Particle Physics
Scientists at the Max Planck Society don't just look at the largest objects in space. They also need to understand what holds the world together at the smallest level. Everything is made up of particles—including us: protons, neutrons, and electrons are the building blocks of the atoms in our bodies. But there are still many unanswered questions about the nature of our existence. In search of answers, particle hunters at CERN's Large Hadron Collider cause proton collisions, search deep underground for dark matter particles, or track high-energy particles from space.
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Fundamentals of particle physics
Everything we see consists of molecules and atoms, which in turn consist of atomic nuclei and electrons. The atomic nuclei contain positively charged protons and uncharged neutrons, which consist of only two versions of the same particle, the quark. What other particles are known? What forces act between the particles, and why do these forces explain why we can walk on Earth and why the particles of our bodies do not merge with those of the Earths surface? Why were particles particularly prevalent shortly after the Big Bang, and how do particle accelerators help to bring the early universe back to life? All the answers can be found in the following background on particle physics:
The roots of particle physics in the Max Planck Society
Although Albert Einstein laid the foundations with his discovery of the photoelectric effect (photovoltaics) as well as Nils Bohr with his atomic model, it was not until 1925 that a breakthrough was achieved in developing a coherent theory of the smallest particles of matter: Werner Heisenberg formulated the mathematical framework of quantum theory. With Heisenberg as former director of the Max Planck Institute for Physics in Munich, the Max Planck Society also looks back on an eventful past. Today, theorists and experimental physicists are conducting research on this topic not only at the Institute for Physics but also at the Max Planck Institutes for Nuclear Physics, Gravitational Physics and Quantum Optics.
Theory of particle physics
The Standard Model of particle physics is a highly successful mathematical construct that describes the structure of the universe – more precisely, the fundamental building blocks of matter throughout the universe. Einstein's theory of gravity, on the other hand, describes how the universe holds together on a large scale. Both, the Standard Model of particle physics and General Relativity explain the universe both at the smallest distances of about 10-19 meters and the largest known distances that we can explore, which is 100 billion light-years. Unifying both theories is one of the biggest research questions.
The fact that matter has mass at all is explained by the Higgs particle, which was discovered in 2012 at CERN's Large Hadron Collider. The Higgs particle represents the so-called Higgs field, which permeates everything. By interacting with this Higgs field, the building blocks of matter acquire their mass. This extension has decisively consolidated the Standard Model of particle physics. And yet there are still many unanswered questions that researchers at the Max Planck Society are investigating, such as how the Higgs could be related to gravity.
Particles from the laboratory
Particle collisions are needed to test the predictions of the Standard Model and understand the interactions of elementary particles. At CERN's Large Hadron Collider (LHC), for example, two proton beams collide with each other from two opposing directions, generating energies at a single point that are high enough to measure the building blocks of protons and other particles. With the LHC, researchers are answering questions about what the young and hot universe looked like shortly after the Big Bang and why it is the way it is today. The Higgs particle, an excitation of the so-called Higgs field that permeates everything, was also found in the LHC's ATLAS detector. However, researchers also use other experiments at CERN to study elementary particles and weigh the mass of the neutrino in a gigantic magnetic cage.
Astroparticle physics
Particles penetrate us at all times, many of which originate in space. When electrons or protons are accelerated in the magnetic fields of extreme astrophysical objects in space, high-energy neutrinos or gamma rays are produced. When these hit the Earth, they can be detected using various methods. Neutrinos hardly interact with matter; however, when passing through the Mediterranean Sea, even a single reaction is enough to cause detectors hanging like strings of pearls below the water's surface to register weak but measurable flashes of light. Instead of seawater, particle physicists also use the atmosphere as a so-called detector volume. When a high-energy particle from the universe collides with the Earth's atmosphere, it triggers a whole chain of subsequent reactions. The new particles that are created in this way can produce a glow that is captured by gamma telescopes such as MAGIC or H.E.S.S. on the ground. By studying these light-signals, they can reconstruct the original particle from space, its energy, origin, and perhaps even how it was created. There are also large water basins on Earth equipped with sensitive detectors that directly detect particles from the atmosphere, or experiments in an underground laboratory that focus entirely on the search for particles that could explain dark matter.
Where does research stand today, and where is it headed?
Despite all the research successes, there are still big and unanswered questions: Why was there a small excess of matter over antimatter in the early universe? It is thanks to this circumstance that we and everything we see around us exist. The matter that accompanies us in everyday life is highly diverse and consists of different materials, which in turn are composed of specific atoms. However, the interior of atoms, the atomic nuclei, consist of only two ingredients: neutrons and protons. Why is that? And will the world remain so simple on a small scale when a new particle accelerator like the potential Future Circular Collider, FCC, penetrates even deeper into it? And finally, a question that also preoccupied Einstein: How can his theory of gravity and quantum mechanics be combined into a universal theory that fully explains nature and its forces at all scales? Even if the Standard Model of particle physics described the world as we know it completely, there is still much to be done. This is because only five percent of the universe is known to us, including planets and stars. The rest is dark matter and dark energy. Both are still unknown, but researchers are eagerly searching for the ingredients of dark matter – may it be particles or even a multitude of smaller black holes.
