Introduction To Particle Physics

How the world of the smallest particles is structured, what is known about fundamental forces, and what particles have to do with the Big Bang

To the point

• Particle physics: This field explains the building blocks of matter and investigates the smallest particles in the universe.
• Big Bang: Physicists use particle accelerators to simulate conditions shortly after the Big Bang and try to understand how the universe developed.
• Standard Model: This model describes the fundamental particles and forces of the universe visible to us, with the Higgs boson responsible for the mass of matter. However, there are still many unanswered questions that make physics beyond this model likely.
• Fundamental forces: There are four fundamental forces: the electromagnetic force and the strong and weak nuclear forces are covered by the Standard Model of particle physics, and gravity by Einstein's general theory of relativity. The unification of both theories is an important field of research, including at the Max Planck Society.

Why particle physics?

If you want to explain what was and what is, you need particle physics. Because everything we see consists of molecules and atoms, and atoms in turn consist of atomic nuclei and electrons. The atomic nuclei contain positively charged protons and uncharged neutrons, which in turn consist of only two elementary particles: the up quark and the down quark. The paradox is that, on the shortest scales, the world always consists of the same particles. The closer you look, the simpler everything becomes. Why this is so is one of the biggest questions, also for researchers at the Max Planck Society.

On the trail of the Big Bang with particle accelerators

Shortly after the Big Bang, the universe was a single hot primordial soup. It was so hot and there was so much energy everywhere that quarks and other elementary particles flew around freely and interacted with each other. Only when the universe expanded rapidly and cooled down did quarks have the opportunity to combine to form protons and neutrons. To explore the beginning of the universe and the formation of matter particles, particle physicists use gigantic particle accelerators - with the highest energy concentrations that can be generated on Earth.

By firing protons at each other and analyzing the fragments of the collisions, researchers create a particle world for an extremely short time and in a small point, as it existed shortly after the Big Bang. In this way, particle physics not only answers the question of which particles exist that still make up matter at the smallest distances today, but also explains the universe as it existed 13.8 billion years ago and how it has developed to this day - without any of us humans having to be there.

What are the basic building blocks of matter?

Visible matter accounts for only about five percent of the universe. The rest is dark matter and dark energy. This small percentage determines everything we know: us, the Earth, the stars, and galaxies. According to current understanding, it consists of two different elementary particles, the so-called up and down quarks, as well as electrons. An electron has a single negative elementary charge of -1, an up quark has a charge of +2/3, and a down quark has a charge of -1/3.

However, other particles also occur in particle reactions, namely muons and tauons, which are heavier variants of the electron, and associated neutrinos. All these particles together form the lepton family. In addition to the up and down quarks, there are two other quark families, each with a charm and strange quark as well as a top and bottom quark. This can be confusing, and it is not without reason that researchers refer to it as the particle zoo. Here is an overview of the basics behind it.

The Standard Model of particle physics

The Standard Model is a highly successful mathematical construct and forms the basic toolkit of particle physics. It describes the structure of the universe - more precisely, the fundamental building blocks that make up matter throughout the universe. What is still missing is Einstein's theory of gravity, which describes what holds the universe together on a large scale. Both theories explain the universe from the smallest distances of about 10^-19 meters to about 100 billion light-years, the size of the universe that can be explored with telescopes. One of the biggest research questions is how both theories could be combined into a single one, the so-called "theory of everything."

The fact that matter has mass at all is thanks to another particle: the Higgs boson, which was discovered in 2012 at CERN's Large Hadron Collider. The Higgs represents the so-called Higgs field, which permeates everything. By interacting with this Higgs field, the building blocks of matter acquire their mass. The discovery of the Higgs boson has decisively consolidated the Standard Model of particle physics. And yet many questions remain, such as how the Higgs is related to gravity.

The fundamental forces of the (particle) world

There are four fundamental forces: the gravitational force, electromagnetic force, and the weak and strong nuclear forces. Even though it is theoretically possible to calculate the gravitational attraction between two people, this force is much smaller than the gravitational pull of the Earth on a person. This is because gravity depends on the product of two masses. This explains why objects of astronomical proportions are particularly affected by it.

Electromagnetism acts on the charged leptons in the Standard Model. It causes negatively charged electron shells to arrange themselves around positively charged atomic nuclei. It determines the atomic structure of molecules and allows the Earth's magnetic field to intercept charged particles from the sun. This force is about a hundred trillion trillion (a one followed by 38 zeros) times stronger than gravity.

The strong interaction or strong nuclear force, on the other hand, only acts at extremely short distances within atomic nuclei. It forms the glue between quarks and is a hundred times stronger than electromagnetism. The weak nuclear force is active at even shorter distances and is about a trillion times weaker than electromagnetism. It is responsible for radioactive decay, for example by allowing an up quark to transform into a down quark.

