**5. Summary**

Color transparency is expected to be present in binary elementary reactions *ab* → *cd* with large scale 1 GeV<sup>2</sup> given by either min(|*t*|, |*u*|) or the (invariant mass)<sup>2</sup> of one of participating particles. It is also expected that channels with mesons in the initial and/or final state are most promising for the observation of CT than pure baryonic processes since a *qq*¯ pair is easier "squeezable" to PLC than a *qqq* triple.

In this work we discussed the results of the Glauber and QDM calculations for the following semiexclusive reactions: *<sup>A</sup>*(*<sup>e</sup>*,*eπ*+), pionic Drell-Yan process *<sup>A</sup>*(*π*<sup>−</sup>, *<sup>l</sup>*−*<sup>l</sup>*+) with *<sup>M</sup>*2*l*−*l*<sup>+</sup> 4 GeV2, and large-angle pion photoproduction *<sup>A</sup>*(*<sup>γ</sup>*, *π*<sup>−</sup> *p*). For these three reactions, strong CT effects are predicted. In the first reaction, CT has been already observed at JLab. The second reaction is suggested to be studied at J-PARC. The third reaction provides also an additional opportunity to study

#### *Particles* **2020**, *3*

the transition to the so-called photon transparency, i.e., the transition from resolved to unresolved (direct) photon with increasing |*t*|. This effect may interfere with CT and needs to be studied in more detail in the future.

CT-like behavior should also persist in inclusive reactions on nuclei at high energies, such as DIS, pA and AA collisions since they are governed by channels with large momentum transfer (large particle multiplicities). In these channels, the FSI is reduced due to a finite hadron formation length resulting in less secondary particles production and less deceleration of pre-hadrons produced in a primary hard collision. We have discussed slow neutron production in hard *γ*<sup>∗</sup>*A* interactions. The hadronization dynamics in these processes can be probed by slow neutrons by using ultraperipheral collisions at the LHC and RHIC.

We have finally discussed proton, pion and neutral hyperon production in pA and AA collisions in the NICA regime. It is demonstrated, that the rapidity and transverse momentum spectra are quite sensitive to the assumptions on the hadron formation model. The effect of hadron formation may influence the formation and equilibration of the resonance matter [42] in the central region of the colliding system. Thus, the studies of hadron formation are complementary to the studies of the nuclear equation of state in heavy ion collisions.

**Author Contributions:** Investigation, A.L. and M.S. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by HIC for FAIR within the framework of the Hessian LOEWE program, by U.S. Department of Energy, Office of Science, Office of Nuclear Physics, under Award No. DE-FG02-93ER40771, and by the German Federal Ministry of Education and Research (BMBF), Grant No. 05P18RGFCA.

**Acknowledgments:** The support of the Frankfurt Center for Scientific Computing is gratefully acknowledged.

**Conflicts of Interest:** The authors declare no conflict of interest.
