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Topic 2: "Cosmic Matter in the Laboratory"


Nuclear and hadron physics deals with some of the most difficult and fascinating problems of contemporary physics. Although the underlying theory of the strong interactions, Quantum Chromo Dynamics (QCD) is well established, the underlying matter and gauge degrees of freedom can never be observed as free particles. They only show up in form of composite objects such as hadrons, nuclei and other forms of strongly interacting matter. In addition, at typical hadronic length scales, the strong force is really strong thus requiring a non-perturbative treatment.


About 99.9 % of the mass of the visible Universe is represented by nucleons. Understanding the origin of its mass is one of the most fascinating and challenging problems in physics. The centennial success of particle physics, discovering the Higgs particle in violent proton-proton collisions at the LHC, indicates that the proposed mechanism for mass generation, by "dressing" originally massless elementary particles by interaction with the Higgs field, is realized in nature. Yet, this mechanism cannot account for most of the mass of the confined, colourless three-quark object, the nucleon; only a few percent of the nucleon mass is due to the current quark mass, which is induced by the Higgs mechanism. The by far bigger part of the nucleon mass was generated about 10 μs after the Big Bang, when non-perturbative effects led to a transition, where the chiral symmetry of QCD is spontaneously broken and quarks and gluons are confined into colourless objects, hadrons. The understanding of the mechanism behind colour confinement and chiral symmetry breaking and in particular their role in the generation of hadron masses is still incomplete and hence remains a central puzzle in high-energy nuclear and hadron physics.


As a first example, the structure of the nucleon, which is the basic block of the matter in the Universe should be mentioned. Even such a fundamental quantity like the charge radius is under debate as different experiments using electron scattering or the muonic Lamb shift lead to incompatible results. Although much has been learned about the nucleons internal structure using high-energy lepton beams and invoking polarization, the experimental determination of nucleon structure observables from antiproton annihilation reactions (time like regime) is still sparse, despite the fact that it represents a natural part of the matrix element. The antiproton beams at FAIR render possible a high precision determination of for example the time like nucleon form factors with the PANDA experiment at FAIR. A precise understanding how the spin is made up is lacking and transversity, the last leading twist observable has yet to be measured directly. The golden channel to study transversity will be the measurement of double polarized Drell-Yan production in proton-antiproton annihilation, and thus constitutes a major motivation to investigate methods to polarize antiprotons by the PAX collaboration.


Another hot topic is the spectrum of QCD. For a long time, it appeared that the only ground states generated by QCD are mesons as quark-antiquark or baryons as three quark states, although QCD allows for a much larger class of bound states. In the last decades, this simple picture has been challenged as a cornucopia of unexpected states has been observed in the charmonium and bottomonium spectrum and for the first time exotic states (that means non qq or 3q states) have been established experimentally. Much more detailed experimental studies of these states are required to pin down their precise nature, such as line shape studies foreseen with the PANDA experiment at FAIR.


With today's accelerators, strongly interacting cosmic matter can be created under extreme conditions in a wide range of temperature and compression in the laboratory. High luminosity in combination with state-of-the-art detector systems allows studying the formed matter with various probes in systematic multi-differential studies. With ALICE at LHC, precision studies of the properties of the quark gluon plasma (QGP) are now being conducted. Matter created in collider experiments at ultra-relativistic energies resembles the situation realized in the early Universe when it was dominated by free quarks and gluons. In fixed-target experiments, baryon-dominated matter is studied at densities several times higher than ground-state matter density with the HADES experiment. These studies of compressed baryonic matter will be refined and extended with the CBM detector in the Compressed Baryonic Matter (CBM) research program, one of the four essential research pillars of FAIR. Its scientific mission is to explore the QCD phase diagram in the region of high net-baryon densities which is still terra incognita. The CBM experiment is designed to perform precision measurements of rare diagnostic probes which are the key observables for the investigation of the highest net-baryon densities created in the laboratory.


Another major line of research in hadron physics is the test of symmetries within and beyond the Standard Model. The CKM model of CP violation has been firmly established in B-meson decays. However, it is well-known that there must be other sources of CP violation to account for the observed matter-antimatter asymmetry in the Universe. One prominent way to look for such physics beyond the Standard Model are high-precision experiments that combined with accurate theory allow one to put constraints on or eventually discover New Physics. Noticeable examples are the electric dipole moments (EDMs) of nucleons or light nuclei. In particular, measurements of the proton or light nuclear EDMs in storage rings hold the promise of improving the so far existing bounds from cold neutron experiments by orders of magnitudes. The search for EDMs of charged particles with unprecedented sensitivity is the goal of the JEDI collaboration. Further, isopsin-symmetry violating reactions that can be studied at hadron colliders involving polarization allow one to gain further bounds on the light quark mass difference, so as to pin down these fundamental parameters with higher accuracy.


The visible matter in the Universe consists primarily of neutrons and protons, which — when bound by the residual strong force — yields the complex structures of atomic nuclei. They are the fuel of stars and the energy released in nuclear reactions enables us to observe the Universe. These reactions not only produce photons, neutrinos or cosmic rays by which we can look far into space and back in time, but also synthesize the elements we and the matter around us are made of. All known stable nuclei which are found on Earth and especially those heavier than lithium have gone through multiple stellar evolutions expelled by star explosions and collected again by gravity. The macroscopic processes in the life-cycle of stars and the element synthesis in stars are intimately connected with the microscopic properties of atomic nuclei. For instance, the understanding of the rapid neutron capture process (r-process), which is responsible for the synthesis of about half of the stable nuclei heavier than iron, requires knowledge of very neutron-rich isotopes, short-lived isotopes in order to constrain the conditions at the r-process sites, which are still not known with certainty, such as neutron flux, temperature, and others. On the other hand, nuclei consist of up to 300 nucleons and form a quantum system, which exhibits a large variety of structural phenomena and effects. Some 8000 different atomic nuclei are expected to exist, while only approximately 3000 of them have been identified or studied; thus, a large terra incognita of very neutron rich and super-heavy nuclei awaits its exploration. The production and study of isotopes up to the neutron and proton driplines and beyond including superheavy elements is an ongoing enterprise. Therefore, the construction and exploitation of the next-generation NuSTAR facilities at FAIR is the most important goal. With the start of the next-generation radioactive ion beam and intense stable-beam facilities ENNA will open a new era of nuclear research on exotic nuclei and super-heavy elements. Simultaneously and in order to improve our understanding of nuclear structure and dynamics the experimental efforts need to be accompanied by theoretical models based on microscopic pictures that utilize as far as possible the existing knowledge on the strong interaction. Therefore, theoretical and experimental efforts aim at the production and study of exotic nuclei and superheavy elements, and on understanding their properties and dynamics with ab-initio theories and the fundamental forces, with the goal to find the ordering principles and to allow for reliable predictions.