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ELEMENTS and its questions

Much of the focus of modern physics is about the understanding of the complex interactions between quarks and gluons that lead to the formation of matter, but also about the equation of state (EOS) and different phases of nuclear matter, i.e., about the dynamical and thermo-dynamical properties of matter under extreme conditions of density, temperature, gravity, and isospin. Attaining such an understanding is a major challenge in contemporary subatomic physics, but, astonishingly, also in the astrophysics and gravitational physics of compact stars. Reaching this understanding will finally provide us with the knowledge of how elements in the Universe have been created, both the “light” ones, whose origin dates back to the Big Bang, and the “heavy” elements, such as gold and platinum, that are instead mostly produced with the merger of neutron stars (NSs).

On the particle-physics side, we know that the Standard Model of elementary-particle physics provides a robust description of the elementary building blocks and fundamental interactions. Yet, the complex properties and phases of matter that are possible in Nature, and how these are intimately connected to the properties of fundamental-particle interactions, are far from being understood. On the astrophysics side, we know that massive stars, the cosmic nuclear reactors transforming light elements into heavier ones, explode producing compact stars, which coalesce through the emission of gravitational waves (GWs) when in binary systems. Their merger releases enormous amounts of energy, both via GWs and through electromagnetic (EM) radiation, leading to the most catastrophic phenomena in astrophysics: “short gamma-ray bursts”. Advanced general-relativistic numerical simulations show that considerable amounts of mass are ejected into the interstellar medium, “polluting” it with its neutron-rich content and synthesizing all of the heavy elements that we encounter in our environment. Despite the qualitative picture described above, our quantitative understanding of these processes is still very limited. A number of fundamental-physics questions still remain unanswered, and are therefore at the very core of the science in ELEMENTS:

Q1. What are the properties of matter under extreme densities and temperatures?

Matter at very high temperatures and densities – such as the one encountered shortly after the Big Bang or when two NSs collide – is governed by the strong interaction and may exist in different phases, often exhibiting strong collective properties. One of these phases is the quark-gluon plasma (QGP), which permeated the early Universe, but can also be reproduced in collisions of heavy ions at the highest available collision energies (which probe very small net baryon densities). When NSs merge emitting strong GWs (which probe very large net baryon densities), more phases may occur, requiring a systematic scan of the QCD phase diagram at different energies. A variety of numerical and analytical concepts have been instrumental to calculate the properties of the QGP and to model the final state of HICs, ranging from first-principles approaches to study QCD thermodynamics on the lattice, from transport theory to viscous relativistic hydrodynamics, from merging NSs to their gravitational and EM emission. To make progress in this multidisciplinary field and advance our understanding of the phase diagram, a close collaboration between experiment and theory is essential and scientists in ELEMENTS can provide the needed expertise. Together, they will construct a renewed picture of the phase diagram, exploring in detail regions not considered before, building a comprehensive picture of matter under extreme conditions. 

Q2. How do particle bound states emerge from strong interactions?

Bound states leading to what is observed as building blocks of matter emerge from strong and EM interactions at various levels. On microscopic scales, ELEMENTS will explore hadronization in heavy-ion collisions (HICs), producing loosely bound states and multi-strange hadrons. At experimental facilities such as the Helmholtzzentrum für Schwerionenforschung GmbH (GSI)/FAIR, ELEMENTS will study the properties of exotic neutron-rich nuclei to the limits of binding, including measurements of their charge radii, neutron skins, and density modulations. This will provide key tests for the strong interaction and constrain the nuclear EOS at the most neutron-rich conditions achievable in the laboratory, with direct consequences for r-process nucleosynthesis and for the structure of NSs. Moreover, ELEMENTS will investigate the properties of bound states in strong EM fields relevant for kilonovae using modern atomic-physics techniques on relativistic ions in storage rings and new theoretical approaches.

Q3. What is the origin of the elements in the Universe?

A central goal of the research carried out by ELEMENTS is to understand the origin of heavy elements in the Universe, based on a unified theoretical description of the nuclear and neutrino microphysics. Key information, such as masses, radii, half-lives, and decay processes, will be measured with high precision, developing new techniques, e.g., using ultracold ion beams in the storage rings at GSI/FAIR or electrofission reactions. This will challenge and contrast ab-initio calculations of nuclei at extreme conditions. Our research will provide quantitative insight into the astronomical sites of the r-process and their radiation signal. In particular, ELEMENTS will provide answers covering not only all aspects of the microphysics of the r-process, but also the macro-physical ones related to the gravitational and EM emission from these phenomena. 

Q4. How can gravitational waves tell us about the fundamental properties of matter?

The first direct detection of GWs from a binary system of NSs has marked the dawn of multi-messenger GW astronomy and has proven that the detection of GWs is an alternative and complementary route to explore the Universe. GWs from merging binary NSs not only are among the strongest signals, but they also provide direct clues to the EOS of nuclear matter. Indeed, the nuclear EOS at non-vanishing density, temperature, and electron fraction is a key ingredient for simulating the merger of two NSs or the merger of a NS and a black hole, since it leaves a clear imprint on the GWs from these systems. ELEMENTS will provide important contributions to the detection of GWs from merging NSs by improving the accuracy of the theoretical predictions of the signal before and after the merger. In addition, it will optimize the extraction of physical information contained in the GW and EM signal from merging binaries and provide ab-initio information on the physical conditions leading to the kilonova emission. 

Over the course of the last 30 years, a number of attempts have been made to provide answers to these long-standing questions. Most of the time, however, these questions have been seen as distinct, so that the answers provided so far do not stem from a comprehensive approach. The collaborative effort to be constructed with ELEMENTS is meant to provide exactly such a comprehensive vision. In this context, High-Performance Computing (HPC) represents an invaluable tool to manage and process the enormous amounts of data that modern accelerators accrue, and also to construct ever more realistic models of the physical phenomena that are revealed by the detectors. In this respect, PIs in ELEMENTS are particularly privileged. In addition, in fact, to the resources that are available to them through the supercomputing Tier-0 facilities in Germany (e.g., at the LRZ in Garching or at the HLRS in Stuttgart), Frankfurt and Darmstadt also offer modern Tier-1 supercomputing facilities at the Center for Scientific Computing (CSC) of Goethe University, or at the Lichtenberg High Performance Computer in Darmstadt. Both facilities will provide the needed local support and resources to carry out the ambitious HPC program within ELEMENTS.