Particle physics

Rutherford Scattering

Beta Negative Decay

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Quark structure proton

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02 Fermilab - Fermi National Accelerator Laboratory - American particle accelerator Fermilab near Chicago Illinois

View inside detector at the CMS cavern LHC CERN

Particle physics is a branch of physics that studies the nature of the particles that constitute matter and radiation. Although the word "particle" can refer to various types of very small objects (e.g., protons, neutrons, and electrons), particle physics usually investigates the irreducibly smallest detectable particles and the fundamental forces necessary to explain their behavior. As such, it is also called "high-energy physics," due to the necessity of using particle accelerators to produce and study these particles.

Overview

Particle physics aims to understand the universe at the smallest scales and highest energies. At the heart of the field is the Standard Model of particle physics, a theory that has successfully explained the properties and interactions of fundamental particles through the framework of quantum mechanics and special relativity. The Standard Model classifies all known subatomic particles into fermions (matter particles) and bosons (force carriers).

Fundamental Particles

Fermions are divided into quarks and leptons. Quarks make up protons and neutrons and are bound together by the strong force, which is mediated by gluons, a type of boson. Leptons include the electron and the neutrino, which interact through the weak force and electromagnetic force but do not feel the strong force. Bosons are particles that mediate the fundamental forces; besides gluons, they include the photon (mediator of the electromagnetic force), the W and Z bosons (mediators of the weak force), and the Higgs boson, which gives other particles mass through the Higgs mechanism.

Fundamental Forces

The four fundamental forces in nature are the strong force, the electromagnetic force, the weak force, and gravity. The Standard Model incorporates the first three, with gravity remaining outside the framework, described instead by general relativity. A grand unified theory, which would include gravity along with the other three forces, is a major goal of theoretical physics.

Experimental Particle Physics

Experimental particle physics involves the use of particle accelerators and particle detectors to study the properties of particles. High-energy accelerators, such as the Large Hadron Collider (LHC), accelerate particles to near the speed of light and collide them, allowing physicists to study the results of these collisions. Detectors capture and analyze the particles produced in these collisions, providing data that tests the predictions of the Standard Model and searches for new physics beyond the Standard Model.

Theoretical Particle Physics

Theoretical particle physics seeks to develop mathematical models that describe the properties and interactions of particles. This includes efforts to reconcile the Standard Model with general relativity and to predict new particles and phenomena that could be observed in experimental settings. Notable theoretical frameworks beyond the Standard Model include string theory and loop quantum gravity.

Challenges and Future Directions

Despite its successes, the Standard Model is not complete. It does not include a quantum theory of gravity, nor does it explain the dark matter and dark energy that appear to constitute most of the universe's mass-energy content. Additionally, the mechanism of neutrino oscillations and the matter-antimatter asymmetry observed in the universe are not fully explained. Future research in particle physics aims to address these and other unanswered questions, potentially leading to new discoveries about the nature of the universe.