep inelastic scattering (DIS). We will discuss the novel way to study hadron formation via slow neutron production induced by a hard photon interaction with a nucleus. Finally, the opportunity to study hadron formation effects in heavy-ion collisions in the NICA regime will be considered.
Highlights
Hard processes, e.g., exclusive meson electroproduction with Q2 1 GeV2, can be only described by taking into account quark-gluon degrees of freedom
Color transparency is expected to be present in binary elementary reactions ab → cd with large scale 1 GeV2 given by either min(|t|, |u|) or the2 of one of participating particles
It is expected that channels with mesons in the initial and/or final state are most promising for the observation of color transparency (CT) than pure baryonic processes since a qq pair is easier “squeezable” to point-like configurations (PLCs) than a qqq triple
Summary
E.g., exclusive meson electroproduction with Q2 1 GeV2 , can be only described by taking into account quark-gluon degrees of freedom. The characteristic transverse size of the incoming and outgoing color-neutral quark configurations in a hard process is rt ∼ 1/Q and, they can be regarded as point-like configurations (PLCs) It can be shown within pQCD [1] that the interaction cross section of the small-rt color singlet qqpair and a proton behaves geometrically at rt → 0, i.e., σqq ∝ rt. Assuming that the quark and antiquark each carry 1/2 of the light cone (LC) momentum of a qqsystem we estimate ∆M2 ' 4(m2q + hk2t i) − m2π ' 0.93 GeV2 for the pion, where mq = 0.340 GeV is the constituent quark mass and hk2t i1/2 ' 0.35 GeV/c is the average transverse momentum of a quark in a hadron [6] These estimates are in a reasonable agreement with the empirical range obtained from the analysis of pionic nuclear transparency at JLab [7], ∆M2 ' 0.7 − 1.1 GeV2 corresponding to lh = 0.4 − 0.6 fm ph.
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