Particle physics

Particle physics, also known as high-energy physics, is the study of the fundamental particles and forces that create matter and radiation. This field explores the most basic building blocks of our universe. It also investigates how these particles combine to form larger structures like protons and neutrons. While nuclear physics focuses on the combinations of protons and neutrons, particle physics looks deeper at the elementary parts themselves. Scientists use two main approaches to study this realm. Experimental particle physics uses machines to observe particles during radioactive processes or high-speed collisions. Theoretical particle physics uses math and quantum theory to understand the cosmos.

To understand how matter works, scientists use a framework called the Standard Model. This model describes how 61 elementary particles interact through different forces. These particles are divided into two main groups: fermions and bosons. Fermions are the matter particles that build everything we see. There are three generations of fermions, though ordinary matter only uses the first generation. This first generation includes up and down quarks, which form protons and neutrons, as well as electrons and electron neutrinos. Bosons are the force-carrying particles, or mediators, that allow these matter particles to interact.

The Standard Model explains three fundamental interactions mediated by specific bosons. Electromagnetism is carried by the photon, which is the quantum of light. The weak interaction, which is responsible for certain types of decay, is mediated by the W and Z bosons. The strong interaction is carried by gluons, which link quarks together. Because of a process called color confinement, gluons and quarks cannot be observed on their own. Quarks possess a property called color charge, labeled as red, green, or blue. When quarks combine to form particles called hadrons, their color charges must combine to become neutral, or "white."

Hadrons are composite particles made of quarks. There are two main types of hadrons: baryons and mesons. Baryons are particles that contain an odd number of quarks, such as the proton and the neutron. Protons consist of two up quarks and one down quark. Neutrons consist of two down quarks and one up quark. Mesons are particles that contain an even number of quarks, specifically one quark and one antiquark. Mesons are very unstable and short-lived. The longest-lived mesons last for only a few hundredths of a microsecond. These are often produced during high-energy collisions in particle accelerators.

Every particle also has a corresponding antiparticle. An antiparticle has the same mass as its particle twin but carries the opposite electric charge. For example, the electron has a negative charge, while its antiparticle, the positron, has a positive charge. If a particle and its antiparticle meet, they undergo annihilation. This process converts their mass into other particles. Some particles, like the photon or the gluon, are their own antiparticles. These particles are actually excitations of underlying quantum fields. These fields govern both the particles and how they interact with one another.

The history of this science shows how our understanding has grown. The idea that matter is made of tiny pieces dates back to at least the 6th century BC. In the 19th century, John Dalton concluded that each element was made of a unique type of particle. Later, physicists discovered that atoms are actually made of smaller parts like electrons. In 1939, Lise Meitner and Otto Hahn proved nuclear fission, while Hans Bethe discovered nuclear fusion. By the 1950s and 1960s, scientists discovered so many new particles that they called it a "particle zoo." The Standard Model was eventually formulated in the 1970s to explain this variety.

Modern research continues to test the limits of the Standard Model. One major success was the discovery of the Higgs boson. This particle was predicted by theory before it was confirmed by experiments at the Large Hadron Collider at CERN in 2012. The Higgs boson is important because it gives mass to the W and Z bosons through the Higgs mechanism. However, the Standard Model may still be incomplete. For instance, it does not currently include gravity, which remains a major challenge for physicists. Recent measurements of neutrino mass have also shown deviations from the model. Scientists are now looking for a more fundamental theory, sometimes called a "Theory of Everything."