Speaker
Description
Pion production in the backward direction in the target rest frame is considered
in proton-nucleus (p+A) collisions. Pions outside the kinematical boundary of
proton-nucleon (p+N) collisions, the so called cumulative effect, is studied.
Basic restrictions on the energy of pions emitted in the backward direction in
p+N and p+A reactions are considered. These are restrictions that follow from
energy-momentum conservation. It is argued that resonances with very high
masses are the only source of the cumulative pions. The resonances are first
created in p+N reactions. Due to successive collisions with nuclear nucleons,
the masses of these resonances may increase and simultaneously their
longitudinal velocities decrease. It is suggested that these two effects give an
explanation of the cumulative pion production. The heavy hadron-like systems with very
high masses (heavy resonances, Hagedorn fireballs, quark-gluon bags, baryon
and meson strings) are of primary importance for properties of strongly
interacting matter at high temperatures and/or baryonic density. They may have
also decisive influence on the transition between hadron matter and quark-gluon
plasma, and define the type of this phase transition and existence of the QCD
critical point itself. We also use the Ultra relativistic Quantum Molecular
Dynamics model to describe the existing data and analyze some microscopic
aspects of cumulative pion production in p+A reactions.
