Looking into matter at different temperatures and densities it’s a journey in Space, Time and Fundamental Forces. There are temperatures at which matter breaks up. When this happens Nature shows us its inner structure and the fundamental features of the Interactions responsible for the stability of our Unvierse at different scales. These are called Phase Transitions. This is also a journey in Time, since the Universe passed through all these Phase Transitions while expanding and cooling after the Big Bang.
Few moments after each Phase Transition (or at Temperatures slightly lower), when matter is not yet broken up in fundamental pieces, it often shows unforeseen behaviours. Phenomena, that we cannot explain with Theories taking into account one-to-one simple interactions (two-body interactions), emerge. It’s a magic moment, when matter starts to behave as a whole and unexpected behaviours appear, teaching us that matter is more complex than we can think.
These are the reasons why we heat up simple nuclei (Helium nuclei in example) by using a beam of mid energy pions (particles made of a quark and a antiquark). This brings them close to the transition phase where protons and neutrons in nuclei are quasi-free (known as Fermi Gas Transition Phase).
In Fig.1 a typical pion-helium nucleus scattering is shown. The incoming pion is entering the detector, filled with helium gas, from right side. The pion is scattered and then decays into a muon (circle) while the helium is broken up into 2 protons and 2 neutrons (being neutral are not visible). The experimental apparatus used is a streamer chamber, a detector that allows to really “see” the scattering event and measure all the remnants after the pion hit the helium nucleus.
What happens during the scattering (time scales of 10^-23 s) is the focus.
==> A not so simple nucleon
The heated pion-helium system emits radiations as it were composed by a huge number of parts, while present nuclear theories describe it as a simple system of 4 nucleons (2 protons and 2 neutrons or a bunch of quarks). The radiation emitted is a Planck blackbody radiation (Fig.2), that can only be emitted by a big number of irradiators put together. What is happening to matter at these temperature scales?
==> A collective behaving nucleus
Fundamental forces allow the existance of short-lived particles, which are called resonances. A well know one, the Delta Resonance, can be excited by a pion and a proton interacting at energies high enough to create its mass (red peak in Fig.3), that is 130% heavier than the proton. When excited in helium nucleus its mass is lower (- 6%) and its life is 3 times longer. The observed resonance seems to be a collective state involving at least 75% of the nucleus (insted of just a proton).
By studying the same phase on heavier nuclei, up to Oxygen, we observed this behaviour up to Carbon nuclei. For bigger nuclei the resonance parameters remains as measured on Helium and Carbon (Fig. 4). This means that the observed collective behaviours are confined in a bag of 4 nucleons (alpha particle). Thus the typical scale of these emerging phenomena in nuclear matter is 1-2 fm, while beyond these scale canonic nuclear theories apply again.