Electromagnetism and the weak and strong nuclear forces exert their effects via so-called exchange particles, also known as bosons. The Higgs boson, however, is special. It represents a field that knows no direction. As a so-called scalar field it spreads over the universe just by differing numbers. The other bosons represent fields that mediate forces and are directional. The photon (i.e., light) mediates electromagnetic interaction, the gluon mediates strong interaction, which binds quarks in protons or neutrons, and the W and Z bosons mediate weak interaction.

Why can we stand on Earth but not on a neutron star?

If we are made up entirely of particles, why doesn't everything flow into each other? Why does matter retain its shape? Why do we stand on the ground and not merge with it? This is where the electromagnetic forces between atoms come into play, which also arrange atoms into molecular structures.

On a neutron star, things are different. Here, the mass of an entire star is contained in a sphere the size of a city. The gravitational pull on a human being would be enormous and would cause their subatomic components to spread out over the surface of the extreme mass. The gravitational forces on a neutron star are so strong that matter is literally compressed into neutrons. On the surface of a neutron star, the strong nuclear force between neutrons dominates.

But does a star actually collapse? When the supply of material for nuclear fusion inside is exhausted, the outward-directed radiation energy is lacking, gravity dominates, and the star begins to collapse. If the star is heavy enough, the forces are strong enough for electrons and protons to combine into neutrons and neutrinos. Incidentally, this reaction is made possible by the weak nuclear force. With the electrons now missing, nothing can keep the star in shape and gravity finally wins out. If the original star is even larger and heavier, the collapse is so violent that even the densest packing of matter in the neutron star can no longer maintain its shape. When the strong nuclear force is overcome, all matter falls into an infinitely small point - a black hole is created.

What are the limits of the Standard Model of particle physics?

The Standard Model works perfectly for known matter, which makes up five percent of the universe. But there are still many unanswered questions that make physics beyond the Standard Model likely. Here are a few examples:

Gravity and the particle world of quantum mechanics can currently only be described in two separate theories. Black holes could be the key to explaining both in a coherent theory. This is because black holes connect both: the large and the smallest scales, the highest energies and the smallest distances. It is also still unclear what dark matter is. Does it permeate the entire universe like the Higgs field? Can it therefore be explained by a particle, the realization of such a field? Have researchers simply not found all the particles yet? Or is dark matter the sum of many light black holes?

And finally, the question: Why do we exist at all? If things had gone just a little differently in the early universe, we probably wouldn't be here today. Back then, shortly after the Big Bang, particles and antiparticles existed in almost identical quantities, with only a tiny bit more matter than antimatter; more precisely, the excess was only 1 in a billion. When matter and antimatter annihilated in a blaze of light only about 13 seconds after the Big Bang, just a little matter remained, just enough to explain everything we see today. Why there was this slight excess of matter is one of the great unanswered questions.

MPG/BEU

Background information

What energy do particle accelerators reach?

In order to study the smallest components of matter, enormous forces that hold atoms together must be overcome. The deeper one wants to penetrate into this world, the more energy one needs to apply. That is why particle hunters accelerate protons in CERN's Large Hadron Collider (LHC) to almost the speed of light and force them into a circular path with a circumference of 27 kilometers using superconducting magnets. Their goal: to collide two such proton beams from opposite directions, as in a rear-end collision. The protons are torn apart into their constituent parts, and in some cases new particles are even created, all of which are identified in a detector the size of a multi-story building, the ATLAS detector.

Particle accelerators specify the collision energy in electron volts. The LHC reaches about 14 TeV (teraelectron volts, a one followed by twelve zeros) at the point of collision. One electron volt corresponds to the energy that an electron experiences when passing through an acceleration voltage of one volt.

The Future Circular Collider: The possible future.

The Future Circular Collider (FCC) is being discussed as the successor to the LHC. It is expected to reach 100 TeV at the collision point. Technically, this would be a huge leap forward. The energy would also allow us to get even closer to the Big Bang. By way of comparison, the LHC and FCC "see" a universe as it existed less than a trillionth of a second after the Big Bang. That's a one with 12 zeros before the decimal point. But a lot happened between that point and the moment of zero. The era of inflation, i.e., the sudden expansion of the universe, ended about one quintillionth of a second after the Big Bang, which is a one with 35 zeros before the decimal point. The energy of the universe at this point was still a billion times higher than the energy at the focus of the FCC.

What are fields and how are they related to particles?

According to Marumi Kado, director at the Max Planck Institute for Physics, particle physics is fundamentally misunderstood. He says that particles are only one side of the coin, the tip of the iceberg even. Particles are merely excitations of something more fundamental, known as fields, at high energies. The Higgs boson, for example, is an excitation of the Higgs field, which permeates everything. This excitation can only be achieved at the high collision energies of the LHC. Marumi Kado compares this relationship to the surface of a lake: "It is like punching your fist into the calm surface of a lake - the splashing water droplets are like Higgs particles, excitations of the Higgs field". This does not make particles any less important, because only those who detect them can prove that the underlying field exists.