In-medium properties of hadrons - observables II

In this review we discuss the observable consequences of in-medium changes of hadronic properties in reactions with elementary probes, and in particular photons, for nuclei. After an outline of the theoretical method used we focus on a discussion of actual observables in photonuclear reactions; we discuss in detail 2 pi meson and vector meson production. We show that the 2 pi(0) photoproduction data can be well described by final state interactions of the pions produced whereas the semi-charged pi(0) pi(+/-) channel exhibits a major discrepancy with theory. For omega production on nuclei in the TAPS/CB@ELSA experiment we analyse the pi(0)gamma decay channel, and illustrate the strength of the method by simulating experimental acceptance problems. Dilepton production in the few GeV range is completely free of final state interactions. We show that the sensitivity of this decay channel to changes of hadronic properties in the medium in photonuclear reactions on nuclei is as large as in ultrarelativistic heavy ion collisions and make predictions for the ongoing G7 experiment at JLAB. Finally we discuss how hadron production in nuclei at 10-20 GeV photon energies can give important information on the hadronization process, and in particular on the timescales involved. We show here detailed calculations for the low energy (12 GeV) run at HERMES and predictions for planned experiments at JLAB. (c) 2005 Elsevier B.V. All rights reserved.