Quarks and gluons are the fundamental constituents of matter, yet under normal conditions they only appear confined to hadrons, e.g. protons or neutrons, by the strong nuclear force.
In the figure here on the side you can see the phase diagram of nuclear matter which show u show nuclear matter behaves at large temperatures and/or densities.
Imagine for example the water. When we heat it up to a certain critical temperature it becomes vapour.
And take now some dinosaur or plant fossils, heat them up to 1.8×10^3 K and put on top of them a pressure of roughly 50.000 kg per cm squared. They becomes diamonds.
Both these elements change their state when we apply high temperatures and pressures.
So what does it happen when we heat and/or squeeze together protons and neutrons?
Lattice QCD calculations have predicted that at temperatures above T~160 MeV or 2×10^12 K (100 000 times hotter than the centre of the sun) quarks and gluons would no longer be confined, but instead form a phase of free quarks and gluons, the quark–gluon plasma (QGP). In ultrarelativistic heavy-ion collisions at the Large Hadron Collider (LHC) one aims to produce such a QGP to learn more about the strong nuclear force.
In order to reach instead very high density we have to look up in the sky.
Neutron stars (NS) are the most compact objects known within our universe. We know that because astronomers can measure their large masses (up to twice as heavy as the sun) and their small radius (between 5 and 15 km only). These numbers translate into densities inside the core of neutron stars that might exceed 4 or 5 times the density of normal nuclear matter. To this end, it is not clear at all what is contained inside the core of neutron stars and which is their Equation of State (EoS). The latter expresses the relationship between the pressure within NS and the density and can be transformed one to one into the relationship between the mass and the radius of the NS through standard considerations based on general relativity. Different hypotheses about the content of NS will lead to different EoS and the corresponding mass/radius relations can be tested against measurements. Newly, the discovery of NS mergers observed through the emission of their gravitational wave signals provide us with even more sophisticated tools to test different hypotheses. What could these hypotheses be? The simplest case would be that mainly neutrons, some protons and electrons and neutrinos exist within NS. This hypothesis becomes although rather improbable already starting from densities of the system as large as twice normal nuclear densities since it is then favorable to replace the neutrons with hyperons: hadrons with at least one strange quark (neutron = ddu, Λ hyperon = uds). The resulting EoS of NS containing nucleons and hyperons will depend from the hyperon-nucleon and hyperon-nucleon-nucleon interactions and how these change when the density of the system increases. In general NS can also contain other hadrons, like for example kaons (K+= u-sbar) or antikaons (K- =s-dbar). In this case the production of this hadron sort is even more favored since they are bosons and can occupy all the same ground level in energy.
Also in this case our job is to study how kaons and antikaons interact with nucleons under different densities and temperatures of the environment.
Our group at TUM participates in the ALICE experiment at the LHC, in the HADES experiment at GSI and in the AMADEUS collaboration to understand how matter behaves under these extreme conditions and how hyperons and kaons interact with nucleons.
-in ALICE: 1) we want to deeply understand the hyperon-nucleon and hyperon-nucleon-nucleon interaction in vacuum by means of femtoscopy since scattering and hypernuclei data are not sufficient to constrain the undergoing interaction. Femtoscopy normally focuses on the investigation of the size and time evolution of the region the particles are emitted from, which happens on the Femtometer scale (10−15 m). But since femtoscopy is also based on final state interactions one can use the method to study strong final state interactions of pairs where not much is known about the interaction and scattering experiments are difficult to realize .
Our specific goal is to use p+p, p+A reactions at different energies ( GeV and TeV) to study the final state interaction of hyperon-nucleon (pΛ,pΞ,pΣ,pΩ) and hyperon-nucleon-nucleon interactions (Λpp) which are fundamental for the EoS of neutron stars.
-in ALICE: 2) we are interested in studying the properties of the QGP and under which initial conditions (i.e. temperature and baryon chemical potential) the QGP is formed in heavy-ion collisions, i.e. the location on the QCD phase diagram show above. One surprising property of the QGP is the surprisingly strong coupling between quarks and gluons: the initial expectation that the QGP would behave like an ideal gas with vanishing interactions. The strong azimuthal anisotropy in the particle production, however, implies a strong coupling that transform initial geometric anisotropies (i.e. the almond shaped overlap region created in non-central heavy-ion collisions) into a momentum space anisotropy. The strength of this anisotropy can be well described by hydrodynamic models with an extremely small shear-viscosity to entropy ratio. Hence the QGP behaves rather like a perfect liquid than an ideal gas. A particular interest of our group is to disentangle initial state geometry effects from the hydrodynamic ones on the observed anisotropies with advanced multiparticle correlation techniques . For these studies it is furthermore crucial to understand precisely the initial conditions at which to start the hydrodynamic models, one of them being the initial temperature. The measurement of black body radiation emitted from the QGP is therefore another active research area of our group .
-in HADES e AMADEUS: 3) we study the modification of hadron properties in nuclear matter.Our group is particularly interested in studying these modifications in terms of interactions between hadrons. Indeed, these interactions will characterize the EoS of the matter we are looking at and hence be relevant for the understanding of dense matter and maybe neutron stars. Another pillar of our research evolves around the study of Kaons, Antikaons and Λ hyperons within normal nuclear matter. There, we look for signatures of the mean value of the interaction of many nucleons with the hadrons of interest to get closer to the high density environment that characterize Neutron Stars. In general the quest of in-medium properties of hadrons is a complicated puzzle where many effects must be taken into account. We try to compose a complete model that takes into account all possible processes that hadrons can undergo within normal nuclear matter: elastic scattering, inelastic scattering, formation of resonances, multi-step processes. Once all obvious and known processes are accounted for we see if something ‘else’ is left to be seen .