Particle Physics

Particle physics (also known as high energy physics) is a branch of physics that study 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, gas particles, or even household dust), particle physics usually investigate the irreducibly smallest detectable particles and the fundamental interactions necessary to explain their behavior. By our current understanding, these elementary particles are excitations of the quantum fields that also govern their interactions. The currently dominant theory explaining these fundamental particles and fields, along with their dynamics, is called the Standard Model. Thus, modern particle physics generally investigates the Standard Model and its various possible extensions, e.g. to the newest "known" particle, the Higgs boson, or even to the oldest known force field, garvity.

Modern particle physics research is focused on subatomic particles, including atomic constituents such as electrons, protons and neutrons (protons and neutrons are composite particles called baryons, made of quarks), produced by radioactive and scattering processes such as photons, neutrinos, and muons, as well as a wide range of exotic particles. Dynamics of particles are also governed by quantum mechanics; they exhibit wave-particle duality, displaying particle-like behavior under certain experimental conditions and wave-like behavior in others. 

In more technical terms, they are described by quantum state vectors in a Hilbert space, which is also treated in quantum field theory. Following the convention of particle physicists, the term elementary particles is applied to those particles that are, according to current understanding, presumed to be indivisible and not composed of other particles.

All particles and their interactions observed to date can be described almost entirely by the Standard Model. The Standard Model, as currently formulated, has 61 elementary particles. Those elementary particles can combine to form composite particles, accounting for the hundreds of species of particles that have been discovered since the 1960s.

The Standard Model has been found to agree with almost all the experimental tests conducted to date. However, most particle physicists believe that it is an incomplete description of nature and that a more fundamental theory awaits discovery. In recent years, measurements of neutrino mass have provided the first experimental deviations from the Standard Model, since neutrinos are massless in the Standard Model.

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